2. GNAT Project Manager — GPR Tools User's Guide 27.0w documentation

# 2. GNAT Project Manager

## 2.1. Introduction

This chapter describes GNAT’s Project Manager, a facility that allows you to manage complex builds involving a number of source files, directories, and options for different system configurations. In particular, project files allow you to specify properties including:

- The directory or set of directories containing the source files, and/or the names of the specific source files themselves;

- The directory in which the compiler’s output (`ALI` files, object files, tree files, etc.) is to be placed;

- The directory in which the executable programs are to be placed;

- Switch settings, which can be applied either globally or to individual compilation units, for any of the project-enabled tools;

- The source files containing the main subprograms to be built;

- The source programming language(s); and

- Source file naming conventions, which can be specified either globally or for individual compilation units (see Naming Schemes).

Project files also allow you to:

- Change any of the above settings depending on external values, thus enabling the reuse of the projects in various scenarios (see Scenarios in Projects); and

- Automatically build libraries as part of the build process (see Library Projects).

Project files are written in an Ada-like syntax, using familiar notions such as packages, context clauses, declarations, default values, assignments, and inheritance (see Project File Reference).

Project files can depend upon other project files in a modular fashion, simplifying complex system integration and project reuse.

- One project can import other projects containing needed source files. More generally, the Project Manager lets you structure large development efforts into possibly interrelated subsystems, where build decisions are delegated to the subsystem level, and thus different compilation environments (switch settings) are used for different subsystems. See Organizing Projects into Subsystems.

- You can organize GNAT projects in a hierarchy: a project can extend a base project, inheriting its source files and optionally overriding any of them with alternative versions. See Project Extension.

Several tools support project files, generally in addition to specifying the information on the command line itself. They share common switches to control the loading of the project (in particular `-Pprojectfile` to define the applicable project file and `-Xvbl=value` to set the value of an external variable).

The Project Manager supports a wide range of development strategies, for systems of all sizes. Here are some typical practices that are easily handled:

- Using a common set of source files and generating object files in different directories via different switch settings. This can be used for instance to generate separate sets of object files for debugging and for production.

- Using a mostly shared set of source files with different versions of some units or subunits. This can be used for instance to group and hide all OS dependencies in a small number of implementation units.

Project files can be used to achieve some of the effects of a source versioning system (for example, defining separate projects for the different sets of sources that comprise different releases) but the Project Manager is independent of any source configuration management tool that might be used by the developers.

The sections below use an example-driven approach to present and illustrate the various concepts related to projects.

## 2.2. Building with Projects

In its simplest form a project may be used in a stand-alone fashion to build a single executable, and this section will focus on such a setup in order to introduce the main ideas. Later sections will extend this basic model to more complex and realistic configurations.

The following concepts are the foundation of project files, and will be further detailed later in this documentation. They are summarized here as a reference.

Project file:

  A text file expressed in an Ada-like syntax, generally with the `.gpr` extension. It defines build-related characteristics of an application. The characteristics include the list of sources, the location of those sources, the location for the generated object files, the name of the main program, and the options for the various tools involved in the build process.

Project attribute:

  A specific project characteristic is defined by an attribute clause. Its value is a string or a sequence of strings. All settings in a project are defined through a list of predefined attributes with precise semantics. See Attributes.

Package in a project:

  Global attributes are defined at the top level of a project. Attributes affecting specific tools are grouped in a package whose name is related to tool’s function. The most common packages are Builder, Compiler, Binder, and Linker. See Packages.

Project variables:

  In addition to attributes, a project can use variables to store intermediate values and avoid duplication in complex expressions. Variables can be initialized with external values coming from the environment. A frequent use of variables is to define scenarios. See External Values, Scenarios in Projects, and Variables.

Source files and source directories:

  A source file is associated with a language through a naming convention. For instance, `foo.c` is typically the name of a C source file; `bar.ads` or `bar.1.ada` are two common naming conventions for a file containing an Ada spec. A compilable entity is often composed of a main source file and potentially several auxiliary ones, such as header files in C. The naming conventions can be user-defined (see Naming Schemes), and will drive the builder to call the appropriate compiler for the given source file.

  Source files are searched for in the source directories associated with the project through the Source_Dirs attribute. By default, all the files (in these source directories) following the naming conventions associated with the declared languages are considered to be part of the project. It is also possible to limit the list of source files using the Source_Files or Source_List_File attributes. Note that those last two attributes only accept basenames with no directory information.

Object files and object directory:

  An object file is an intermediate file produced by the compiler from a compilation unit. It is used by post-compilation tools to produce final executables or libraries. Object files produced in the context of a given project are stored in a single directory that can be specified by the Object_Dir attribute. In order to store objects in two or more object directories, the system must be split into distinct subsystems, each with its own project file.

The following subsections introduce the attributes of interest for simple build needs. Here is the basic setup that will be used in the following examples:

> The Ada source files `pack.ads`, `pack.adb`, and `proc.adb` are in the `common/` directory. The file `proc.adb` contains an Ada main subprogram `Proc` that `with`s package `Pack`. We want to compile these source files with the switch `-O2`, and place the resulting files in the `common/obj/` directory. Here is the directory structure:

>     common/
>       pack.ads
>       pack.adb
>       proc.adb
>     common/obj/
>       proc.ali, proc.o pack.ali, pack.o, proc.exe

Our project is to be called Build. The name of the file is the name of the project (case-insensitive) with the `.gpr` extension, therefore the project file name is `build.gpr`. This is not mandatory, but a warning is issued when this convention is not followed.

This is a very simple example, and as stated above, a single project file is sufficient. We will thus create a new file, `build.gpr`, that initially contains an empty project declaration:

>     project Build is
>     end Build;

Note that repeating the project name after `end` is mandatory.

### 2.2.1. Source Files and Directories

When you create a new project, the first task is to specify where the corresponding source files are located. These are the only settings that are needed by all the tools that will use this project (builder, compiler, binder and linker for the compilation, IDEs to edit the source files, etc.).

The first step is thus to declare the source directories, which are the directories to be searched to find source files. In the current example, the `common` directory is the only source directory.

There are several ways to specify the source directories:

- When the attribute Source_Dirs is not defined, a project contains a single source directory which is the one where the project file itself resides. In our example, if `build.gpr` is placed in the `common` directory, the project will have the needed implicit source directory.

- The attribute Source_Dirs can be set to a list of path names, one for each of the source directories. Such paths can either be absolute names (for instance `"/usr/local/common/"` on Unix), or relative to the directory in which the project file resides (for instance `"."` if `build.gpr` is inside `common/`, or `"common"` if it is one level up). Each of the source directories must exist and be readable.

  The syntax for directories is platform specific. For portability, however, the project manager will always properly translate Unix-like path names to the native format of the specific platform. For instance, when the same project file is to be used both on Unix and Windows, `"/"` should be used as the directory separator rather than `"\"`.

- The attribute Source_Dirs can automatically include subdirectories using a special syntax inspired by some Unix shells. If any of the paths in the list ends with “`**`”, then that path and all its subdirectories (recursively) are included in the list of source directories. For instance, “`**`” and “`./**`” represent the complete directory tree rooted at the directory in which the project file resides.

When using the `Source_Dirs` construct, you may sometimes find it convenient to also use the attribute `Excluded_Source_Dirs`, which is also a list of paths. Each entry specifies a directory whose immediate content, not including subdirs, is to be excluded. It is also possible to exclude a complete directory subtree using the `**` notation.

It is often desirable to remove, from the source directories, directory subtrees rooted at some subdirectories. An example is the subdirectories created by a Version Control System such as Subversion that creates directory subtrees rooted at a subdirectory named `.svn`. To do that, attribute Ignore_Source_Sub_Dirs can be used. It specifies the list of simple file names or patterns for the roots of these undesirable directory subtrees.

>     for Source_Dirs use ("./**");
>     for Ignore_Source_Sub_Dirs use (".svn", "@*");

With the declaration of attribute Ignore_Source_Sub_Dirs above, .svn subtrees as well as subtrees rooted at subdirectories with a name starting with ‘@’ are not part of the source directories of the project.

When applied to the simple example, and because we generally prefer to have the project file at the top-level directory rather than mixed with the sources, we will add the relevant definition for the `Source_Dirs` attribute to our `build.gpr` project file:

>     project Build is
>        for Source_Dirs use ("common");  --  <<<<
>     end Build;

Once the source directories have been specified, you may need to indicate specific source files of interest. By default, all source files present in the source directories are considered by the Project Manager. When this is not desired, it is possible to explicitly specify the list of sources to consider. In such a case, only source file base names are indicated and not their absolute or relative path names. The project manager is in charge of locating the specified source files in the specified source directories.

- By default, the project manager searches for all source files of all specified languages in all the source directories.

  Since the project manager was initially developed for Ada environments, the default language is usually Ada and the above project file is complete: it defines without ambiguity the sources composing the project: that is, all the sources in subdirectory `common` for the default language (Ada) using the default naming convention.

  However, when compiling a multi-language application, or a pure C application, the project manager must be told which languages are of interest, which is done by setting the Languages attribute to a list of strings, each of which is the name of a language.

  Even when only Ada is used, the default naming might not be suitable. Indeed, how does the project manager distinguish an Ada source file from any other file? Project files can describe the naming scheme used for source files, and override the default (see Naming Schemes). The default is the standard GNAT extension (`.adb` for bodies and `.ads` for specs), which is what is used in our example, and thus no naming scheme is explicitly specified. See Naming Schemes.

- Source_Files. In some cases, source directories might contain files that should not be included in a project. One can specify the explicit list of file names to be considered through the Source_Files attribute. When this attribute is defined, instead of looking at every file in the source directories, the project manager takes only those names into consideration and reports errors if they cannot be found in the source directories or do not correspond to the naming scheme.

- It is sometimes useful to have a project with no sources (most of the time because the attributes defined in the project file will be reused in other projects, as explained in Organizing Projects into Subsystems. To do this, the attribute `Source_Files` is set to the empty list, i.e. `()`. Alternatively, `Source_Dirs` can be set to the empty list, with the same result.

- Source_List_File. If there is a large number of files, it might be more convenient to use the attribute Source_List_File, which specifies the full path of a file. This file must contain a list of source file names (one per line, no directory information) that are searched as if they had been defined through `Source_Files`. Such a file can easily be created through external tools.

  A warning is issued if both attributes `Source_Files` and `Source_List_File` are given explicit values. In this case, the attribute `Source_Files` prevails.

- Excluded_Source_Files. Specifying an explicit list of files is not always convenient. Instead it might be preferable to use the default search rules with specific exceptions. This can be done through the attribute Excluded_Source_Files (or its synonym Locally_Removed_Files). Its value is the list of file names that should not be taken into account. This attribute is often used when extending a project, see Project Extension. A similar attribute Excluded_Source_List_File plays the same role but takes the name of file containing file names similarly to `Source_List_File`.

In most simple cases, such as the above example, the default source file search behavior provides the expected result, and we do not need to add anything after setting `Source_Dirs`. The Project Manager automatically finds `pack.ads`, `pack.adb`, and `proc.adb` as source files of the project.

Note that by default a warning is issued when a project has no sources attached to it and this is not explicitly indicated in the project file.

### 2.2.2. Duplicate Sources in Projects

If the order of the source directories is known statically, that is if `"/**"` is not used in the string list for `Source_Dirs`, then there may be several files with the same name situated in different directories of the project. In this case, only the file in the first directory is considered as a source of the project and the others are hidden. If `"/**"` is used in the string list for `Source_Dirs`, it is an error to have several files with the same name in the same directory `"/**"` subtree, since there would be an ambiguity as to which one should be used.

If there are two sources with the same name in different directories of the same `"/**"` subtree, one way to resolve the problem is to exclude the directory of the file that should not be used as a source of the project.

### 2.2.3. Object and Exec Directory

Another consideration when designing a project is to decide where the compiler should place the object files. In fact, the compiler and other tools might create several different kinds of files (for GNAT, there is the object file and the ALI file). One of the important concepts in projects is that most tools may consider source directories as read-only and thus do not attempt to create new or temporary files there. Instead, all such files are created in the object directory. (This is not true for project-aware IDEs, one of whose purposes is to create the source files.)

The object directory is specified through the Object_Dir attribute. Its value is the path to the object directory, either absolute or relative to the directory containing the project file. This directory must already exist and be readable and writable, although some tools have a switch to create the directory if needed (See the switch `-p` for gprbuild).

If the attribute `Object_Dir` is not specified, it defaults to the directory containing the project file.

For our example, we can specify the object directory in this way (assuming that the project file will reside in the parent directory of `common`):

>     project Build is
>        for Source_Dirs use ("common");
>        for Object_Dir use "common/obj";   --  <<<<
>     end Build;

As mentioned earlier, there is a single object directory per project. As a result, if you have an existing system where the object files are spread across several directories, one option is to move all of them into the same directory if you want to build it with a single project file. An alternative approach is described below (see Organizing Projects into Subsystems), allowing each separate object directory to be associated with a corresponding subsystem of the application.

Incidentally, the directory designated by the `Object_Dir` attribute may be used by project aware tools other than the compilation toolchain to store reports or intermediate files.

When the linker is called, it usually creates an executable. By default, this executable is placed in the project’s object directory. However in some situations it may be convenient to store it in elsewhere. This can be done through the Exec_Dir attribute, which, like `Object_Dir` contains a single absolute or relative path and must point to an existing and writable directory, unless you ask the tool to create it on your behalf. If neither `Object_Dir` nor `Exec_Dir` is specified then the executable is placed in the directory containing the project file.

In our example, let’s specify that the executable is to be placed in the same directory as the project file `build.gpr`. The project file is now:

>     project Build is
>        for Source_Dirs use ("common");
>        for Object_Dir use "obj";
>        for Exec_Dir use ".";  --   <<<<
>     end Build;

### 2.2.4. Main Subprograms

An important role of a project file is to identify the executable(s) that will be built. It does this by specifying the source file for the main subprogram (for Ada) or the file that contains the `main` function (for C).

There can be any number of such main files within a given project, and thus several executables can be built from a single project file. Of course, a given executable might not (and in general will not) need all the source files referenced by the project. As opposed to other build mechanisms such as through a Makefile, you do not need to specify the list of dependencies of each executable. The project-aware builder knows enough of the semantics of the languages to build and link only the necessary elements.

The list of main files is specified via the Main attribute. It contains a list of file names (no directories). If a file name is specified without extension, it is completed using the naming convention defined in the package Naming. If a project defines this attribute, it is not necessary to identify main files on the command line when invoking a builder, and editors like GPS will be able to create extra menus to spawn or debug the corresponding executables.

>     project Build is
>        for Source_Dirs use ("common");
>        for Object_Dir use "obj";
>        for Exec_Dir use ".";
>        for Main use ("proc.adb");  --   <<<<
>     end Build;

If this attribute is defined in the project, then spawning the builder with a command such as

>     gprbuild -Pbuild

automatically builds all the executables corresponding to the files listed in the Main attribute. It is possible to specify one or more executables on the command line to build a subset of them.

One or more spaces may be placed between the `-P` and the project name, and the project name may be a simple name (no file extension) or a path for the project file. Thus each of the following is equivalent to the command above:

>     gprbuild -P build
>     gprbuild -P build.gpr
>     gprbuild -P ./build.gpr

### 2.2.5. Tools Options in Project Files

We now have a project file that fully describes our environment, and it can be used to build the application with a simple GPRbuild command as shown above. In fact, the empty project that we saw at the beginning (with no attribute definitions) could already achieve this effect if it was placed in the `common` directory.

Of course, we might want more control. This section shows you how to specify the compilation switches that the various tools involved in the building of the executable should use.

Since source names and locations are described in the project file, it is not necessary to use switches on the command line for this purpose (such as `-I` for gcc). This removes a major source of command line length overflow. Clearly, the builders will have to communicate this information one way or another to the underlying compilers and tools they call, but they usually use various text files, such as response files, for this purpose and thus are not subject to command line overflow.

Several tools are used to create an executable: the compiler produces object files from the source files; the binder (when the language is Ada) creates a “source” file that, among other things, takes care of elaboration issues and global variable initialization; and the linker gathers everything into a single executable. All these tools are known to the project manager and will be invoked with user-defined switches from the project files. To obtain this effect, a project file feature known as a package is used.

A project file contains zero or more packages, each of which defines the attributes specific to one tool (or one set of tools). Project files use an Ada-like syntax for packages. Package names permitted in project files are restricted to a predefined set (see Packages), and the contents of packages are limited to a small set of constructs and attributes (see Attributes).

Our example project file below includes several empty packages. At this stage, they could all be omitted since they are empty, but they show which packages would be involved in the build process.

>     project Build is
>        for Source_Dirs use ("common");
>        for Object_Dir use "obj";
>        for Exec_Dir use ".";
>        for Main use ("proc.adb");

>        package Builder is  --<<<  for gprbuild
>        end Builder;

>        package Compiler is --<<<  for the compiler
>        end Compiler;

>        package Binder is   --<<<  for the binder
>        end Binder;

>        package Linker is   --<<<  for the linker
>        end Linker;
>     end Build;

Let’s first examine the compiler switches. As stated in the initial description of the example, we want to compile all files with `-O2`. This is a compiler switch, although it is typical, on the command line, to pass it to the builder which then passes it to the compiler. We recommend directly using the correct package, which will make the setup easier to understand.

Several attributes can be used to specify the switches:

Default_Switches:

> This illustrates the concept of an indexed attribute. When such an attribute is defined, you must supply an index in the form of a literal string. In the case of Default_Switches, the index is the name of the language to which the switches apply (since a different compiler will likely be used for each language, and each compiler has its own set of switches). The value of the attribute is a list of switches.

> In this example, we want to compile all Ada source files with the switch `-O2`; the resulting Compiler package is as follows:

>     package Compiler is
>       for Default_Switches ("Ada") use ("-O2");
>     end Compiler;

Switches:

> In some cases, we might want to use specific switches for one or more files. For instance, compiling `proc.adb` might not be desirable at a high level of optimization. In such a case, the Switches attribute (indexed by the file name) can be used and will override the switches defined by Default_Switches. The Compiler package in our project file would become:

>     package Compiler is
>        for Default_Switches ("Ada")
>            use ("-O2");
>        for Switches ("proc.adb")
>            use ("-O0");
>     end Compiler;

> Switches may take a pattern as an index, such as in:

>     package Compiler is
>       for Default_Switches ("Ada")
>           use ("-O2");
>       for Switches ("pkg*")
>           use ("-O0");
>     end Compiler;

> Sources `pkg.adb` and `pkg-child.adb` would be compiled with `-O0`, not `-O2`.

> Switches can also be given a language name as index instead of a file name in which case it has the same semantics as Default_Switches. However, indexes with wild cards are never valid for language name.

Local_Configuration_Pragmas:

> This attribute may specify the path of a file containing configuration pragmas for use by the Ada compiler, such as pragma Restrictions (No_Tasking). These pragmas will be used for all the sources of the project.

The switches for the other tools are defined in a similar manner through the Default_Switches and Switches attributes, respectively in the Builder package (for GPRbuild), the Binder package (binding Ada executables) and the Linker package (for linking executables).

### 2.2.6. Compiling with Project Files

Now that our project file is written, let’s build our executable. Here is the command we would use from the command line:

>     gprbuild -Pbuild

This will automatically build the executables specified in the Main attribute: for each, it will compile or recompile the sources for which the object file does not exist or is not up-to-date; it will then run the binder; and finally run the linker to create the executable itself.

The GPRbuild builder can automatically manage C files the same way: create the file `utils.c` in the `common` directory, set the attribute Languages to “(Ada, C)”, and re-run

>     gprbuild -Pbuild

GPRbuild knows how to recompile the C files and will recompile them only if one of their dependencies has changed. No direct indication on how to build the various elements is given in the project file, which describes the project properties rather than a set of actions to be executed. Here is the invocation of GPRbuild when building a multi-language program:

>     $ gprbuild -Pbuild
>     gcc -c proc.adb
>     gcc -c pack.adb
>     gcc -c utils.c
>     gprbind proc
>     ...
>     gcc proc.o -o proc

Notice the three steps described earlier:

- The first three gcc commands correspond to the compilation phase.

- The gprbind command corresponds to the post-compilation phase.

- The last gcc command corresponds to the final link.

The default output of GPRbuild is reasonably simple and easy to understand. In particular, some of the less frequently used commands are not shown, and some parameters are abbreviated. Thus it is not possible to rerun the effect of the GPRbuild command by cut-and-pasting its output. The `-v` option to GPRbuild provides a much more verbose output which includes, among other information, more complete compilation, post-compilation and link commands.

### 2.2.7. Executable File Names

By default, the executable name corresponding to a main file is computed from the main source file name. Through the attribute Executable in package `Builder`, it is possible to change this default.

For instance, instead of building an executable named `"proc"` (or `"proc.exe"` on Windows), we could configure our project file to build `proc1` (respectively `proc1.exe`) as follows:

>     project Build is
>        ...  --  same as before
>        package Builder is
>           for Executable ("proc.adb") use "proc1";
>        end Builder
>     end Build;

Attribute Executable_Suffix, when specified, changes the suffix of the executable files when no attribute `Executable` applies: its value replaces the platform-specific executable suffix. The default executable suffix is the empty string empty on Unix and `".exe"` on Windows.

It is also possible to change the name of the produced executable by using the command line switch `-o`. However, when several main programs are defined in the project, it is not possible to use the `-o` switch; then the only way to change the names of the executable is through the attributes `Executable` and `Executable_Suffix`.

### 2.2.8. Using Variables to Avoid Duplication

To illustrate some other project capabilities, here is a slightly more complex project using similar sources and a main program in C:

>     project C_Main is
>        for Languages    use ("Ada", "C");
>        for Source_Dirs  use ("common");
>        for Object_Dir   use  "obj";
>        for Main         use ("main.c");
>        package Compiler is
>           C_Switches := ("-pedantic");
>           for Default_Switches ("C")   use C_Switches;
>           for Default_Switches ("Ada") use ("-gnaty");
>           for Switches ("main.c") use C_Switches & ("-g");
>        end Compiler;
>     end C_Main;

This project has many similarities with the previous one. As expected, its `Main` attribute now refers to a C source file. The attribute `Exec_Dir` is now omitted, thus the resulting executable will be put in the object directory `obj`.

The most noticeable difference is the use of a variable in the `Compiler` package to store settings used in several attributes. This avoids text duplication and eases maintenance (a single place to modify if we want to add new switches for C files). We will later revisit the use of variables in the context of scenarios (see Scenarios in Projects).

In this example, we see that the file `main.c` will be compiled with the switches used for all the other C files, plus `-g`. In this specific situation the use of a variable could have been replaced by a reference to the `Default_Switches` attribute:

>     for Switches ("c_main.c") use Compiler'Default_Switches ("C") & ("-g");

Note the tick character “`'`”, which is used to refer to attributes defined in a package.

Here is the output of the GPRbuild command using this project:

>     $ gprbuild -Pc_main
>     gcc -c -pedantic -g main.c
>     gcc -c -gnaty proc.adb
>     gcc -c -gnaty pack.adb
>     gcc -c -pedantic utils.c
>     gprbind main.bexch
>     ...
>     gcc main.o -o main

The default switches for Ada sources, the default switches for C sources (in the compilation of `lib.c`), and the specific switches for `main.c` have all been taken into account.

### 2.2.9. Naming Schemes

Sometimes an Ada software system needs to be ported from one compilation environment to another (such as GNAT), but the files might not be named using the default GNAT conventions. Instead of changing all the file names, which for a variety of reasons might not be possible, you can define the relevant file naming scheme in the Naming package of your project file.

The naming scheme has two distinct goals for the Project Manager: it allows source files to be located when searching in the source directories, and given a source file name it makes it possible to infer the associated language, and thus which compiler to use.

Note that the Ada compiler’s use of pragma Source_File_Name is not supported when using project files. You must use the features described here. You can, however, specify other configuration pragmas.

The following attributes can be defined in package Naming:

Casing:

> Its value must be one of `"lowercase"` (the default if unspecified), `"uppercase"` or `"mixedcase"`. It describes the casing of file names with regard to the Ada unit name.

> Given an Ada package body My_Unit, the base file name (i.e. minus the extension, which is controlled by other attributes described below) will respectively be:

> - for “lowercase”: “my_unit”

> - for “uppercase”: “MY_UNIT”

> - for “mixedcase”: any spelling with indifferent casing such as “My_Unit”, “MY_Unit”, “My_UnIT” etc… The case insensitive name must be unique, otherwise an error will be reported. For example, there cannot be two source file names such as “My_Unit.adb” and “MY_UnIT.adb”.

> On Windows, file names are case insensitive, so this attribute is irrelevant.

Dot_Replacement:

> This attribute specifies the string that should replace the `"."` in unit names. Its default value is `"-"` so that a unit `Parent.Child` is expected to be found in the file `parent-child.adb`. The replacement string must satisfy the following requirements to avoid ambiguities in the naming scheme:

> - It must not be empty

> - It cannot start or end with an alphanumeric character

> - It cannot be a single underscore

> - It cannot start with an underscore followed by an alphanumeric

> - It cannot contain a dot `'.'` unless the entire string is `"."`

> - It cannot include a space or a character that is not printable ASCII

Spec_Suffix and Specification_Suffix:

> For Ada, these attributes specify the suffix used in file names that contain specifications. For other languages, they give the extension for files that contain declarations (header files in C for instance). The attribute is indexed by the language name. The two attributes are equivalent, but `Specification_Suffix` is obsolescent.

> If the value of the attribute is the empty string, it indicates to the Project Manager that the only specifications/header files for the language are those specified with attributes `Spec` or `Specification_Exceptions`.

> If `Spec_Suffix ("Ada")` is not specified, then the default is `".ads"`.

> A non empty value must satisfy the following requirements:

> - It must include at least one dot

> - If `Dot_Replacement` is a single dot, then it cannot include more than one dot.

Body_Suffix and Implementation_Suffix:

> These attributes are equivalent and specify the extension used for file names that contain code (bodies in Ada). They are indexed by the language name. `Implementation_Suffix` is obsolescent and fully replaced by the first attribute.

> For each language of a project, one of these two attributes needs to be specified, either in the project itself or in the configuration project file.

> If the value of the attribute is the empty string, it indicates to the Project Manager that the only source files for the language are those specified with attributes `Body` or `Implementation_Exceptions`.

> These attributes must satisfy the same requirements as `Spec_Suffix`. In addition, they must be different from any of the values in `Spec_Suffix`. If `Body_Suffix ("Ada")` is not specified, then the default is `".adb"`.

> If `Body_Suffix ("Ada")` and `Spec_Suffix ("Ada")` end with the same string, then a file name that ends with the longest of these two suffixes will be a body if the longest suffix is `Body_Suffix ("Ada")`, or a spec if the longest suffix is `Spec_Suffix ("Ada")`.

> If the suffix does not start with a `'.'`, a file with a name exactly equal to the suffix will also be part of the project (for instance if you define the suffix as `Makefile.in`, a file called `Makefile.in` will be part of the project. This capability is usually not of interest when building. However, it might become useful when a project is also used to find the list of source files in an editor, like the GNAT Programming System (GPS).

> Note

> Attributes `Body_Suffix` and `Spec_Suffix` have case-insensitive values. This means different languages should not share the same attribute value in a single project.

> For instance :

>     package Naming is
>       for Body_Suffix ("c") use ".c";
>       for Body_Suffix ("c++") use ".C";
>       for Spec_Suffix ("c") use ".h";
>       for Spec_Suffix ("c++") use ".H";
>     end Naming;

> will result in :

>     Body_Suffix (".c") for language c is also defined for language c++.
>     Spec_Suffix (".h") for language c is also defined for language c++.

> In that case, having each language inside its own project and individually imported to a master project allows such project architecture.

Separate_Suffix:

> This attribute is specific to Ada. It denotes the suffix used in file names for files that contain subunits (separate bodies). If it is not specified, then it defaults to same value as `Body_Suffix ("Ada")`.

> The value of this attribute cannot be the empty string.

> Otherwise, the same rules apply as for the `Body_Suffix` attribute.

Spec or Specification:

> These attributes are equivalent. The `Spec` attribute can be used to define the source file name for a given Ada compilation unit’s spec. The index is the literal name of the Ada unit (case insensitive). The value is the literal base name of the file that contains this unit’s spec (case sensitive or insensitive depending on the operating system). This attribute allows the definition of exceptions to the general naming scheme, in case some files do not follow the usual convention.

> When a source file contains several units, the relative position of the unit can be indicated. The first unit in the file is at position 1.

> >     for Spec ("MyPack.MyChild") use "mypack.mychild.spec";
> >     for Spec ("top") use "foo.a" at 1;
> >     for Spec ("foo") use "foo.a" at 2;

Body or Implementation:

> These attribute play the same role as `Spec`, but for Ada bodies.

Specification_Exceptions and Implementation_Exceptions:

> These attributes define exceptions to the naming scheme for languages other than Ada. They are indexed by the language name, and contain a list of file names respectively for headers and source code.

As an example of several of these attributes, the following package models the Apex file naming rules:

>     package Naming is
>       for Casing               use "lowercase";
>       for Dot_Replacement      use ".";
>       for Spec_Suffix ("Ada")  use ".1.ada";
>       for Body_Suffix ("Ada")  use ".2.ada";
>     end Naming;

## 2.3. Organizing Projects into Subsystems

A subsystem is a coherent part of the complete system to be built. It is represented by a set of sources and a single object directory. A system can consist of a single subsystem when it is simple as we have seen in the earlier examples. Complex systems are usually composed of several interdependent subsystems. A subsystem is dependent on another subsystem if knowledge of the other one is required to build it, and in particular if visibility on some of the sources of this other subsystem is required. Each subsystem is usually represented by its own project file.

In this section, we’ll enhance the previous example. Let’s assume some sources of our `Build` project depend on other sources. For instance, when building a graphical interface, it is usual to depend upon a graphical library toolkit such as GtkAda. Furthermore, we also need sources from a logging module we had previously written.

### 2.3.1. Importing Projects

GtkAda comes with its own project file (appropriately called `gtkada.gpr`), and we will assume we have already built a project called `logging.gpr` for the logging module. With the information provided so far in `build.gpr`, building the application would fail with an error indicating that the gtkada and logging units that are relied upon by the sources of this project cannot be found.

This is solved by defining `build.gpr` to import the gtkada and logging projects: this is done by adding the following `with` clauses at the beginning of our project:

>     with "gtkada.gpr";
>     with "a/b/logging.gpr";
>     project Build is
>       ...  --  as before
>     end Build;

When such a project is compiled, gprbuild will automatically check the imported projects and recompile their sources when needed. It will also recompile the sources from Build when needed, and finally create the executable.

In some cases, the implementation units needed to recompile a project are not available, or come from some third party and you do not want to recompile it yourself. In this case, set the attribute Externally_Built to `"true"`, indicating to the builder that this project can be assumed to be up-to-date, and should not be considered for recompilation. In Ada, if the sources of this externally built project were compiled with another version of the compiler or with incompatible options, the binder will issue an error.

The project’s `with` clause has several effects. It provides source visibility between projects during the compilation process. It also guarantees that the necessary object files from `Logging` and `GtkAda` are available when linking `Build`.

As can be seen in this example, the syntax for importing projects is similar to the syntax for importing compilation units in Ada. However, project files use literal strings instead of names, and the `with` clause identifies project files rather than packages.

Each literal string after `with` is the path (absolute or relative) to a project file. The `.gpr` extension is optional, but we recommend adding it. If no extension is specified, and no project file with the `.gpr` extension is found, then the file is searched for exactly as written in the `with` clause, that is with no extension.

As mentioned above, the path after a `with` has to be a literal string, and you cannot use concatenation, or lookup the value of external variables to change the directories from which a project is loaded. A solution if you need something like this is to use aggregate projects (see Aggregate Projects).

When a relative path or a base name is used, the project files are searched relative to each of the directories in the project path. This path includes all the directories found by the following procedure, in decreasing order of priority; the first matching file is used:

- First, the file is searched relative to the directory that contains the current project file.

- Then it is searched relative to all the directories specified in the environment variables `GPR_PROJECT_PATH_FILE`, `GPR_PROJECT_PATH` and `ADA_PROJECT_PATH` (in that order) if they exist. The value of `GPR_PROJECT_PATH_FILE`, when defined, is the path name of a text file that contains project directory path names, one per line. `GPR_PROJECT_PATH` and `ADA_PROJECT_PATH`, when defined, contain project directory path names separated by directory separators. `ADA_PROJECT_PATH` is used for compatibility, it is recommended to use `GPR_PROJECT_PATH_FILE` or `GPR_PROJECT_PATH`.

- Finally, it is searched relative to the default project directories. The following locations are searched, in the specified order:

  - `<compiler_prefix>/<target>/<runtime>/share/gpr`

  - `<compiler_prefix>/<target>/<runtime>/lib/gnat`

  - `<compiler_prefix>/<target>/share/gpr`

  - `<compiler_prefix>/<target>/lib/gnat`

  - `<compiler_prefix>/share/gpr/`

  - `<compiler_prefix>/lib/gnat/`

The first two paths are only added if the explicit runtime is specified either via `--RTS` switch or via `Runtime` attribute. <target> can be communicated via `--target` switch or `Target` attribute, otherwise default target will be used. <compiler_prefix> is typically discovered automatically based on target, runtime and language information.

In our example, `gtkada.gpr` is found in the predefined directory if it was installed at the same root as GNAT.

Some tools also support extending the project path from the command line, generally through the `-aP`. You can see the value of the project path by using the `gprls -v` command.

Any symbolic link will be fully resolved in the directory of the importing project file before the imported project file is examined.

Any source file in the imported project can be used by the sources of the importing project, transitively. Thus if A imports B, which imports C, the sources of A may depend on the sources of C, even if A does not import C explicitly. However, this is not recommended, because if and when B ceases to import C, some sources in A will no longer compile. GPRbuild has a switch `--no-indirect-imports` that will report such indirect dependencies.

Project import closure

The project import closure for a given project proj is the set of projects consisting of proj itself, together with each project that is directly or indirectly imported by proj. The import may be from either a `with` or, as will be explained below, a `limited with`.

Note

One very important aspect of a project import closure is that a given source can only belong to one project in this set (otherwise the project manager would not know which settings apply to it and when to recompile it). Thus different project files do not usually share source directories, or, when they do, they need to specify precisely which project owns which sources using the attribute Source_Files or equivalent. By contrast, two projects can each own a source with the same base file name as long as they reside in different directories. The latter is not true for Ada sources because of the correlation between source files and Ada units.

### 2.3.2. Cyclic Project Dependencies

In general, cyclic import dependencies are forbidden: if project A `with`s project B (directly or indirectly) then B is not allowed to `with` A. However, there are cases when cyclic dependencies at the project level are necessary, as dependencies at the source level may exist both ways between A’s sources and B’s sources. For these cases, another form of import between projects is supplied: the limited with. A project A that imports a project B with a simple `with` may also be imported, directly or indirectly, by B through a `limited with`.

The difference between a simple `with` and `limited with` is that the name of a project imported with a `limited with` cannot be used in the importing project. In particular, its packages cannot be renamed and its variables cannot be referenced.

>     with "b.gpr";
>     with "c.gpr";
>     project A is
>         for Exec_Dir use B'Exec_Dir; -- OK
>     end A;

>     limited with "a.gpr";   --  Cyclic dependency: A -> B -> A
>     project B is
>        for Exec_Dir use A'Exec_Dir; -- not OK
>     end B;

>     with "d.gpr";
>     project C is
>     end C;

>     limited with "a.gpr";  --  Cyclic dependency: A -> C -> D -> A
>     project D is
>        for Exec_Dir use A'Exec_Dir; -- not OK
>     end D;

### 2.3.3. Sharing between Projects

When building an application, it is common to have similar needs in several of the projects corresponding to the subsystems under construction. For instance, they might all have the same compilation switches.

As seen above (see Tools Options in Project Files), setting compilation switches for all sources of a subsystem is simple: it is just a matter of adding a `Compiler'Default_Switches` attribute to each project file with the same value. However, that would entail duplication of data, and both places would need to be changed in order to recompile the whole application with different switches. This may be a serious issue if there are many subsystems and thus many project files to edit.

There are two main approaches to avoiding this duplication:

- Since `build.gpr` imports `logging.gpr`, we could change the former to reference the attribute in Logging, either through a package renaming, or by referencing the attribute. The following example shows both cases:

      project Logging is
         package Compiler is
            for Switches ("Ada")
                use ("-O2");
         end Compiler;
         package Binder is
            for Switches ("Ada")
                use ("-E");
         end Binder;
      end Logging;

      with "logging.gpr";
      project Build is
         package Compiler renames Logging.Compiler;
         package Binder is
            for Switches ("Ada") use Logging.Binder'Switches ("Ada");
         end Binder;
      end Build;

  The solution used for Compiler gets the same value for all attributes of the package, but you cannot modify anything from the package (adding extra switches or some exceptions). The solution for the Binder package is more flexible, but more verbose.

  If you need to refer to the value of a variable in an imported project, rather than an attribute, the syntax is similar but uses a `"."` rather than an apostrophe. For instance:

      with "imported";
      project Main is
         Var1 := Imported.Var;
      end Main;

- The second approach is to define the switches in a separate project. That project does not contain any source files (thus, as opposed to the first example, none of the projects plays a special role), and will only be used to define the attributes. Such a project is typically named `shared.gpr`.

      abstract project Shared is
         for Source_Files use ();   --  no sources
         package Compiler is
            for Switches ("Ada")
                use ("-O2");
         end Compiler;
      end Shared;

      with "shared.gpr";
      project Logging is
         package Compiler renames Shared.Compiler;
      end Logging;

      with "shared.gpr";
      project Build is
         package Compiler renames Shared.Compiler;
      end Build;

  As with the first example, we could have chosen to set the attributes one by one rather than to rename a package. The reason we explicitly indicate that Shared has no sources is so that it can be created in any directory, and we are sure it shares no sources with Build or Logging, which would be invalid.

  Note the additional use of the abstract qualifier in `shared.gpr`. This qualifier is optional, but helps convey the message that we do not intend this project to have source files (see Qualified Projects for additional information about project qualifiers).

### 2.3.4. Global Attributes

We have already seen many examples of attributes used to specify a particular option for one of the tools involved in the build process. Most of those attributes are project specific. That is to say, they only affect the invocation of tools on the sources of the project where they are defined.

There are a few additional attributes that, when defined for a “main” project proj, also apply to all other projects in the project import closure of proj. A main project is a project explicitly specified on the command line.

Such attributes are known as global attributes; here are several that are commonly used:

Builder’Global_Configuration_Pragmas:

> This attribute specifies a file that contains configuration pragmas to use when building executables. These pragmas apply to all executables built from this project import closure. As noted earlier, additional pragmas can be specified on a per-project basis by setting the `Compiler'Local_Configuration_Pragmas` attribute.

Builder’Global_Compilation_Switches:

> This attribute is a list of compiler switches that apply when compiling any source file in the project import closure. These switches are used in addition to the ones defined in the `Compiler` package, which only apply to the sources of the corresponding project. This attribute is indexed by the name of the language.

Using such global capabilities is convenient, but care is needed since it can also lead to unexpected behavior. An example is when several subsystems are shared among different main projects but the different global attributes are not compatible. Note that using aggregate projects can be a safer and more powerful alternative to global attributes.

## 2.4. Scenarios in Projects

Various project properties can be modified based on scenarios. These are user-defined modes (the values of project variables and attributes) that determine the behavior of a project, based on the values of externally defined variables. Typical examples are the setup of platform-specific compiler options, or the use of a debug and a release mode (the former would activate the generation of debug information, while the latter would request an increased level of code optimization).

Let’s enhance our example to support debug and release modes. The issue is to let the user choose which kind of system to build: use `-g` as a compiler switch in debug mode and `-O2` in release mode. We will also set up the projects so that we do not share the same object directory in both modes; otherwise switching from one to the other might trigger more recompilations than needed or mix objects from the two modes.

One approach is to create two different project files, say `build_debug.gpr` and `build_release.gpr`, that set the appropriate attributes as explained in previous sections. This solution does not scale well, because in the presence of multiple projects depending on each other, you will also have to duplicate the complete set of projects and adapt the project files accordingly.

Instead, project files support the notion of scenarios controlled by the values of externally defined variables. Such values can come from several sources (in decreasing order of priority):

Command line:

  When launching gprbuild, the user can pass `-X` switches to define the external variables. In our case, the command line might look like

      gprbuild -Pbuild.gpr -Xmode=release

  which defines the external variable named `mode` and sets its value to `"release"`.

Environment variables:

  When the external value does not come from the command line, it can come from the value of an environment variable of the appropriate name. In our case, if an environment variable named `mode` exists, its value will be used.

Tool mode:

  In the special case of the `GPR_TOOL` variable, if its value has not been specified via the command line or as an environment variable, the various tools set this variable to a value proper to each tool. gprbuild sets this value to `gprbuild`. See the documentation of other tools to find out which value they set this variable to.

External function second parameter.

  Once an external variable is defined, its value needs to be obtained by the project. The general form is to use the predefined function `external`, which returns the current value of the external variable. For instance, we could set up the object directory to point to either `obj/debug` or `obj/release` by changing our project to

      project Build is
          for Object_Dir use "obj/" & external ("mode", "debug");
          ... --  as before
      end Build;

  The second parameter to `external` is optional, and is the default value to use if `mode` is not set from the command line or the environment. If the second parameter is not supplied, and there is no external or environment variable named by the first parameter, then an error is reported.

In order to set the switches according to the different scenarios, other constructs are needed, such as typed variables and case constructions.

A typed variable is a variable that can take only a limited number of values, similar to variable from an enumeration type in Ada. Such a variable can then be used in a case construction, resulting in conditional sections in the project. The following example shows how this can be done:

>     project Build is
>        type Mode_Type is ("debug", "release");         -- all possible values
>        Mode : Mode_Type := external ("mode", "debug"); -- a typed variable

>        package Compiler is
>           case Mode is
>              when "debug" =>
>                 for Switches ("Ada")
>                     use ("-g");
>              when "release" =>
>                 for Switches ("Ada")
>                     use ("-O2");
>           end case;
>        end Compiler;
>     end Build;

This project is larger than the ones we have seen previously, but it has become much more flexible. The `Mode_Type` type defines the only valid values for the `Mode` variable. If any other value is read from the environment, an error is reported and the project is considered as invalid.

The `Mode` variable is initialized with an external value defaulting to `"debug"`. This default could be omitted and that would force the user to define the value. Finally, we can use a case construction to set the switches depending on the scenario the user has chosen.

Most aspects of a project can depend on scenarios. The notable exception is the identity of an imported project (via a `with` or `limited with` clause), which cannot depend on a scenario.

Scenarios work analogously across projects in a project import closure. You can either duplicate a variable similar to `Mode` in each of the projects (as long as the first argument to `external` is always the same and the type is the same), or simply set the variable in the `shared.gpr` project (see Sharing between Projects).

## 2.5. Library Projects

So far, we have seen examples of projects that create executables. However, it is also possible to create libraries instead. A library is a specific type of subsystem where, for convenience, objects are grouped together using system-specific means such as archives or Windows DLLs.

Library projects provide a system- and language-independent way of building both static and dynamic libraries. They also support the concept of standalone libraries (SAL) which offer two significant properties: the elaboration (e.g. initialization) of the library is either automatic or very simple; a change in the implementation part of the library implies minimal post-compilation actions on the complete system and potentially no action at all for the rest of the system in the case of dynamic SALs.

There is a restriction on shared library projects: by default, they are only allowed to import other shared library projects. They are not allowed to import non-library projects or static library projects.

The GNAT Project Manager takes complete care of the library build, rebuild and installation tasks, including recompilation of the source files for which objects do not exist or are not up to date, assembly of the library archive, and installation of the library (i.e., copying associated source, object and `ALI` files to the specified location).

### 2.5.1. Building Libraries

Let’s enhance our example and transform the logging subsystem into a library. In order to do so, a few changes need to be made to `logging.gpr`. Some attributes need to be defined: at least Library_Name and Library_Dir; in addition, some other attributes can be used to specify specific aspects of the library. For readability, it is also recommended (although not mandatory), to use the qualifier library in front of the project keyword.

Library_Name:

> This attribute is the name of the library to be built. There is no restriction on the name of a library imposed by the project manager, except for stand-alone libraries whose names must follow the syntax of Ada identifiers; however, there may be system-specific restrictions on the name. In general, we recommend using only alphanumeric characters (and possibly single underscores), to help portability.

Library_Dir:

> This attribute is the path (absolute or relative) of the directory where the library is to be installed. In the process of building a library, the sources are compiled and the object files are placed in the explicitly- or implicitly specified `Object_Dir` directory. When all sources of a library are compiled, some of the compilation artifacts, including the library itself, are copied to the library_dir directory. This directory must exist and be writable. It must also be different from the object directory so that cleanup activities in the Library_Dir do not affect recompilation needs.

Here is the new version of `logging.gpr` that makes it a library:

>     library project Logging is          --  "library" is optional
>        for Library_Name use "logging";  --  will create "liblogging.a" on Unix
>        for Object_Dir   use "obj";
>        for Library_Dir  use "lib";      --  different from object_dir
>     end Logging;

Once the above two attributes are defined, the library project is valid and is sufficient for building a library with default characteristics. Other library-related attributes can be used to change the defaults:

Library_Kind:

> The value of this attribute must be either `"static"`, `"static-pic"`, `"dynamic"` or `"relocatable"` (the last is a synonym for `"dynamic"`). It indicates which kind of library should be built (the default is to build a static library, that is an archive of object files that can potentially be linked into a static executable). A static-pic library is also an archive, but the code is Position Independent Code, usually compiled with the switch -fPIC. When the library is set to be dynamic, a separate image is created that will be loaded independently, usually at the start of the main program execution. Support for dynamic libraries is very platform specific, for instance on Windows it takes the form of a DLL while on GNU/Linux, it is a dynamic elf image whose suffix is usually `.so`. Library project files, on the other hand, can be written in a platform independent way so that the same project file can be used to build a library on different operating systems.

> If you need to build both a static and a dynamic library, we recommend using two different object directories, since in some cases some extra code needs to be generated for the latter. For such cases, one can either define two different project files, or a single one that uses scenarios to indicate the various kinds of library to be built and their corresponding object_dir.

Library_ALI_Dir:

> This attribute may be specified to indicate the directory where the ALI files of the library are installed. By default, they are copied into the `Library_Dir` directory, but as for the executables where we have a separate Exec_Dir attribute, you might want to put them in a separate directory since there may be hundreds of such files. The same restrictions as for the `Library_Dir` attribute apply.

Library_Version:

> This attribute is platform dependent, and has no effect on Windows. On Unix, it is used only for dynamic libraries as the internal name of the library (the “soname”). If the library file name (built from the `Library_Name`) is different from the `Library_Version`, then the library file will be a symbolic link to the actual file whose name will be `Library_Version`. This follows the usual installation schemes for dynamic libraries on many Unix systems.

>     project Logging is
>        Version := "1";
>        for Library_Dir use "lib";
>        for Library_Name use "logging";
>        for Library_Kind use "dynamic";
>        for Library_Version use "liblogging.so." & Version;
>     end Logging;

> After the compilation, the directory `lib` will contain both a `liblogging.so.1` library and a symbolic link to it called `liblogging.so`.

Library_GCC:

> This attribute is the name of the tool to use instead of `gcc` to link shared libraries. A common use of this attribute is to define a wrapper script that accomplishes specific actions before calling `gcc` (which itself calls the linker to build the library image).

Library_Options:

> This attribute may be used to specify additional switches (“last switches”) when linking a shared library or a static standalone library. In the case of a simple static library, the values for this attribute are restricted to paths to object files. Those paths may be absolute or relative to the object directory.

Leading_Library_Options:

> This attribute, which is taken into account only by GPRbuild, may be used to specify leading options (“first switches”) when linking a shared library.

### 2.5.2. Using Library Projects

When the builder detects that a project file is a library project file, it recompiles all sources of the project that need recompilation and rebuilds the library if any of the sources have been recompiled. It then groups all object files into a single file, which is a shared or a static library. This library can later on be linked with multiple executables. Note that the use of shared libraries reduces the size of the final executable and can also reduce the memory footprint at execution time when the library is shared among several executables.

GPRbuild also allows building multi-language libraries when specifying sources from multiple languages.

A non-library project NLP can import a library project LP. When the builder is invoked on NLP, it always rebuilds LP even if all of the latter’s files are up to date. For instance, let’s assume in our example that `logging` has the following sources: `log1.ads`, `log1.adb`, `log2.ads` and `log2.adb`. If `log1.adb` has been modified, then the library `liblogging` will be rebuilt when compiling all the sources of `Build` even if `proc.ads`, `pack.ads` and `pack.adb` do not include a `"with Log1"`.

To ensure that all the sources in the `Logging` library are up to date, and that all the sources of `Build` are also up to date, the following two commands need to be used:

>     gprbuild -Plogging.gpr
>     gprbuild -Pbuild.gpr

All `ALI` files will also be copied from the object directory to the library directory. To build executables, GPRbuild will use the library rather than the individual object files.

Library projects can also be useful to specify a library that needs to be used but, for some reason, cannot be rebuilt. Such a situation may arise when some of the library sources are not available. Such library projects need to use the `Externally_Built` attribute as in the example below:

>     library project Extern_Lib is
>        for Languages    use ("Ada", "C");
>        for Source_Dirs  use ("lib_src");
>        for Library_Dir  use "lib2";
>        for Library_Kind use "dynamic";
>        for Library_Name use "l2";
>        for Externally_Built use "true";  --  <<<<
>     end Extern_Lib;

In the case of externally built libraries, the `Object_Dir` attribute does not need to be specified because it will never be used.

The main effect of using such an externally built library project is mostly to affect the linker command in order to reference the desired library. It can also be achieved by using `Linker'Linker_Options` or `Linker'Switches` in the project corresponding to the subsystem needing this external library. This latter method is more straightforward in simple cases but when several subsystems depend upon the same external library, finding the proper place for the `Linker'Linker_Options` might not be easy and if it is not placed properly, the final link command is likely to present ordering issues. In such a situation, it is better to use the externally built library project so that all other subsystems depending on it can declare this dependency through a project `with` clause, which in turn will trigger the builder to find the proper order of libraries in the final link command.

### 2.5.3. Stand-alone Library Projects

A stand-alone library is a library that contains the necessary code to elaborate the Ada units that are included in the library. A stand-alone library is a convenient way to add an Ada subsystem to a more global system whose main is not in Ada since it makes the elaboration of the Ada part mostly transparent. However, stand-alone libraries are also useful when the main is in Ada: they provide a means for minimizing relinking and redeployment of complex systems when localized changes are made.

The name of a stand-alone library, specified with attribute `Library_Name`, must have the syntax of an Ada identifier.

The most prominent characteristic of a stand-alone library is that it offers a distinction between interface units and implementation units. Only the former are visible to units outside the library. A stand-alone library project is thus characterized by a third attribute, usually `Library_Interface`, in addition to the two attributes that make a project a Library Project (Library_Name and Library_Dir). This third attribute may also be `Interfaces`. `Library_Interface` only works when the interface is in Ada and takes a list of units as parameter. `Interfaces` works for any supported language and takes a list of sources as parameter.

Library_Interface:

> This attribute defines an explicit subset of the units of the project. Units from projects importing this library project may only “with” units whose sources are listed in the Library_Interface. Other sources are considered implementation units.

>     for Library_Dir use "lib";
>     for Library_Name use "logging";
>     for Library_Interface use ("lib1", "lib2");  --  unit names

Interfaces

> This attribute defines an explicit subset of the source files of a project. Sources from projects importing this project, can only depend on sources from this subset. This attribute can be used on non library projects. It can also be used as a replacement for attribute `Library_Interface`, in which case, units have to be replaced by source files. For multi-language library projects, it is the only way to make the project a Stand-Alone Library project whose interface is not purely Ada.

Library_Standalone:

> This attribute defines the kind of stand-alone library to build. Values are either `standard` (the default), `no` or `encapsulated`. When `standard` is used the code to elaborate and finalize the library is embedded, when `encapsulated` is used the library can furthermore depend only on static libraries (including the GNAT runtime). This attribute can be set to `no` to make it clear that the library should not be stand-alone in which case the `Library_Interface` should not defined. Note that this attribute only applies to shared libraries, so `Library_Kind` must be set to dynamic or relocatable.

>     for Library_Dir use "lib";
>     for Library_Name use "logging";
>     for Library_Kind use "dynamic";
>     for Library_Interface use ("lib1", "lib2");  --  unit names
>     for Library_Standalone use "encapsulated";

In order to include the elaboration code in the stand-alone library, the binder is invoked on the closure of the library units creating a package whose name depends on the library name (`b~logging.ads/b` in the example). This binder-generated package includes initialization and finalization procedures whose names depend on the library name (`logginginit` and `loggingfinal` in the example). The object corresponding to this package is included in the library.

Library_Auto_Init:

> A dynamic stand-alone Library is automatically initialized if automatic initialization of stand-alone Libraries is supported on the platform and if attribute `Library_Auto_Init` is not specified or is specified with the value `"true"`. Whether a static stand-alone Library is automatically initialized is platform dependent. Specifying `"false"` for the `Library_Auto_Init` attribute prevents automatic initialization.

> When a non-automatically initialized stand-alone library is used in an executable, its initialization procedure must be called before any service of the library is used. When the main subprogram is in Ada, it may mean that the initialization procedure has to be called during elaboration of another package.

Library_Dir:

> For a stand-alone library, only the `ALI` files of the interface units (those that are listed in attribute Library_Interface) are copied to the library directory. As a consequence, only the interface units may be imported from Ada units outside of the library. If other units are imported, the binding phase will fail.

Binder’Default_Switches:

> When a stand-alone library is bound, the switches that are specified in the attribute `Binder'Default_Switches ("Ada")` are used in the call to gnatbind.

Library_Src_Dir:

> This attribute defines the location (absolute or relative to the project directory) where the sources of the interface units are copied at installation time. These sources includes the specs of the interface units along with the closure of sources necessary to compile them successfully. That may include bodies and subunits, when pragmas Inline are used, or when there are generic units in specs. This directory cannot point to the object directory or one of the source directories, but it can point to the library directory, which is the default value for this attribute.

Library_Symbol_Policy:

> This attribute controls the export of symbols on some platforms (like Windows, GNU/Linux). It is not supported on all platforms (where it will just have no effect). It may have one of the following values:

> - `"restricted"`: The exported symbols will be restricted to the one from the interface of the stand-alone library. This is either computed automatically or using the `Library_Symbol_File` if specified.

> - `"unrestricted"`: All symbols from the stand-alone library are exported.

Library_Symbol_File

> This attribute may define the name of the symbol file to be used when building a stand-alone library when the symbol policy is `"restricted"`, on platforms that support symbol control. This file must contain one symbol per line and only those symbols will be exported from the stand-alone library.

### 2.5.4. Installing a Library with Project Files

When using project files, a usable version of the library is created in the directory specified by the `Library_Dir` attribute of the library project file. Thus no further action is needed in order to make use of the libraries that are built as part of the general application build.

You may want to install a library in a context different from where the library is built. This situation arises with third party suppliers, who may want to distribute a library in binary form where the user is not expected to be able to recompile the library. The simplest option in this case is to provide a project file slightly different from the one used to build the library, by using the `Externally_Built` attribute. See Using Library Projects.

Another option is to use gprinstall to install the library in a different context than the build location. The gprinstall tool automatically generates a project to use this library, and also copies the minimum set of sources needed to use the library to the install location. See Package Install Attributes.

## 2.6. Project Extension

During development of a large system, it is sometimes necessary to use modified versions of some of the source files, without changing the original sources. This can be achieved through the project extension facility.

Suppose that our example `Build` project is built every night for the whole team, in some shared directory. A developer usually needs to work on a small part of the system, and might not want to have a copy of all the sources and all the object files since that could require too much disk space and too much time to recompile everything. A better approach is to override some of the source files in a separate directory, while still using the object files generated at night for the non-overridden shared sources.

Another use case is a large software system with multiple implementations of a common interface; in Ada terms, multiple versions of a package body for the same spec, or perhaps different versions of a package spec that have the same visible part but different private parts. For example, one package might be safe for use in tasking programs, while another might be used only in sequential applications.

A third example is different versions of the same system. For instance, assume that a `Common` project is used by two development branches. One of the branches has now been frozen, and no further change can be done to it or to `Common`. However, on the other development branch the sources in `Common` are still evolving. A new version of the subsystem is needed, which reuses as much as possible from the original.

Each of these can be implemented in GNAT using project extension:

> If one project extends another project (the base project) then by default all source files of the base project are inherited by the extending project, but the latter can override any of the base project’s source files with a new version, and can also add new files or remove unnecessary ones. A project can extend at most one base project.

This facility is somewhat analogous to class extension (with single inheritance) in object-oriented programming. Project extension hierarchies are permitted (an extending project may itself serve as a base project and be extended), and a project that extends a project can also import other projects.

An extending project implicitly inherits all the sources and objects from its base project. It is possible to create a new version of some of the sources in one of the additional source directories of the extending project. Those new versions hide the original versions. As noted above, adding new sources or removing existing ones is also possible. Here is an example of how to extend the project Build from previous examples:

>     project Work extends "../bld/build.gpr" is
>     end Work;

The project after the `extends` keyword is the base project being extended. As usual, it can be specified using an absolute path, or a path relative to any of the directories in the project path. The `Work` project does not specify source or object directories, so the default values for these attributes will be used; that is, the current directory (where project `Work` is placed). We can compile that project with

>     gprbuild -Pwork

If no sources have been placed in the current directory, this command has no effect, since this project does not change the sources it inherited from `Build` and thus all the object files in `Build` and its dependencies are still valid and are reused automatically.

Suppose we now want to supply an alternative version of `pack.adb` but use the existing versions of `pack.ads` and `proc.adb`. We can create the new file in the `Work` project’s directory (for example by copying the one from the `Build` project and making changes to it). If new packages are needed at the same time, we simply create new files in the source directory of the extending project.

When we recompile, GPRbuild will now automatically recompile this file (thus creating `pack.o` in the current directory) and any file that depends on it (thus creating `proc.o`). Finally, the executable is also linked locally.

Note that we could have obtained the desired behavior using project import rather than project inheritance. Some project `proj` would contain the sources for `pack.ads` and `proc.adb`, and `Work` would import `proj` and add `pack.adb`. In this situation `proj` cannot contain the original version of `pack.adb` since otherwise two versions of the same unit would be in project import closure of `proj`, which is not allowed. In general we do not recommended placing the spec and body of a unit in different projects, since this affects their autonomy and reusability.

In a project file that extends another project, it is possible to indicate that an inherited source is not part of the sources of the extending project. This is necessary, for example, when a package spec has been overridden in such a way that a body is forbidden. In this case, it is necessary to indicate that the inherited body is not part of the sources of the project, otherwise there will be a compilation error.

Two attributes are available for this purpose:

- Excluded_Source_Files, whose value is a list of file names, and

- Excluded_Source_List_File, whose value is the path of a text file containing one file name per line.

      project Work extends "../bld/build.gpr" is
         for Source_Files use ("pack.ads");
         --  New spec of Pkg does not need a completion
         for Excluded_Source_Files use ("pack.adb");
      end Work;

All tool packages that are not declared in the extending project are inherited from the base project, with their attributes, with the exception of `Linker'Linker_Options` which is never inherited. In particular, an extending project retains all the switches specified in its base project.

At the project level, if they are not declared in the extending project, some attributes are inherited from the base project. They are: `Languages`, `Main` (for a root non library project) and `Library_Name` (for a project extending a library project).

### 2.6.1. Importing and Project Extension

One of the fundamental restrictions for project extension is the following:

> A project is not allowed to import, directly or indirectly, both an extending project P and also some project that P extends either directly or indirectly

In the absence of this rule, two imports might access different versions of the same source file, or different sets of tool switches for the same source file (one from the base project and the other from an extending project).

As an example of this problem, consider the following set of project files:

- `a.gpr` which contains the source files `foo.ads` and `foo.adb`, among others

- `b.gpr` which imports `a.gpr` (one of its source files `with`s `foo`)

- `c.gpr` which imports `b.gpr`

Suppose we want to extend the projects as follows:

- `a_ext.gpr` extends `a.gpr` and overrides `foo.adb`

- `c_ext.gpr` extends `c.gpr`, overriding one of its source files

Since `c_ext.gpr` needs to access sources in `b.gpr`, it will import `b.gpr`

Finally, `main.gpr` needs to access the overridden source files in `a_ext.gpr` and `c_ext.gpr` and thus will import these two projects.

This project structure is shown in `figure #`.

Fig. 2.1 Example of Source File Ambiguity from imports/extends Violation

This violates the restriction above, since `main.gpr` imports the extending project `a_ext.gpr` and also (indirectly through `c_ext.gpr` and `b.gpr`) the project `a.gpr` that `a_ext.gpr` extends. The problem is that the import path through `c_ext.gpr` and `b.gpr` would build with the version of `foo.adb` from `a.gpr`, whereas the import path through `a_ext.gpr` would use that project’s version of `foo.adb`. The error will be detected and reported by `gprbuild`.

A solution is to introduce an “empty” extension of `b.gpr`, which is imported by `c_ext.gpr` and imports `a_ext.gpr`:

>     with "a_ext.gpr";
>     project B_Ext extends "b.gpr" is
>     end B_Ext;

This project structure is shown in `figure #`.

Fig. 2.2 Using “Empty” Project Extension to Avoid imports/extends Violation

There is now no ambiguity over which version of `foo.adb` to use; it will be the one from `a_ext.gpr`.

When extending a large system spanning multiple projects, it is often inconvenient to extend every project in the project import closure that is impacted by a small change introduced in a low layer. In such cases, it is possible to create an implicit extension of an entire hierarchy using the extends all relationship.

When a project `P` is extended using extends all inheritance, all projects that are imported by `P`, both directly and indirectly, are considered virtually extended. That is, the project manager creates implicit projects that extend every project in the project import closure; all these implicit projects do not control sources on their own and use the object directory of the extends all project.

It is possible to explicitly extend one or more projects in the import closure in order to adapt the sources. These extending projects must be imported by the `extends all` project, which will replace the corresponding virtual projects with the explicit ones.

When building such a project closure extension, the project manager will ensure recompilation of both the modified sources and the sources in implicit extending projects that depend on them.

To illustrate the `extends all` feature, here’s a slight variation on the earlier examples. We have a `Main` project that imports project `C`, which imports `B`, which imports `A`. The source files in `Main` refer to compilation units whose sources are in `C` and `A`. (Recall that `imports` is transitive, so `A` is implicitly accessible in `Main`.)

This project structure is shown in `figure #`.

Fig. 2.3 Simple Project Structure before Extension

Suppose that we want to extend `a.gpr`, overriding one of its source files, and create a new version of `main.gpr` that can access the overridden file in the extending project `a_ext.gpr` and otherwise use the sources in `b.gpr` and `c.gpr`.

Instead of explicitly defining empty projects to extend `b.gpr` and `c.gpr`, we can create a new project `main_ext.gpr` that does an `extends all` of `main.gpr` and imports `a_ext.gpr`. The `extends_all` will implicitly create the empty projects `b_ext.gpr` and `c_ext.gpr` as well as the relevant import relationships:

- `c_ext.gpr` will import `b_ext.gpr`, which will import `a_ext.gpr`

- `main_ext.gpr` will implicitly import `c_ext.gpr` since `main.gpr` imports `c.gpr`.

The resulting project structure is shown in `figure #`, where the italicized labels, dashed arrows, and dashed boxes indicate what was added implicitly as an effect of the `extends_all`.

Fig. 2.4 Project Structure with `extends_all`

When project `main_ext.gpr` is built, the entire modified project space is considered for recompilation, including the sources from `b.gpr` and `c.gpr` that are affected by the changes to `a.gpr`.

## 2.7. Child Projects

In order to more clearly express the relationship between a project `Q` and some other project `P` that `Q` either imports or extends, you can use the notation `P.Q` to declare `Q` as a child of `P`. The project `P` is then referred to as the parent of `Q`. This is useful, for example, when the purpose of the child is to serve as a testing subsystem for the parent.

The visibility of the child on the sources and other properties of the parent is determined by whether the child imports or extends the parent. No additional visibility is obtained by declaring the project as a child; the parent.child notation serves solely as a naming convention to convey to the reader the closeness of the relationship between the projects.

For example:

>     -- math_proj.gpr
>     project Math_Proj is
>        ...
>     end Math_Proj;

>     ---------------

>     with "math_proj.gpr";
>     project Math_Proj.Tests is      -- Legal; child imports parent
>        ...
>     end Math_Proj.Tests;

>     ---------------

>     project Math_Proj.High_Performance
>        extends "math_proj.gpr" is   -- Legal; child extends parent
>        ...
>     end Math_Proj.High_Performance;

>     ---------------

>     project GUI_Proj.Tests is       -- Illegal
>        ...
>     end GUI_Proj.Tests;

Child projects may in turn be the parents of other projects, so in general a project hierarchy can be created. A project may be the parent of many child projects, but a child project can only have one parent.

Note that child projects have slightly different semantics from their Ada language analog (child units). An Ada child unit implicitly `with`s its parent, whereas a child project must have an explicit `with` clause (or else extend its parent). The need to explicitly `with` or extend the parent project helps avoid the error of unintentionally creating a child of some project that happens to be on the project path.

## 2.8. Aggregate Projects

Aggregate projects are an extension of the project paradigm, and are designed to handle a few specific situations that cannot be solved directly using standard projects. This section will present several such use cases.

### 2.8.1. Building all main programs from a single project closure

A large application is typically organized into modules and submodules, which are conveniently represented as a project graph (the project import closure): a “root” project A `with`s the projects for modules B and C, which in turn `with` projects for submodules.

Very often, modules will build their own executables (for testing purposes for instance) or libraries (for easier reuse in various contexts).

However, if you build your project through GPRbuild, using a syntax similar to

>     gprbuild -PA.gpr

this will only rebuild the main programs of project A, not those of the imported projects B and C. Therefore you have to spawn several GPRbuild commands, one per project, to build all executables. This is somewhat inconvenient, but more importantly is inefficient because GPRbuild needs to do duplicate work to ensure that sources are up-to-date, and cannot easily compile things in parallel when using the `-j` switch.

Also, libraries are always rebuilt when building a project.

To solve this problem you can define an aggregate project `Agg` that groups `A`, `B` and `C`:

>     aggregate project Agg is
>        for Project_Files use ("a.gpr", "b.gpr", "c.gpr");
>     end Agg;

Then, when you build with

>     gprbuild -PAgg.gpr

this will build all main programs from `A`, `B` and `C`.

If B or C do not define any main program (through their Main attribute), all their sources are built. When you do not group them in an aggregate project, only those sources that are needed by A will be built.

If you add a main to a project `P` not already explicitly referenced in the aggregate project, you will need to add `p.gpr` in the list of project files for the aggregate project, or the main will not be built when building the aggregate project.

### 2.8.2. Building a set of projects with a single command

Another application of aggregate projects is when you have multiple applications and libraries that are built independently (but can be built in parallel). For instance, you might have a project graph rooted at `A`, and another one (which might share some subprojects) rooted at `B`.

Using only GPRbuild, you could do

>     gprbuild -PA.gpr
>     gprbuild -PB.gpr

to build both. But again, GPRbuild has to do some duplicate work for those files that are shared between the two, and cannot truly build things in parallel efficiently.

If the two projects are really independent, share no sources other than through a common subproject, and have no source files with a common basename, you could create a project `C` that imports `A` and `B`. But these restrictions are often too strong, and one has to build them independently. An aggregate project does not have these limitations and can aggregate two project graphs that have common sources:

>     aggregate project Agg is
>        for Project_Files use ("a.gpr", "b.gpr");
>     end Agg;

This scenario is particularly useful in environments like VxWorks 653 where the applications running in the multiple partitions can be built in parallel through a single GPRbuild command. This also works well with Annex E of the Ada Language Reference Manual.

### 2.8.3. Defining a build environment

The environment variables at the time you launch GPRbuild will influence the view these tools have of the project (for example `PATH` to find the compiler, `ADA_PROJECT_PATH` or `GPR_PROJECT_PATH` to find the projects, and environment variables that are referenced in project files through the `external` built-in function). Several command line switches can be used to override those (`-X` or `-aP`), but on some systems and with some projects, this might make the command line too long, and on all systems often make it hard to read.

An aggregate project can be used to set the environment for all projects built through that aggregate. One of the benefits is that you can put the aggregate project under configuration management, and make sure all your users have a consistent environment when building. For example:

>     aggregate project Agg is
>        for Project_Files use ("A.gpr", "B.gpr");
>        for Project_Path use ("../dir1", "../dir1/dir2");
>        for External ("BUILD") use "PRODUCTION";

>        package Builder is
>           for Global_Compilation_Switches ("Ada") use ("-g");
>        end Builder;
>     end Agg;

Another use of aggregate projects is to simulate the referencing of external variables in `with` clauses, For technical reasons the following project file is not allowed:

>     with external("SETUP") & "path/prj.gpr";   --  ILLEGAL
>     project MyProject is
>        ...
>     end MyProject;

However, you can use aggregate projects to obtain an equivalent effect:

>     aggregate project Agg is
>         for Project_Path use (external("SETUP") & "path");
>         for Project_Files use ("myproject.gpr");
>     end Agg;

>     with "prj.gpr";  --  searched on Agg'Project_Path
>     project MyProject is
>        ...
>     end MyProject;

### 2.8.4. Improving builder performance

The loading of aggregate projects is optimized in GPRbuild, so that all files are searched for only once on the disk (thus reducing the number of system calls and yielding faster compilation times, especially on systems with sources on remote servers). As part of the loading, GPRbuild computes how and where a source file should be compiled, and even if it is located several times in the aggregated projects it will be compiled only once.

Since there is no ambiguity as to which switches should be used, individual compilations, binds and links can be performed in parallel (through the usual `-j` switch) and this can be done while maximizing the use of CPUs (compared to launching multiple GPRbuild commands in parallel). The -j option can control parallelization of compilation, binding, and linking separately with -jc, -jb, and -jl variants accordingly.

### 2.8.5. Syntax of aggregate projects

An aggregate project follows the general syntax of project files. The recommended extension is still `.gpr`. However, a special `aggregate` qualifier must appear before the keyword `project`.

An aggregate project cannot `with` any other project (standard or aggregate), except an abstract project (which can be used to share attribute values). Also, aggregate projects cannot be extended or imported though a `with` clause by any other project. Building other aggregate projects from an aggregate project is done through the `Project_Files` attribute (see below).

An aggregate project does not have any source files directly (only through other standard projects). Therefore a number of the standard attributes and packages are forbidden in an aggregate project. Here is a (non exhaustive) list:

- `Languages`

- `Source_Files`, `Source_List_File` and other attributes dealing with list of sources.

- `Source_Dirs` and `Exec_Dir`

- `Library_Dir`, `Library_Name` and other library-related attributes

- `Main`

- `Roots`

- `Externally_Built`

- `Inherit_Source_Path`

- `Excluded_Source_Dirs`

- `Locally_Removed_Files`

- `Excluded_Source_Files`

- `Excluded_Source_List_File`

- `Interfaces`

The `Object_Dir` attribute is allowed and used by some analysis tools such as gnatcheck to store intermediate files and aggregated results. The attribute value is just ignored by the compilation toolchain, for which every artifact of interest is best associated with the leaf non aggregate projects and stored in the corresponding `Object_Dir`.

The package `Naming` and packages that control the compilation process (`Compiler`, `Binder`, `Linker` and `Install`) are forbidden.

The following three attributes can be used only in an aggregate project:

Project_Files:

> This attribute is compulsory. It specifies a list of constituent `.gpr` files that are grouped in the aggregate. The list may be empty. The project files can be any projects except configuration or abstract projects; they can be other aggregate projects. When grouping standard projects, you can have both the root of a project import closure (and you do not need to specify all its imported projects), and any project within the closure.

> The basic idea is to specify all those projects that have main programs you want to build and link, or libraries you want to build. You can specify projects that do not use the `Main` attribute or the `Library_*` attributes, and the result will be to build all their source files (not just the ones needed by other projects).

> The file can include paths (absolute or relative). Paths are relative to the location of the aggregate project file itself (if you use a base name, the `.gpr` file is expected in the same directory as the aggregate project file). The environment variables `ADA_PROJECT_PATH`, `GPR_PROJECT_PATH` and `GPR_PROJECT_PATH_FILE` are not used to find the project files. The extension `.gpr` is mandatory, since this attribute contains file names, not project names.

> Paths can also include the `"*"` and `"**"` globbing patterns. The latter indicates that any subdirectory (recursively) will be searched for matching files. The `"**"` pattern can only occur at the last position in the directory part (i.e. `"a/**/*.gpr"` is supported, but not `"**/a/*.gpr"`). Starting the pattern with `"**"` is equivalent to starting with `"./**"`.

> At present the pattern `"*"` is only allowed in the filename part, not in the directory part. This is mostly for efficiency reasons to limit the number of system calls that are needed.

> Here are a few examples:

> >     for Project_Files use ("a.gpr", "subdir/b.gpr");
> >     --  two specific projects relative to the directory of agg.gpr

> >     for Project_Files use ("**/*.gpr");
> >     --  all projects recursively, except in the current directory

> >     for Project_Files use ("**/*.gpr", "*.gpr");
> >     --  all projects recursively

Project_Path:

> This attribute can be used to specify a list of directories in which to search for project files in `with` clauses.

> When you specify a project in `Project_Files` (say `x/y/a.gpr`), and `a.gpr` imports a project `b.gpr`, only `b.gpr` is searched in the project path. The file `a.gpr` must be exactly at `dir of the aggregate/x/y/a.gpr`.

> This attribute, however, does not affect the search for the aggregated project files specified with `Project_Files`.

> Each aggregate project has its own `Project_Path` (thus if `agg1.gpr` includes `agg2.gpr`, they can potentially both have a different Project_Path).

> This project path is defined as the concatenation, in this order, of:

> - the current directory;

> - followed by the command line `-aP` switches;

> - then the directories from the `GPR_PROJECT_PATH` and `ADA_PROJECT_PATH` environment variables;

> - then the directories from the `Project_Path` attribute;

> - and finally the predefined directories.

> In the example above, the project path for `agg2.gpr` is not influenced by the attribute agg1’Project_Path, nor is agg1 influenced by agg2’Project_Path.

> This can potentially lead to errors. Consider the example in `figure #`.

> Fig. 2.5 Example of `Project_Path` Error

> When looking for `p.gpr`, both aggregates find the same physical file on the disk. However, it might happen that with their different project paths, both aggregate projects would in fact find a different `r.gpr`. Since we have a common project `p.gpr` `with`ing two different `r.gpr`, this will be reported as an error by the builder.

> Directories are relative to the location of the aggregate project file.

> Example:

> >     for Project_Path use ("/usr/local/gpr", "gpr/");

External:

> This attribute can be used to set the value of environment variables as retrieved through the `external` function in projects. It does not affect the environment variables themselves (so for instance you cannot use it to change the value of your `PATH` as seen from the spawned compiler).

> This attribute affects the external values as seen in the rest of the aggregate project, and in the aggregated projects.

> The exact value of an external variable comes from one of three sources (each level overrides the previous levels):

> - An External attribute in aggregate project, for instance for External (“BUILD_MODE”) use “DEBUG”;

> - Environment variables. These override the value given by the attribute, so that users can override the value set in the (presumably shared with others team members) aggregate project.

> - The `-X` command line switch to gprbuild. This always takes precedence.

> This attribute is only taken into account in the main aggregate project (i.e. the one specified on the command line to GPRbuild), and ignored in other aggregate projects. It is invalid in standard projects. The goal is to have a consistent value in all projects that are built through the aggregate, which would not be the case in a “diamond” situation: `A` groups the aggregate projects `B` and `C`, which both (either directly or indirectly) build the project `P`. If `B` and `C` could set different values for the environment variables, we would have two different views of `P`, which in particular might impact the list of source files in `P`.

### 2.8.6. package Builder in aggregate projects

When used in an aggregate project, only the following attributes of this package are valid:

Switches:

> This attribute gives the list of switches to use for GPRbuild. Because no mains can be specified for aggregate projects, the only possible index for attribute `Switches` is `others`. All other indexes will be ignored.

> Example:

>     for Switches (others) use ("-v", "-k", "-j8");

> These switches are only read from the main aggregate project (the one passed on the command line), and ignored in all other aggregate projects or projects.

> It can only contain builder switches, not compiler switches.

Global_Compilation_Switches

> This attribute gives the list of compiler switches for the various languages. For instance,

>     for Global_Compilation_Switches ("Ada") use ("O1", "-g");
>     for Global_Compilation_Switches ("C")   use ("-O2");

> This attribute is only taken into account in the aggregate project specified on the command line, not in other aggregate projects.

> In the projects grouped by that aggregate, the attribute `Builder'Global_Compilation_Switches` is also ignored. However, the attribute `Compiler'Default_Switches` will be taken into account (but that of the aggregate has higher priority). The attribute `Compiler'Switches` is also taken into account and can be used to override the switches for a specific file. As a result, it always has priority.

> The rules are meant to avoid ambiguities when compiling. For instance, aggregate project `Agg` groups the projects `A` and `B`, which both depend on `C`. Here is an example for all of these projects:

>     aggregate project Agg is
>         for Project_Files use ("a.gpr", "b.gpr");
>         package Builder is
>            for Global_Compilation_Switches ("Ada") use ("-O2");
>         end Builder;
>     end Agg;

>     with "c.gpr";
>     project A is
>         package Builder is
>            for Global_Compilation_Switches ("Ada") use ("-O1");
>            --  ignored
>         end Builder;

>         package Compiler is
>            for Default_Switches ("Ada")
>                use ("-O1", "-g");
>            for Switches ("a_file1.adb")
>                use ("-O0");
>         end Compiler;
>     end A;

>     with "c.gpr";
>     project B is
>         package Compiler is
>            for Default_Switches ("Ada") use ("-O0");
>         end Compiler;
>     end B;

>     project C is
>         package Compiler is
>            for Default_Switches ("Ada")
>                use ("-O3",
>                     "-gnatn");
>            for Switches ("c_file1.adb")
>                use ("-O0", "-g");
>         end Compiler;
>     end C;

> The following switches are used:

> - all files from project `A` except `a_file1.adb` are compiled with `-O2 -g`, since the aggregate project has priority.

> - the file `a_file1.adb` is compiled with :option”-O0, since `Compiler'Switches` has priority

> - all files from project `B` are compiled with `-O2`, since the aggregate project has priority

> - all files from `C` are compiled with `-O2 -gnatn`, except for `c_file1.adb` which is compiled with `-O0 -g`

> Even though `C` is seen through two paths (through `A` and through `B`), the switches used by the compiler are unambiguous.

Global_Configuration_Pragmas

> This attribute can be used to specify a file containing configuration pragmas, to be passed to the Ada compiler. Since we ignore the package `Builder` in other aggregate projects and projects, only those pragmas defined in the main aggregate project will be taken into account.

> Projects can locally add to those by using the `Compiler'Local_Configuration_Pragmas` attribute if they need.

Global_Config_File

> This attribute, indexed with a language name, can be used to specify a config when compiling sources of the language. For Ada, these files are configuration pragmas files.

For projects that are built through the aggregate mechanism, the package `Builder` is ignored, except for the `Executable` attribute which specifies the name of the executables resulting from the link of the main programs, and for the `Executable_Suffix`.

## 2.9. Aggregate Library Projects

Aggregate library projects make it possible to build a single library using object files built using other standard or library projects. This gives the flexibility to describe an application as having multiple modules (for example a GUI, database access, and other) using different project files (so possibly built with different compiler options) and yet create a single library (static or relocatable) out of the corresponding object files.

### 2.9.1. Building aggregate library projects

For example, we can define an aggregate project `Agg` that groups `A`, `B` and `C`:

>     aggregate library project Agg is
>        for Project_Files use ("a.gpr", "b.gpr", "c.gpr");
>        for Library_Name  use "agg";
>        for Library_Dir   use "lagg";
>     end Agg;

Then, when you build with:

>     gprbuild agg.gpr

this will build all units from projects `A`, `B` and `C` and will create a static library named `libagg.a` in the `lagg` directory. An aggregate library project has the same set of restrictions as a standard library project.

Note that a shared aggregate library project cannot aggregate a static library project. In platforms where a compiler option is required to create relocatable object files, a `Builder package` in the aggregate library project may be used:

>     aggregate library project Agg is
>        for Project_Files use ("a.gpr", "b.gpr", "c.gpr");
>        for Library_Name use ("agg");
>        for Library_Dir use ("lagg");
>        for Library_Kind use "relocatable";

>        package Builder is
>           for Global_Compilation_Switches ("Ada") use ("-fPIC");
>        end Builder;
>     end Agg;

With the above aggregate library Builder package, the `-fPIC` option will be passed to the compiler when building any source code from projects `a.gpr`, `b.gpr` and `c.gpr`.

### 2.9.2. Syntax of aggregate library projects

An aggregate library project follows the general syntax of project files. The recommended extension is still `.gpr`. However, a special `aggregate library` qualifier must appear before the keyword `project`.

An aggregate library project cannot `with` any other project (standard or aggregate), except an abstract project which can be used to share attribute values.

An aggregate library project does not have any source files directly (only through other standard projects). Therefore a number of the standard attributes and packages are forbidden in an aggregate library project. Here is a (non-exhaustive) list:

- `Languages`

- `Source_Files`, `Source_List_File` and other attributes dealing with a list of sources.

- `Source_Dirs` and `Exec_Dir`

- `Main`

- `Roots`

- `Externally_Built`

- `Inherit_Source_Path`

- `Excluded_Source_Dirs`

- `Locally_Removed_Files`

- `Excluded_Source_Files`

- `Excluded_Source_List_File`

The only package that is allowed (and optional) is `Builder`.

The `Project_Files` attribute is used to describe the aggregated projects whose object files have to be included into the aggregate library. The environment variables `ADA_PROJECT_PATH`, `GPR_PROJECT_PATH` and `GPR_PROJECT_PATH_FILE` are not used to find the project files.

As for regular (not library) aggregate projects, the `Object_Dir` attribute is allowed and used by some analysis tools in the same fashion.

## 2.10. Project File Reference

This section describes the syntactic structure of project files, explains the various constructs that can be used, and summarizes the available attributes.

The syntax is presented in a notation similar to what is used in the Ada Language Reference Manual. Curly braces ‘{’ and ‘}’ indicate 0 or more occurrences of the enclosed construct, and square brackets ‘[’ and ‘]’ indicate 0 or 1 occurrence of the enclosed construct. Reserved words are enclosed between apostrophes.

### 2.10.1. Project Declaration

Project files have an Ada-like syntax. The minimal project file is:

>     project Empty is
>     end Empty;

The identifier `Empty` is the name of the project. This project name must be present after the reserved word `end` at the end of the project file, followed by a semicolon.

Identifiers (i.e., the user-defined names such as project or variable names) have the same syntax as Ada identifiers: they must start with a letter, and be followed by zero or more letters, digits or underscore characters; it is also illegal to have two underscores next to each other. Identifiers are always case-insensitive (`"Name"` is the same as `"name"`).

    simple_name ::= identifier
    name        ::= simple_name { . simple_name }

Strings are used for values of attributes or as indexes for these attributes. They are in general case sensitive, except when noted otherwise (in particular, strings representing file names will be case insensitive on some systems, so that `"file.adb"` and `"File.adb"` both represent the same file).

Reserved words are the standard Ada 95 reserved words, plus several others listed below, and cannot be used for identifiers. In particular, the following Ada 95 reserved words are currently used in project files:

>     abstract  all     at       case
>     end       for     is       limited
>     null      others  package  renames
>     type      use     when     with

The additional project file reserved words are:

>     extends external external_as_list project

Note that `aggregate` and `library` are qualifiers that may appear before the keyword `project`, but they are not themselves keywords.

To avoid possible compatibility issues in the future, we recommend that the reserved words introduced by Ada 2005 and Ada 2012 not be used as identifiers in project files. Note also that new reserved words may be added to the project file syntax in a later release.

Comments in project files have the same syntax as in Ada, two consecutive hyphens through the end of the line.

A project may be an independent project, entirely defined by a single project file. Any source file in an independent project depends only on the predefined library and other source files in the same project. Alternatively, a project may depend on other projects in various ways:

- by importing them through context clauses (`with` clauses), or

- by extending at most one other project (its base project).

A given project may exhibit either or both of these dependencies; for example:

>     with "imported_proj.gpr";
>     project My_Project extends "base_proj.gpr" is
>     end My_Project;

The import dependencies form a directed graph, potentially cyclic when using limited with. The subgraph reflecting the extends relationship is a tree (hierarchy).

A path name denotes a project file. It can be absolute or relative. An absolute path name includes a sequence of directories, in the syntax of the host operating system, that uniquely identifies the project file in the file system. A relative path name identifies the project file, relative to the directory that contains the current project, or relative to a directory listed in the environment variables `ADA_PROJECT_PATH` and `GPR_PROJECT_PATH`. Path names are case sensitive if file names in the host operating system are case sensitive. As a special case, the directory separator can always be `'/'` even on Windows systems, so that project files can be made portable across architectures. The syntax of the environment variables `ADA_PROJECT_PATH` and `GPR_PROJECT_PATH` is a list of directory names separated by colons on Unix and semicolons on Windows.

A given project name can appear only once in a context clause, and may not appear in different context clauses for the same project.

It is illegal for a project imported by a context clause to refer, directly or indirectly, to the project in which this context clause appears (the dependency graph cannot contain cycles), except when one of the `with` clauses in the cycle is a `limited with`.

A project’s immediate sources are the source files directly defined by that project, either implicitly by residing in the project source directories, or explicitly through any of the source-related attributes. More generally, a project’s sources are the immediate sources of the project together with the immediate sources (unless overridden) of any project on which it depends directly or indirectly.

     project        ::= context_clause project_declaration

     context_clause ::= {with_clause}
     with_clause    ::= [ 'limited' ] 'with' path_name { , path_name } ;
     path_name      ::= string_literal

     project_declaration ::= simple_project_declaration | project_extension

     simple_project_declaration ::=
       [ qualifier ] 'project' <project_>name 'is'
         {declarative_item}
       'end' <project_>name ;

     project_extension ::=
      [ qualifier ] 'project' <project_>name 'extends' [ 'all' ] <base_project_>name 'is'
        {declarative_item}
      'end' <project_>name ;

    qualifier ::=
      'abstract' | identifier [ identifier ]

### 2.10.2. Qualified Projects

Immediately preceding the reserved `project`, a qualifier may be specified which identifies the nature of the project. The following qualifiers are allowed:

standard:

  A standard project is a non-library project with source files. This is the default (implicit) qualifier.

abstract:

  A project with no source files. Such a project must either have no declaration for attributes `Source_Dirs`, `Source_Files`, `Languages` or `Source_List_File`, or one of `Source_Dirs`, `Source_Files`, or `Languages` must be declared as empty. If it extends another project, the base project must also be an abstract project.

aggregate:

  A project whose sources are aggregated from other project files.

aggregate library:

  A library whose sources are aggregated from other project or library project files.

library:

  A library project must define both of the attributes Library_Name and Library_Dir.

configuration:

  A configuration project cannot be in a project tree. It describes compilers and other tools to gprbuild.

### 2.10.3. Declarations

Declarations introduce new entities that denote types, variables, attributes, and packages. Some declarations can only appear immediately within a project declaration. Others can appear within a project or within a package.

    declarative_item ::= simple_declarative_item
      | typed_string_declaration
      | package_declaration

    simple_declarative_item ::= variable_declaration
      | typed_variable_declaration
      | attribute_declaration
      | case_construction
      | empty_declaration

    empty_declaration ::= 'null' ;

An empty declaration is allowed anywhere a declaration is allowed. It has no effect.

### 2.10.4. Packages

A project file may contain packages, which group attributes (typically all the attributes that are used by one of the GNAT tools).

A package with a given name may only appear once in a project file. The following packages are currently supported in project files (See Attributes for the list of attributes that each can contain).

Binder

  This package specifies characteristics useful when invoking the binder either directly via the gnat driver or when using GPRbuild. See Main Subprograms.

Builder

  This package specifies the compilation options used when building an executable or a library for a project. Most of the options should be set in one of `Compiler`, `Binder` or `Linker` packages, but there are some general options that should be defined in this package. See Main Subprograms, and Executable File Names in particular.

Check

  This package specifies the options used when calling the coding standard verification tool gnatcheck. Its attributes `Default_Switches` and `Switches` have the same semantics as for the package `Builder`. The first string should always be `-rules` to specify that all the other options belong to the `-rules` section of the parameters to gnatcheck.

Clean

  This package specifies the options used when cleaning a project or a project tree using the tools gnatclean or gprclean.

Compiler

  This package specifies the compilation options used by the compiler for each language. See Tools Options in Project Files.

Cross_Reference

  This package specifies the options used when calling the library tool gnatxref via the gnat driver. Its attributes `Default_Switches` and `Switches` have the same semantics as for the package `Builder`.

Documentation

  This package specifies the options used when calling the tool gnatdoc.

Eliminate

  This package specifies the options used when calling the tool gnatelim. Its attributes `Default_Switches` and `Switches` have the same semantics as for the package `Builder`.

Finder

  This package specifies the options used when calling the search tool gnatfind via the gnat driver. Its attributes `Default_Switches` and `Switches` have the same semantics as for the package `Builder`.

Gnatls

  This package specifies the options to use when invoking gnatls via the gnat driver.

Gnatstub

  This package specifies the options used when calling the tool gnatstub. Its attributes `Default_Switches` and `Switches` have the same semantics as for the package `Builder`.

IDE

  This package specifies the options used when starting an integrated development environment, for instance GPS or GNATbench.

Install

  This package specifies the options used when installing a project with gprinstall. See Package Install Attributes.

Linker

  This package specifies the options used by the linker. See Main Subprograms.

Metrics

  This package specifies the options used when calling the tool gnatmetric. Its attributes `Default_Switches` and `Switches` have the same semantics as for the package `Builder`.

Naming

  This package specifies the naming conventions that apply to the source files in a project. In particular, these conventions are used to automatically find all source files in the source directories, or given a file name to find out its language for proper processing. See Naming Schemes.

Pretty_Printer

  This package specifies the options used when calling the formatting tool gnatpp. Its attributes `Default_Switches` and `Switches` have the same semantics as for the package `Builder`.

Remote

  This package is used by GPRbuild to describe how distributed compilation should be done.

Stack

  This package specifies the options used when calling the tool gnatstack. Its attributes Default_Switches and Switches have the same semantics as for the package Builder.

Synchronize

  This package specifies the options used when calling the tool gnatsync via the gnat driver.

In its simplest form, a package may be empty:

>     project Simple is
>       package Builder is
>       end Builder;
>     end Simple;

A package may contain attribute declarations, variable declarations and case constructions, as will be described below.

When there is ambiguity between a project name and a package name, the name always designates the project. To avoid possible confusion, it is always a good idea to avoid naming a project with one of the names allowed for packages or any name that starts with gnat.

Package renaming

A package may be defined by a renaming declaration. The new package renames a package declared in a different project file, and has the same attributes as the package it renames. The name of the renamed package must be the same as the name of the renaming package. The project must contain a package declaration with this name, and the project must appear in the context clause of the current project, or be its base or parent project. It is not possible to add or override attributes to the renaming project. If you need to do so, you should use an extending declaration (see below).

Packages that are renamed in other project files often come from project files that have no sources: they are just used as templates. Any modification in the template will be reflected automatically in all the project files that rename a package from the template. This is a very common way to share settings between projects.

Package extension

A package can also be defined by an extending declaration. This is similar to a renaming declaration, except that it is possible to add or override attributes.

    package_declaration ::= package_spec | package_renaming | package_extension

    package_spec ::=
      'package' <package_>simple_name 'is'
         { simple_declarative_item }
      'end' package_identifier ;

    package_renaming ::=
      'package' <package_>simple_name 'renames'
            <project_>simple_name.package_identifier ;

    package_extension ::=
      'package' <package_>simple_name 'extends'
            <project_>simple_name.package_identifier 'is'
         { simple_declarative_item }
      'end' package_identifier ;

### 2.10.5. Expressions

An expression is any value that can be assigned to an attribute or a variable. It is either a literal value, or a construct requiring run-time computation by the Project Manager. In a project file, the computed value of an expression is either a string or a list of strings.

A string value is one of:

- A literal string, for instance `"comm/my_proj.gpr"`

- The name of a variable that evaluates to a string (see Variables)

- The name of an attribute that evaluates to a string (see Attributes)

- An external reference (see The function external)

- A concatenation of the above, as in `"prefix_" & Var`.

A list of strings is one of the following:

- A parenthesized comma-separated list of zero or more string expressions, for instance `(File_Name, "gnat.adc", File_Name & ".orig")` or `()`.

- The name of a variable that evaluates to a list of strings

- The name of an attribute that evaluates to a list of strings

- A concatenation of a list of strings and a string (as defined above), for instance `("A", "B") & "C"`

- A concatenation of two lists of strings

The following is the grammar for expressions

    string_literal ::= "{string_element}"  --  Same as Ada

    string_expression ::= string_literal
        | <variable_>name
        | external_value
        | attribute_reference
        | ( string_expression { & string_expression } )

    string_list  ::= ( string_expression { , string_expression } )
       | <string_variable>_name
       | <string_>attribute_reference

    term ::= string_expression | string_list

    expression ::= term { & term }     --  Concatenation

Concatenation involves strings and list of strings. As soon as a list of strings is involved, the result of the concatenation is a list of strings. The following Ada declarations show the existing operators:

>     function "&" (X : String;      Y : String)      return String;
>     function "&" (X : String_List; Y : String)      return String_List;
>     function "&" (X : String_List; Y : String_List) return String_List;

Here are some specific examples:

>     List := () & File_Name; --  One string in this list
>     List2 := List & (File_Name & ".orig"); -- Two strings
>     Big_List := List & Lists2;  --  Three strings
>     Illegal := "gnat.adc" & List2;  --  Illegal, must start with list

### 2.10.6. Built-in Functions

Built-in functions may be used in expression. The names of built-in functions are not reserved words and may also be used as variable names. In an expression, a built-in function is recognized if its name is immediately followed by an open parenthesis (‘(‘).

#### 2.10.6.1. The function `external`

An external value is an expression whose value is obtained from the command that invoked the processing of the current project file (typically a gprbuild command).

The syntax of a single string external value is:

    external_value ::= 'external' ( string_literal [, string_literal] )

The first string_literal is the name of the external variable, whose value (a string) may be specified by an environment variable with this name, or on the command line via the `-Xname=value` option. The command line takes precedence if the name is defined in both contexts, thus allowing the user to locally override an environment variable. The second string_literal, if present, is the default to use if there is no specification for this external value either on the command line or in the environment. If the value of the external variable is not obtained from an environment variable or the command line, and the invocation of the `external` function does not supply a second parameter, then an error is reported.

An external reference may be part of a string expression or of a string list expression, and can therefore appear in a variable declaration or an attribute declaration.

This construct is typically used to initialize typed variables, which are then used in case constructions to control the value assigned to attributes in various scenarios. Thus such variables are often called scenario variables.

#### 2.10.6.2. The function `external_as_list`

An external value is an expression whose value is obtained from the command that invoked the processing of the current project file (typically a gprbuild command).

The syntax for a string list external value is:

    external_value ::= 'external_as_list' ( string_literal , string_literal )

The first string_literal is the name of the external variable, with the same interpretation as for the `external` function; it is looked up first on the command line (as the name in a `-Xname=value` option) and, if not so specified, then as an environment variable. If it is not defined by either of these, then the function returns an empty list. The second string_literal is the separator between each component of the string list. An empty list is returned if the separator is an empty string or if the external value is only one separator.

Any separator at the beginning or at the end of the external value is discarded. Then, if there is no separator in the external value, the result is a string list with only one string. Otherwise, any string between the beginning and the first separator, between two consecutive separators and between the last separator and the end are components of the string list.

Note the following differences between `external` and `external_as_list`:

- The `external_as_list` function has no default value for the external variable

- The `external_as_list` function returns an empty list, and does not report an error, when the value of the external variable is undefined.

These differences reflect the different use cases for the two functions. External variables evaluated by the `external` function are often used for configuration control, and misspellings should be detected as errors rather than silently returning the empty string. If the user intended an empty string as the result when the external variable was undefined, then this could easily be obtained:

>     external ("SOME_VAR", "")

In contrast, the `external_as_list` function more typically is used for external variables that may or may not have definitions (for example, lists of options or paths) and then the desired result in the undefined case is an empty list, not a reported error.

Here is an example of the `external_as_list` function:

>     external_as_list ("SWITCHES", ",")

If the external value of `SWITCHES` is `"-O2,-g"`, the result is `("-O2", "-g")`.

If the external value is `",-O2,-g,"`, the result is also `("-O2", "-g")`.

if the external value is `"-gnatv"`, the result is `("-gnatv")`.

If the external value is `",,"`, the result is (`""`).

If the external value is `","`, the result is `()`, the empty string list.

#### 2.10.6.3. Split

Function Split takes two single string parameters and return a string list.

Example:

>     Split ("-gnatf,-gnatv", ",")

>     => ("-gnatf", "gnatv")

The first string argument is the string to be split. The second argument is the separator. Each occurrence of the separator in the first argument is a place where it is split. If the first argument is an empty string or contains only occurrences of the separator, then the result is an empty string list. If the argument does not contains any occurrence of the separator, then the result is a list with only one string: the first argument. Empty strings are not included in the result.

>     Split ("-gnatf   -gnatv", " ")

>     => ("-gnatf", "gnatv")

### 2.10.7. Typed String Declaration

A type declaration introduces a discrete set of string literals. If a string variable is declared to have this type, its value is restricted to the given set of literals. These are the only named types in project files. A type declaration may only appear at the project level, not inside a package.

    typed_string_declaration ::=
      'type' <typed_string_>simple_name 'is' ( string_literal {, string_literal} );

The string literals in the list are case sensitive and must all be different. They may include any graphic characters allowed in Ada, including spaces. Here is an example of a string type declaration:

>     type OS is ("GNU/Linux", "Unix", "Windows", "VMS");

Variables of a string type are called typed variables; all other variables are called untyped variables. Typed variables are particularly useful in case constructions, to support conditional attribute declarations. (See Case Constructions).

A string type may be referenced by its name if it has been declared in the same project file, or by an expanded name whose prefix is the name of the project in which it is declared.

### 2.10.8. Variables

Variables store values (strings or list of strings) and can appear as part of an expression. The declaration of a variable creates the variable and assigns the value of the expression to it. The name of the variable is available immediately after the assignment symbol, if you need to reuse its old value to compute the new value. Before the completion of its first declaration, the value of a variable defaults to the empty string (`""`).

A typed variable can be used as part of a case expression to compute the value, but it can only be declared once in the project file, so that all case constructions see the same value for the variable. This provides more consistency and makes the project easier to understand. The syntax for its declaration is identical to the Ada syntax for an object declaration. In effect, a typed variable acts as a constant.

An untyped variable can be declared and overridden multiple times within the same project. It is declared implicitly through an Ada assignment. The first declaration establishes the kind of the variable (string or list of strings) and successive declarations must respect the initial kind. Assignments are executed in the order in which they appear, so the new value replaces the old one and any subsequent reference to the variable uses the new value.

A variable may be declared at the project file level, or within a package.

    typed_variable_declaration ::=
      <typed_variable_>simple_name : <typed_string_>name := string_expression;

    variable_declaration ::= <variable_>simple_name := expression;

Here are some examples of variable declarations:

>     This_OS : OS := external ("OS"); --  a typed variable declaration
>     That_OS := "GNU/Linux";          --  an untyped variable declaration

>     Name      := "readme.txt";
>     Save_Name := Name & ".saved";

>     Empty_List := ();
>     List_With_One_Element := ("-gnaty");
>     List_With_Two_Elements := List_With_One_Element & "-gnatg";
>     Long_List := ("main.ada", "pack1_.ada", "pack1.ada", "pack2_.ada");

A variable reference may take several forms:

- The simple variable name, for a variable in the current package (if any) or in the current project

- An expanded name, whose prefix is a context name.

A context may be one of the following:

- The name of an existing package in the current project

- The name of an imported project of the current project

- The name of a direct or indirect base project (i.e., a project extended by the current project, either directly or indirectly)

- An expanded name whose prefix is an imported/parent project name, and whose selector is a package name in that project.

### 2.10.9. Case Constructions

A case construction is used in a project file to effect conditional behavior. Through this construction, you can set the value of attributes and variables depending on the value previously assigned to a typed variable.

All choices in a choice list must be distinct. Unlike Ada, the choice lists of all alternatives do not need to include all values of the type. An others choice must appear last in the list of alternatives.

The syntax of a `case` construction is based on the Ada case construction (although the `null` declaration for empty alternatives is optional).

The case expression must be a string variable, either typed or not, whose value is often given by an external reference (see The function external).

Each alternative starts with the reserved word `when`, either a list of literal strings separated by the `"|"` character or the reserved word `others`, and the `"=>"` token. When the case expression is a typed string variable, each literal string must belong to the string type that is the type of the case variable. After each `=>`, there are zero or more declarations. The only declarations allowed in a case construction are other case constructions, attribute declarations, and variable declarations. String type declarations and package declarations are not allowed. Variable declarations are restricted to variables that have already been declared before the case construction.

    case_construction ::=
      'case' <variable_>name 'is' {case_item} 'end' 'case' ;

    case_item ::=
      'when' discrete_choice_list =>
        {case_declaration
          | attribute_declaration
          | variable_declaration
          | empty_declaration}

    discrete_choice_list ::= string_literal {| string_literal} | 'others'

Here is a typical example, with a typed string variable:

>     project MyProj is
>        type OS_Type is ("GNU/Linux", "Unix", "Windows", "VMS");
>        OS : OS_Type := external ("OS", "GNU/Linux");

>        package Compiler is
>          case OS is
>            when "GNU/Linux" | "Unix" =>
>              for Switches ("Ada")
>                  use ("-gnath");
>            when "Windows" =>
>              for Switches ("Ada")
>                  use ("-gnatP");
>            when others =>
>              null;
>          end case;
>        end Compiler;
>     end MyProj;

### 2.10.10. Attributes

A project (and its packages) may have attributes that define the project’s properties. Some attributes have values that are strings; others have values that are string lists.

    attribute_declaration ::=
       simple_attribute_declaration | indexed_attribute_declaration

    simple_attribute_declaration ::= 'for' attribute_designator 'use' expression ;

    indexed_attribute_declaration ::=
      'for' *<indexed_attribute_>*simple_name ( string_literal) 'use' expression ;

    attribute_designator ::=
      <simple_attribute_>simple_name
      | <indexed_attribute_>simple_name ( string_literal )

There are two categories of attributes: simple attributes and indexed attributes. Each simple attribute has a default value: the empty string (for string attributes) and the empty list (for string list attributes). An attribute declaration defines a new value for an attribute, and overrides the previous value. The syntax of a simple attribute declaration is similar to that of an attribute definition clause in Ada.

Some attributes are indexed. These attributes are mappings whose domain is a set of strings. They are declared one association at a time, by specifying a point in the domain and the corresponding image of the attribute. Like untyped variables and simple attributes, indexed attributes may be declared several times. Each declaration supplies a new value for the attribute, and replaces the previous setting.

Here are some examples of attribute declarations:

>     --  simple attributes
>     for Object_Dir use "objects";
>     for Source_Dirs use ("units", "test/drivers");

>     --  indexed attributes
>     for Body ("main") use "Main.ada";
>     for Switches ("main.ada")
>         use ("-v", "-gnatv");
>     for Switches ("main.ada") use Builder'Switches ("main.ada") & "-g";

>     --  indexed attributes copy (from package Builder in project Default)
>     --  The package name must always be specified, even if it is the current
>     --  package.
>     for Default_Switches use Default.Builder'Default_Switches;

When an attribute is defined in the configuration project but not in the user project, it is inherited in the user project.

When a single string attribute is defined in both the configuration project and the user project, its value in the user project is as declared; the value in the configuration project does not matter.

For string list attributes, there are two cases. Some of these attributes are configuration concatenable. For these attributes, when they are declared in both the configuration project and the user project, the final value is the concatenation of the value in the configuration project with the value in the user project. The configuration concatenable attributes are indicated in the list below.

Attributes references may appear anywhere in expressions, and are used to retrieve the value previously assigned to the attribute. If an attribute has not been set in a given package or project, its value defaults to the empty string or the empty list, with some exceptions.

    attribute_reference ::=
      attribute_prefix ' <simple_attribute>_simple_name [ (string_literal) ]

    attribute_prefix ::= 'project'
      | <project_>simple_name
      | package_identifier
      | <project_>simple_name . package_identifier

Here are some examples:

>     project'Object_Dir
>     Naming'Dot_Replacement
>     Imported_Project'Source_Dirs
>     Imported_Project.Naming'Casing
>     Builder'Default_Switches ("Ada")

The exceptions to the empty defaults are:

- `Object_Dir`: default is `"."`

- `Exec_Dir`: default is `'Object_Dir`, that is, the value of attribute `Object_Dir` in the same project, declared or defaulted

- `Source_Dirs`: default is `(".")`

The prefix of an attribute may be:

- `project` for an attribute of the current project

- The name of an existing package of the current project

- The name of an imported project

- The name of a parent project that is extended by the current project

- An expanded name whose prefix is imported/base/parent project name, and whose selector is a package name

In the following sections, all predefined attributes are succinctly described, first the project level attributes (that is, those attributes that are not in a package), then the attributes in the different packages.

It is possible for different tools to dynamically create new packages with attributes, or new attributes in predefined packages. These attributes are not documented here.

The attributes under Configuration headings are usually found only in configuration project files.

The characteristics of each attribute are indicated as follows:

- Type of value

  The value of an attribute may be a single string, indicated by the word “single”, or a string list, indicated by the word “list”.

- Read-only

  When the attribute is read-only – that is when a declaration for the attribute is forbidden – this is indicated by the “read-only”.

- Optional index

  If an optional index is allowed in the value of the attribute (both single and list), this is indicated by the words “optional index”.

- Indexed attribute

  An indexed attribute is indicated by the word “indexed”.

- Case-sensitivity of the index

  For an indexed attribute, if the index is case-insensitive, this is indicated by the words “case-insensitive index”.

- File name index

  For an indexed attribute, when the index is a file name, this is indicated by the words “file name index”. The index may or may not be case-sensitive, depending on the platform.

- others allowed in index

  For an indexed attribute, if it is allowed to use others as the index, this is indicated by the words “others allowed”.

  When others is used as the index of an indexed attribute, the value of the attribute indexed by others is used when no other index would apply.

- configuration concatenable

  For a string list attribute, the final value if the attribute is declared in both the configuration project and the user project is the concatenation of the two value, configuration then user.

#### 2.10.10.1. Project Level Attributes

- General

  - Name: single, read-only

    The name of the project.

  - Project_Dir: single, read-only

    The path name of the project directory.

  - Main: list

    The list of main sources for the executables.

  - Languages: list

    The list of languages of the sources of the project.

  - Roots: list, indexed, file name index

    The index is the file name of an executable source. Indicates the list of units from the main project that need to be bound and linked with their closures with the executable. The index is either a file name, a language name or “*”. The roots for an executable source are those in Roots with an index that is the executable source file name, if declared. Otherwise, they are those in Roots with an index that is the language name of the executable source, if present. Otherwise, they are those in Roots (“*”), if declared. If none of these three possibilities are declared, then there are no roots for the executable source.

  - Externally_Built: single

    Indicates if the project is externally built. Only case-insensitive values allowed are “true” and “false”, the default.

  - Warning_Message: single

    Causes gprbuild to emit a user-defined warning message.

- Directories

  - Object_Dir: single

    Indicates the object directory for the project.

  - Exec_Dir: single

    Indicates the exec directory for the project, that is the directory where the executables are.

  - Create_Missing_Dirs: single

    Indicates if the missing object, library and executable directories should be created automatically by the project-aware tool. Taken into account only in the main project. Only authorized case-insensitive values are “true” and “false”.

  - Source_Dirs: list

    The list of source directories of the project.

  - Inherit_Source_Path: list, indexed, case-insensitive index

    Index is a language name. Value is a list of language names. Indicates that in the source search path of the index language the source directories of the languages in the list should be included.

    Example:

        for Inherit_Source_Path ("C++") use ("C");

  - Exclude_Source_Dirs: list

    The list of directories that are included in Source_Dirs but are not source directories of the project.

  - Ignore_Source_Sub_Dirs: list

    Value is a list of simple names or patterns for subdirectories that are removed from the list of source directories, including their subdirectories.

- Source Files

  - Source_Files: list

    Value is a list of source file simple names.

  - Locally_Removed_Files: list

    Obsolescent. Equivalent to Excluded_Source_Files.

  - Excluded_Source_Files: list

    Value is a list of simple file names that are not sources of the project. Allows to remove sources that are inherited or found in the source directories and that match the naming scheme.

  - Source_List_File: single

    Value is a text file name that contains a list of source file simple names, one on each line.

  - Excluded_Source_List_File: single

    Value is a text file name that contains a list of file simple names that are not sources of the project.

  - Interfaces: list

    Value is a list of file names that constitutes the interfaces of the project.

- Aggregate Projects

  - Project_Files: list

    Value is the list of aggregated projects.

  - Project_Path: list

    Value is a list of directories that are added to the project search path when looking for the aggregated projects.

  - External: single, indexed

    Index is the name of an external reference. Value is the value of the external reference to be used when parsing the aggregated projects.

- Libraries

  - Library_Dir: single

    Value is the name of the library directory. This attribute needs to be declared for each library project.

  - Library_Name: single

    Value is the name of the library. This attribute needs to be declared or inherited for each library project.

  - Library_Kind: single

    Specifies the kind of library: static library (archive) or shared library. Case-insensitive values must be one of “static” for archives (the default), “static-pic” for archives of Position Independent Code, or “dynamic” or “relocatable” for shared libraries.

  - Library_Version: single

    Value is the name of the library file.

  - Library_Interface: list

    Value is the list of unit names that constitutes the interfaces of a Stand-Alone Library project.

  - Library_Standalone: single

    Specifies if a Stand-Alone Library (SAL) is encapsulated or not. Only authorized case-insensitive values are “standard” for non encapsulated SALs, “encapsulated” for encapsulated SALs or “no” for non SAL library project.

  - Library_Encapsulated_Options: list, configuration concatenable

    Value is a list of options that need to be used when linking an encapsulated Stand-Alone Library.

  - Library_Encapsulated_Supported: single

    Indicates if encapsulated Stand-Alone Libraries are supported. Only authorized case-insensitive values are “true” and “false” (the default).

  - Library_Auto_Init: single

    Indicates if a Stand-Alone Library is auto-initialized. Only authorized case-insensitive values are “true” and “false”.

  - Leading_Library_Options: list, configuration concatenable

    Value is a list of options that are to be used at the beginning of the command line when linking a shared library.

  - Library_Options: list, configuration concatenable

    Value is a list of options that are to be used when linking a shared library.

  - Library_Rpath_Options: list, indexed, case-insensitive index, configuration concatenable

    Index is a language name. Value is a list of options for an invocation of the compiler of the language. This invocation is done for a shared library project with sources of the language. The output of the invocation is the path name of a shared library file. The directory name is to be put in the run path option switch when linking the shared library for the project.

  - Library_Src_Dir: single

    Value is the name of the directory where copies of the sources of the interfaces of a Stand-Alone Library are to be copied.

  - Library_ALI_Dir: single

    Value is the name of the directory where the ALI files of the interfaces of a Stand-Alone Library are to be copied. When this attribute is not declared, the directory is the library directory.

  - Library_gcc: single

    Obsolescent attribute. Specify the linker driver used to link a shared library. Use instead attribute Linker’Driver.

  - Library_Symbol_File: single

    Value is the name of the library symbol file.

  - Library_Symbol_Policy: single

    Indicates the symbol policy kind. Only authorized case-insensitive values are “restricted”, “unrestricted”.

  - Library_Reference_Symbol_File: single

    Value is the name of the reference symbol file.

- Configuration - General

  - Default_Language: single

    Value is the case-insensitive name of the language of a project when attribute Languages is not specified.

  - Run_Path_Option: list

    Value is the list of switches to be used when specifying the run path option in an executable.

  - Run_Path_Origin: single

    Value is the string that may replace the path name of the executable directory in the run path options.

  - Separate_Run_Path_Options: single

    Indicates if there may be several run path options specified when linking an executable. Only authorized case-insensitive values are “true” or “false” (the default).

  - Toolchain_Version: single, indexed, case-insensitive index

    Index is a language name. Specify the version of a toolchain for a language.

  - Required_Toolchain_Version: single, indexed, case-insensitive index

    Index is a language name. Specify the value expected for the Toolchain_Version attribute for this language, typically provided by an auto-generated configuration project. If Required_Toolchain_Version and Toolchain_Version do not match, the project processing aborts with an error.

  - Toolchain_Description: single, indexed, case-insensitive index

    Obsolescent. No longer used.

  - Object_Generated: single, indexed, case-insensitive index

    Index is a language name. Indicates if invoking the compiler for a language produces an object file. Only authorized case-insensitive values are “false” and “true” (the default).

  - Objects_Linked: single, indexed, case-insensitive index

    Index is a language name. Indicates if the object files created by the compiler for a language need to be linked in the executable. Only authorized case-insensitive values are “false” and “true” (the default).

  - Target: single

    Value is the name of the target platform. Taken into account only in the main project.

    Note that when the target is specified on the command line (usually with a switch –target=), the value of attribute reference ‘Target is the one specified on the command line.

  - Runtime: single, indexed, case-insensitive index

    Index is a language name. Indicates the runtime directory that is to be used when using the compiler of the language. Taken into account only in the main project, or its extended projects if any.

    Note that when the runtime is specified for a language on the command line (usually with a switch –RTS), the value of attribute reference ‘Runtime for this language is the one specified on the command line.

  - Runtime_Dir: single, indexed, case-insensitive index

    Index is a language name. Value is the path name of the runtime directory for the language.

  - Runtime_Library_Dirs: list, indexed, case-insensitive index

    Index is a language name. Value is the path names of the directories where the runtime libraries are located. This attribute is not normally declared.

  - Runtime_Library_Dir: single, indexed, case-insensitive index

    Index is a language name. Value is the path name of the directory where the runtime libraries are located. This attribute is obsolete.

  - Runtime_Source_Dirs: list, indexed, case-insensitive index

    Index is a language name. Value is the path names of the directories where the sources of runtime libraries are located. This attribute is not normally declared.

  - Runtime_Source_Dir: single, indexed, case-insensitive index

    Index is a language name. Value is the path name of the directory where the sources of runtime libraries are located. This attribute is obsolete.

  - Runtime_Library_Version: single, indexed, case-insensitive index

    Index is a language name. Value is library version for the language. This attribute is not normally declared.

  - Toolchain_Name: single, indexed, case-insensitive index

    Index is a language name. Indicates the toolchain name that is to be used when using the compiler of the language. Taken into account only in the main project, or its extended projects if any.

- Configuration - Libraries

  - Library_Builder: single

    Value is the path name of the application that is to be used to build libraries. Usually the path name of “gprlib”.

  - Library_Support: single

    Indicates the level of support of libraries. Only authorized case-insensitive values are “static_only”, “full” or “none” (the default).

- Configuration - Archives

  - Archive_Builder: list

    Value is the name of the application to be used to create a static library (archive), followed by the options to be used.

  - Archive_Builder_Append_Option: list

    Value is the list of options to be used when invoking the archive builder to add project files into an archive.

  - Archive_Indexer: list

    Value is the name of the archive indexer, followed by the required options.

  - Archive_Suffix: single

    Value is the extension of archives. When not declared, the extension is “.a”.

  - Library_Partial_Linker: list

    Value is the name of the partial linker executable, followed by the required options.

- Configuration - Shared Libraries

  - Shared_Library_Prefix: single

    Value is the prefix in the name of shared library files. When not declared, the prefix is “lib”.

  - Shared_Library_Suffix: single

    Value is the extension of the name of shared library files. When not declared, the extension is “.so”.

  - Symbolic_Link_Supported: single

    Indicates if symbolic links are supported on the platform. Only authorized case-insensitive values are “true” and “false” (the default).

  - Library_Major_Minor_Id_Supported: single

    Indicates if major and minor ids for shared library names are supported on the platform. Only authorized case-insensitive values are “true” and “false” (the default).

  - Library_Auto_Init_Supported: single

    Indicates if auto-initialization of Stand-Alone Libraries is supported. Only authorized case-insensitive values are “true” and “false” (the default).

  - Shared_Library_Minimum_Switches: list, configuration concatenable

    Value is the list of required switches when linking a shared library.

  - Library_Version_Switches: list, configuration concatenable

    Value is the list of switches to specify a internal name for a shared library.

  - Library_Install_Name_Option: single

    Value is the name of the option that needs to be used, concatenated with the path name of the library file, when linking a shared library.

#### 2.10.10.2. Package Binder Attributes

- General

  - Default_Switches: list, indexed, case-insensitive index, configuration concatenable

    Index is a language name. Value is the list of switches to be used when binding code of the language, if there is no applicable attribute Switches.

  - Switches: list, optional index, indexed, case-insensitive index, others allowed, configuration concatenable

    Index is either a language name or a source file name. Value is the list of switches to be used when binding code. Index is either the source file name of the executable to be bound or the language name of the code to be bound.

- Configuration - Binding

  - Driver: single, indexed, case-insensitive index

    Index is a language name. Value is the name of the application to be used when binding code of the language.

  - Required_Switches: list, indexed, case-insensitive index, configuration concatenable

    Index is a language name. Value is the list of the required switches to be used when binding code of the language.

  - Prefix: single, indexed, case-insensitive index

    Index is a language name. Value is a prefix to be used for the binder exchange file name for the language. Used to have different binder exchange file names when binding different languages.

  - Objects_Path: single,indexed, case-insensitive index

    Index is a language name. Value is the name of the environment variable that contains the path for the object directories.

  - Object_Path_File: single,indexed, case-insensitive index

    Index is a language name. Value is the name of the environment variable. The value of the environment variable is the path name of a text file that contains the list of object directories.

#### 2.10.10.3. Package Builder Attributes

- Default_Switches: list, indexed, case-insensitive index,

    configuration concatenable

  Index is a language name. Value is the list of builder switches to be used when building an executable of the language, if there is no applicable attribute Switches.

- Switches: list, optional index, indexed, case-insensitive index,

    others allowed, configuration concatenable

  Index is either a language name or a source file name. Value is the list of builder switches to be used when building an executable. Index is either the source file name of the executable to be built or its language name.

- Global_Compilation_Switches: list, optional index, indexed,

    case-insensitive index, configuration concatenable

  Index is a language name. Value is the list of compilation switches to be used when building an executable. Index is either the source file name of the executable to be built or its language name.

- Executable: single, indexed, case-insensitive index

  Index is an executable source file name. Value is the simple file name of the executable to be built.

- Executable_Suffix: single

  Value is the extension of the file names of executable. When not specified, the extension is the default extension of executables on the platform.

- Global_Configuration_Pragmas: single

  Value is the file name of a configuration pragmas file that is specified to the Ada compiler when compiling any Ada source in the project tree.

- Global_Config_File: single, indexed, case-insensitive index

  Index is a language name. Value is the file name of a configuration file that is specified to the compiler when compiling any source of the language in the project tree.

#### 2.10.10.4. Package Check Attributes

- Default_Switches: list, indexed, case-insensitive index, configuration concatenable

  Index is a language name. Value is a list of switches to be used when invoking gnatcheck for a source of the language, if there is no applicable attribute Switches.

- Switches: list, optional index, indexed, case-insensitive index, others allowed, configuration concatenable

  Index is a source file name. Value is the list of switches to be used when invoking gnatcheck for the source.

#### 2.10.10.5. Package Clean Attributes

- Switches: list, configuration concatenable

  Taken into account only in the main project. Value is a list of switches to be used by the cleaning application.

- Source_Artifact_Extensions: list, indexed, case-insensitive index

  Index is a language names. Value is the list of extensions for file names derived from object file names that need to be cleaned in the object directory of the project.

- Object_Artifact_Extensions: list, indexed, case-insensitive index

  Index is a language names. Value is the list of extensions for file names derived from source file names that need to be cleaned in the object directory of the project.

- Artifacts_In_Object_Dir: single

  Value is a list of file names expressed as regular expressions that are to be deleted by gprclean in the object directory of the project.

- Artifacts_In_Exec_Dir: single

  Value is list of file names expressed as regular expressions that are to be deleted by gprclean in the exec directory of the main project.

#### 2.10.10.6. Package Compiler Attributes

- General

  - Default_Switches: list, indexed, case-insensitive index, configuration concatenable

    Index is a language name. Value is a list of switches to be used when invoking the compiler for the language for a source of the project, if there is no applicable attribute Switches.

  - Switches: list, optional index, indexed, case-insensitive index, others allowed, configuration concatenable

    Index is a source file name or a language name. Value is the list of switches to be used when invoking the compiler for the source or for its language.

  - Local_Configuration_Pragmas: single

    Value is the file name of a configuration pragmas file that is specified to the Ada compiler when compiling any Ada source in the project.

  - Local_Config_File: single, indexed, case-insensitive index

    Index is a language name. Value is the file name of a configuration file that is specified to the compiler when compiling any source of the language in the project.

- Configuration - Compiling

  - Driver: single, indexed, case-insensitive index

    Index is a language name. Value is the name of the executable for the compiler of the language.

  - Language_Kind: single, indexed, case-insensitive index

    Index is a language name. Indicates the kind of the language, either file based or unit based. Only authorized case-insensitive values are “unit_based” and “file_based” (the default).

  - Dependency_Kind: single, indexed, case-insensitive index

    Index is a language name. Indicates how the dependencies are handled for the language. Only authorized case-insensitive values are “makefile”, “ali_file”, “ali_closure” or “none” (the default).

  - Required_Switches: list, indexed, case-insensitive index, configuration concatenable

    Equivalent to attribute Leading_Required_Switches.

  - Leading_Required_Switches: list, indexed, case-insensitive index, configuration concatenable

    Index is a language name. Value is the list of the minimum switches to be used at the beginning of the command line when invoking the compiler for the language.

  - Trailing_Required_Switches: list, indexed, case-insensitive index, configuration concatenable

    Index is a language name. Value is the list of the minimum switches to be used at the end of the command line when invoking the compiler for the language.

  - PIC_Option: list, indexed, case-insensitive index

    Index is a language name. Value is the list of switches to be used when compiling a source of the language when the project is a shared library project.

  - Source_File_Switches: single, indexed, case-insensitive index configuration concatenable

    Index is a language name. Value is a list of switches to be used just before the path name of the source to compile when invoking the compiler for a source of the language.

  - Object_File_Suffix: single, indexed, case-insensitive index

    Index is a language name. Value is the extension of the object files created by the compiler of the language. When not specified, the extension is the default one for the platform.

  - Object_File_Switches: list, indexed, case-insensitive index, configuration concatenable

    Index is a language name. Value is the list of switches to be used by the compiler of the language to specify the path name of the object file. When not specified, the switch used is “-o”.

  - Multi_Unit_Switches: list, indexed, case-insensitive index, configuration concatenable

    Index is a language name. Value is the list of switches to be used to compile a unit in a multi unit source of the language. The index of the unit in the source is concatenated with the last switches in the list.

  - Multi_Unit_Object_Separator: single, indexed, case-insensitive index

    Index is a language name. Value is the string to be used in the object file name before the index of the unit, when compiling a unit in a multi unit source of the language.

- Configuration - Mapping Files

  - Mapping_File_Switches: list, indexed, case-insensitive index, configuration concatenable

    Index is a language name. Value is the list of switches to be used to specify a mapping file when invoking the compiler for a source of the language.

  - Mapping_Spec_Suffix: single, indexed, case-insensitive index

    Index is a language name. Value is the suffix to be used in a mapping file to indicate that the source is a spec.

  - Mapping_Body_Suffix: single, indexed, case-insensitive index

    Index is a language name. Value is the suffix to be used in a mapping file to indicate that the source is a body.

- Configuration - Config Files

  - Config_File_Switches: list: single, indexed, case-insensitive index, configuration concatenable

    Index is a language name. Value is the list of switches to specify to the compiler of the language a configuration file.

  - Config_Body_File_Name: single, indexed, case-insensitive index

    Index is a language name. Value is the template to be used to indicate a configuration specific to a body of the language in a configuration file.

  - Config_Body_File_Name_Index: single, indexed, case-insensitive index

    Index is a language name. Value is the template to be used to indicate a configuration specific to the body a unit in a multi unit source of the language in a configuration file.

  - Config_Body_File_Name_Pattern: single, indexed, case-insensitive index

    Index is a language name. Value is the template to be used to indicate a configuration for all bodies of the languages in a configuration file.

  - Config_Spec_File_Name: single, indexed, case-insensitive index

    Index is a language name. Value is the template to be used to indicate a configuration specific to a spec of the language in a configuration file.

  - Config_Spec_File_Name_Index: single, indexed, case-insensitive index

    Index is a language name. Value is the template to be used to indicate a configuration specific to the spec a unit in a multi unit source of the language in a configuration file.

  - Config_Spec_File_Name_Pattern: single, indexed, case-insensitive index

    Index is a language name. Value is the template to be used to indicate a configuration for all specs of the languages in a configuration file.

  - Config_File_Unique: single, indexed, case-insensitive index

    Index is a language name. Indicates if there should be only one configuration file specified to the compiler of the language. Only authorized case-insensitive values are “true” and “false” (the default).

- Configuration - Dependencies

  - Dependency_Switches: list, indexed, case-insensitive index, configuration concatenable

    Index is a language name. Value is the list of switches to be used to specify to the compiler the dependency file when the dependency kind of the language is file based, and when Dependency_Driver is not specified for the language.

  - Dependency_Driver: list, indexed, case-insensitive index

    Index is a language name. Value is the name of the executable to be used to create the dependency file for a source of the language, followed by the required switches.

- Configuration - Search Paths

  - Include_Switches: list, indexed, case-insensitive index, configuration concatenable

    Index is a language name. Value is the list of switches to specify to the compiler of the language to indicate a directory to look for sources.

  - Include_Path: single, indexed, case-insensitive index

    Index is a language name. Value is the name of an environment variable that contains the path of all the directories that the compiler of the language may search for sources.

  - Include_Path_File: single, indexed, case-insensitive index

    Index is a language name. Value is the name of an environment variable the value of which is the path name of a text file that contains the directories that the compiler of the language may search for sources.

  - Object_Path_Switches: list, indexed, case-insensitive index, configuration concatenable

    Index is a language name. Value is the list of switches to specify to the compiler of the language the name of a text file that contains the list of object directories. When this attribute is not declared, the text file is not created.

- Configuration - Response Files

  - Max_Command_Line_Length: single

    Value is the maximum number of character in the command line when invoking a compiler that supports response files.

  - Response_File_Format: single, indexed, case-insensitive index

    Indicates the kind of response file to create when the length of the compiling command line is too large. The index is the name of the language for the compiler. Only authorized case-insensitive values are “none”, “gnu”, “object_list”, “gcc_gnu”, “gcc_option_list” and “gcc_object_list”.

  - Response_File_Switches: list, indexed, case-insensitive index, configuration concatenable

    Value is the list of switches to specify a response file for a compiler. The index is the name of the language for the compiler.

#### 2.10.10.7. Package Cross_Reference Attributes

- Default_Switches: list, indexed, case-insensitive index, configuration concatenable

  Index is a language name. Value is a list of switches to be used when invoking gnatxref for a source of the language, if there is no applicable attribute Switches.

- Switches: list, optional index, indexed, case-insensitive index, others allowed, configuration concatenable

  Index is a source file name. Value is the list of switches to be used when invoking gnatxref for the source.

#### 2.10.10.8. Package Documentation Attributes

Please refer to GNATdoc documentation for the list of supported attributes and their meaning.

#### 2.10.10.9. Package Finder Attributes

- Default_Switches: list, indexed, case-insensitive index, configuration concatenable

  Index is a language name. Value is a list of switches to be used when invoking gnatfind for a source of the language, if there is no applicable attribute Switches.

- Switches: list, optional index, indexed, case-insensitive index, others allowed, configuration concatenable

  Index is a source file name. Value is the list of switches to be used when invoking gnatfind for the source.

#### 2.10.10.10. Package Gnatls Attributes

- Switches: list

  Taken into account only in the main project. Value is a list of switches to be used when invoking gnatls.

#### 2.10.10.11. Package gnatstub Attributes

- Default_Switches: list, indexed, case-insensitive index, configuration concatenable

  Index is a language name. Value is a list of switches to be used when invoking gnatstub for a source of the language, if there is no applicable attribute Switches.

- Switches: list, optional index, indexed, case-insensitive index, others allowed, configuration concatenable

  Index is a source file name. Value is the list of switches to be used when invoking gnatstub for the source.

#### 2.10.10.12. Package IDE Attributes

Please refer to your IDE documentation for the list of supported attributes and their meaning.

#### 2.10.10.13. Package Install Attributes

- Artifacts: list, indexed

  An indexed attribute to declare a set of files not part of the sources to be installed. The array index is the directory where the file is to be installed. If a relative directory then Prefix (see below) is prepended. Note also that if the same file name occurs multiple time in the attribute list, the last one will be the one installed. If an artifact is not found a warning is displayed.

- Required_Artifacts: list, indexed

  As above, but artifacts must be present or an error is reported.

- Prefix: single

  Value is the install destination directory. If the value is a relative path, it is taken as relative to the global prefix directory. That is, either the value passed to –prefix option or the default installation prefix.

- Sources_Subdir: single

  Value is the sources directory or subdirectory of Prefix.

- Exec_Subdir: single

  Value is the executables directory or subdirectory of Prefix.

- ALI_Subdir: single

  Value is ALI directory or subdirectory of Prefix.

- Lib_Subdir: single

  Value is library directory or subdirectory of Prefix.

- Project_Subdir: single

  Value is the project directory or subdirectory of Prefix.

- Active: single

  Indicates that the project is to be installed or not. Case-insensitive value “false” means that the project is not to be installed, all other values mean that the project is to be installed.

- Mode: single

  Value is the installation mode, it is either dev (default) or usage.

- Install_Name: single

  Specify the name to use for recording the installation. The default is the project name without the extension.

- Side_Debug: single

  Indicates that the project’s executable and shared libraries are to be stripped of the debug symbols. Those debug symbols are written into a side file named after the original file with the “.debug” extension added. Case-insensitive value “false” (default) disables this feature. Set it to “true” to activate.

- Install_Project: single

  Indicates that a project is to be generated and installed. The value is either “true” to “false”. Default is “true”.

#### 2.10.10.14. Package Linker Attributes

- General

  - Required_Switches: list, configuration concatenable

    Value is a list of switches that are required when invoking the linker to link an executable.

  - Default_Switches: list, indexed, case-insensitive index, configuration concatenable

    Index is a language name. Value is a list of switches for the linker when linking an executable for a main source of the language, when there is no applicable Switches.

  - Leading_Switches: list, optional index, indexed, case-insensitive index, others allowed, configuration concatenable

    Index is a source file name or a language name. Value is the list of switches to be used at the beginning of the command line when invoking the linker to build an executable for the source or for its language.

  - Switches: list, optional index, indexed, case-insensitive index, others allowed, configuration concatenable

    Index is a source file name or a language name. Value is the list of switches to be used when invoking the linker to build an executable for the source or for its language.

  - Trailing_Switches: list, optional index, indexed, case-insensitive index, others allowed, configuration concatenable

    Index is a source file name or a language name. Value is the list of switches to be used at the end of the command line when invoking the linker to build an executable for the source or for its language. These switches may override the Required_Switches.

  - Linker_Options: list, configuration concatenable

    This attribute specifies a list of additional switches to be given to the linker when linking an executable. It is ignored when defined in the main project and taken into account in all other projects that are imported directly or indirectly. These switches complement the `Linker'Switches` defined in the main project. This is useful when a particular subsystem depends on an external library: adding this dependency as a `Linker_Options` in the project of the subsystem is more convenient than adding it to all the `Linker'Switches` of the main projects that depend upon this subsystem.

  - Map_File_Option: single

    Value is the switch to specify the map file name that the linker needs to create.

  - Unconditionally_Linked: single, indexed, case-insensitive index

    Index is a language name. Indicates that all object files of this language going to be linked unconditionally. Only case-insensitive values allowed are “true” and “false”, the default.

- Configuration - Linking

  - Driver: single

    Value is the name of the linker executable.

- Configuration - Response Files

  - Max_Command_Line_Length: single

    Value is the maximum number of character in the command line when invoking the linker to link an executable.

  - Response_File_Format: single

    Indicates the kind of response file to create when the length of the linking command line is too large. Only authorized case-insensitive values are “none”, “gnu”, “object_list”, “gcc_gnu”, “gcc_option_list” and “gcc_object_list”.

  - Response_File_Switches: list, configuration concatenable

    Value is the list of switches to specify a response file to the linker.

#### 2.10.10.15. Package Metrics Attribute

- Default_Switches: list, indexed, case-insensitive index, configuration concatenable

  Index is a language name. Value is a list of switches to be used when invoking gnatmetric for a source of the language, if there is no applicable attribute Switches.

- Switches: list, optional index, indexed, case-insensitive index, others allowed, configuration concatenable

  Index is a source file name. Value is the list of switches to be used when invoking gnatmetric for the source.

#### 2.10.10.16. Package Naming Attributes

- Specification_Suffix: single, indexed, case-insensitive index

  Equivalent to attribute Spec_Suffix.

  Warning

  Also has case-insensitive values in gprbuild and GNATcool.Project-based tools

- Spec_Suffix: single, indexed, case-insensitive index

  Index is a language name. Value is the extension of file names for specs of the language.

  Warning

  Also has case-insensitive values in gprbuild and GNATcool.Project-based tools

- Implementation_Suffix: single, indexed, case-insensitive index

  Equivalent to attribute Body_Suffix.

  Warning

  Also has case-insensitive values in gprbuild and GNATcool.Project-based tools

- Body_Suffix: single, indexed, case-insensitive index

  Index is a language name. Value is the extension of file names for bodies of the language.

  Warning

  Also has case-insensitive values in gprbuild and GNATcool.Project-based tools

- Separate_Suffix: single

  Value is the extension of file names for subunits of Ada.

- Casing: single

  Indicates the casing of sources of the Ada language. Only authorized case-insensitive values are “lowercase”, “uppercase” and “mixedcase”.

- Dot_Replacement: single

  Value is the string that replace the dot of unit names in the source file names of the Ada language.

- Specification: single, optional index, indexed, case-insensitive index

  Equivalent to attribute Spec.

- Spec: single, optional index, indexed, case-insensitive index

  Index is a unit name. Value is the file name of the spec of the unit.

- Implementation: single, optional index, indexed, case-insensitive index

  Equivalent to attribute Body.

- Body: single, optional index, indexed, case-insensitive index

  Index is a unit name. Value is the file name of the body of the unit.

- Specification_Exceptions: list, indexed, case-insensitive index

  Index is a language name. Value is a list of specs for the language that do not necessarily follow the naming scheme for the language and that may or may not be found in the source directories of the project.

- Implementation_Exceptions: list, indexed, case-insensitive index

  Index is a language name. Value is a list of bodies for the language that do not necessarily follow the naming scheme for the language and that may or may not be found in the source directories of the project.

#### 2.10.10.17. Package Pretty_Printer Attributes

- Default_Switches: list, indexed, case-insensitive index, configuration concatenable

  Index is a language name. Value is a list of switches to be used when invoking gnatpp for a source of the language, if there is no applicable attribute Switches.

- Switches: list, optional index, indexed, case-insensitive index, others allowed, configuration concatenable

  Index is a source file name. Value is the list of switches to be used when invoking gnatpp for the source.

#### 2.10.10.18. Package Remote Attributes

- Included_Patterns: list

  If this attribute is defined it sets the patterns to synchronized from the master to the slaves. It is exclusive with Excluded_Patterns, that is it is an error to define both.

- Included_Artifact_Patterns: list

  If this attribute is defined it sets the patterns of compilation artifacts to synchronized from the slaves to the build master. This attribute replace the default hard-coded patterns.

- Excluded_Patterns: list

  Set of patterns to ignore when synchronizing sources from the build master to the slaves. A set of predefined patterns are supported (e.g. *.o, *.ali, *.exe, etc.), this attributes make it possible to add some more patterns.

- Root_Dir: single

  Value is the root directory used by the slave machines.

#### 2.10.10.19. Package Stack Attributes

- Switches: list, configuration concatenable

  Taken into account only in the main project. Value is the list of switches to be used when invoking gnatstack.

#### 2.10.10.20. Package Synchronize Attributes

- Default_Switches: list, indexed, case-insensitive index

  Index is a language name. Value is a list of switches to be used when invoking gnatsync for a source of the language, if there is no applicable attribute Switches.

- Switches: list, optional index, indexed, case-insensitive index, others allowed

  Index is a source file name. Value is the list of switches to be used when invoking gnatsync for the source.

### 2.10.11. Environment Variables

Project processing can be affected by environment variables.

- GPR_CONFIG

  When declared with a non empty name, use its value as the default configuration project file name in native platforms, instead of “default.cgpr”.

- GPR_RUNTIME_PATH

  Path where to look for a non empty runtime directory.

- PATH

  The path, that may be modified to add the directories related to the compilers.

- GPR_PROJECT_PATH_FILE

  The path to a file containing project directory path names

- GPR_PROJECT_PATH

  The path where to look for projects

- ADA_PROJECT_PATH

  The path where to look for projects

- TMPDIR

  Directories where to create temporary files

- TEMP

  Directories where to create temporary files

- TMP

  Directories where to create temporary files

- GPR_VERBOSITY

  Value for the quiet mode or the verbosity level. Overriden with switches -q, -v and -vP?

- USER

  Used to communicate with a slave in distributed gprbuild.

- USERNAME

  Used to communicate with a slave in distributed gprbuild.

- GPRBIND_DEBUG

  When value is “TRUE”, keep a copy of the binder exchange file sent to gprbind as main.bexch__saved

- GPRLIB_DEBUG

  When value is “TRUE”, keep a copy of the library exchange file sent to gprlib as main.lexch__saved

## 2.11. Glossary

Abstract project

  A project with no source files, typically used to define common attributes that are shared by other project files. See Sharing between Projects.

Aggregate project

  A project that in effect combines several projects in order to efficiently support concurrent builds or builds of all main programs from the constituent projects, or the convenient definition of a common environment for the constituent projects. See Aggregate Projects.

Attribute

  A named property of a project or one of its packages. See Attributes.

Base project

  A project that is extended by some other project. See Project Extension.

Child project

  A project that is defined by a name `Parent_proj.Child_proj` where `Child_proj` either imports or extends `Parent_Proj`. This feature is typically used to show a close relationship between the two projects, for example where the child project serves as a testbed for the parent. See Child Projects.

Configuration project

  A project that describes compilers and other tools, for use by GPRbuild. See Configuration Project.

Extending a project

  The reuse and possible adaption by one project of the source files from another project (the base project). Somewhat analogous to (single) class inheritance in object-oriented programming. See Project Extension.

External variable

  A variable that is defined on the command line (by the `-X` switch), as the value of an environment variable, or, by default, as the second parameter to the `external` function. See Scenarios in Projects.

Global attribute

  An attribute that applies to all projects in the project import closure of a main project. See Global Attributes.

Importing a project

  The usage of a `with` or `limited with` clause on a project file in order to reuse properties of some other project file. See Importing Projects.

Independent project

  A project defined by a single project file and thus not dependent on any other projects. See Independent Project.

Library project

  A project that is used to define a library rather than an executable program. See Library Projects.

Main project

  A project that is specified on the command line. See Global Attributes.

Package

  A grouping of attribute definitions related to a particular GNAT tool. See Packages.

Parent project

  A project that has one or more child projects. See Child Projects.

Project

  A set of named properties and their values, associated with the GNAT tools that are used during the development of software in Ada and other languages. Properties include directories for source files, object files, and executables; the switch settings for the various tools; and the naming scheme for source files.

Project extension

  See glossary item Extending a project

Project file

  A textual representation of a project, which uses an Ada-like notation. The syntax is presented in Project File Reference.

Project import closure

  The project import closure for a given project proj is the set of projects consisting of proj itself, together with each project that is directly or indirectly imported by proj. The import may be from either a `with` or a `limited with`. See Project Import Closure.

Scenario

  The values of a project’s variables and attributes, as determined by the settings of external variables referenced by a project. A scenario typically defines a particular mode of usage for the project. See Scenarios in Projects.

Scenario variable

  An external variable, typically assigned to a typed variable and queried in a case construction. See Scenario variable.

Standard project

  A non-library project with source files. See Standard project

Typed variable

  A project variable that can take any of a specified set of values, analogous to a variable of an Ada enumeration type but where the values are string literals. See Scenarios in Projects.