execveat(2)

NAME
execveat - execute program relative to a directory file descriptor

LIBRARY
Standard C library (libc, -lc)

SYNOPSIS
#include <linux/fcntl.h>      /* Definition of AT_* constants */
#include <unistd.h>

int execveat(int dirfd, const char *path,
             char *const _Nullable argv[],
             char *const _Nullable envp[],
             int flags);

DESCRIPTION
The execveat() system call executes the program referred to by the combination of dirfd and path. It operates in exactly the same way as execve(2), except for the differences described in this manual page.

If path is relative, then it is interpreted relative to the directory referred to by the file descriptor dirfd (rather than relative to the current working directory of the calling process, as is done by execve(2) for a relative pathname).

If path is relative and dirfd is the special value AT_FDCWD, then path is interpreted relative to the current working directory of the calling process (like execve(2)).

If path is absolute, then dirfd is ignored.

If path is an empty string and the AT_EMPTY_PATH flag is specified, then the file descriptor dirfd specifies the file to be executed (i.e., dirfd refers to an executable file, rather than a directory).

The flags argument is a bit mask that can include zero or more of the following flags:

AT_EMPTY_PATH
    If path is an empty string, operate on the file referred to by dirfd (which may have been obtained using the open(2) O_PATH flag).

AT_SYMLINK_NOFOLLOW
    If the file identified by dirfd and a non-NULL path is a symbolic link, then the call fails with the error ELOOP.

RETURN VALUE
On success, execveat() does not return. On error, -1 is returned, and errno is set to indicate the error.

ERRORS
The same errors that occur for execve(2) can also occur for execveat(). The following additional errors can occur for execveat():

path
    is relative but dirfd is neither AT_FDCWD nor a valid file descriptor.

EINVAL
    Invalid flag specified in flags.

ELOOP
    flags includes AT_SYMLINK_NOFOLLOW and the file identified by dirfd and a non-NULL path is a symbolic link.

ENOENT
    The program identified by dirfd and path requires the use of an interpreter program (such as a script starting with "#!"), but the file descriptor dirfd was opened with the O_CLOEXEC flag, with the result that the program file is inaccessible to the launched interpreter. See BUGS.

ENOTDIR
    path is relative and dirfd is a file descriptor referring to a file other than a directory.

STANDARDS
Linux.

HISTORY
Linux 3.19, glibc 2.34.

NOTES
In addition to the reasons explained in openat(2), the execveat() system call is also needed to allow fexecve(3) to be implemented on systems that do not have the /proc filesystem mounted.

When asked to execute a script file, the argv[0] that is passed to the script interpreter is a string of the form /dev/fd/N or /dev/fd/N/P, where N is the number of the file descriptor passed via the dirfd argument. A string of the first form occurs when AT_EMPTY_PATH is employed. A string of the second form occurs when the script is specified via both dirfd and path; in this case, P is the value given in path.

For the same reasons described in fexecve(3), the natural idiom when using execveat() is to set the close-on-exec flag on dirfd. (But see BUGS.)

BUGS
The ENOENT error described above means that it is not possible to set the close-on-exec flag on the file descriptor given to a call of the form:

    execveat(fd, "", argv, envp, AT_EMPTY_PATH);

However, the inability to set the close-on-exec flag means that a file descriptor referring to the script leaks through to the script itself. As well as wasting a file descriptor, this leakage can lead to file-descriptor exhaustion in scenarios where scripts recursively employ execveat().

SEE ALSO
execve(2), openat(2), fexecve(3)