RECV(3P)

PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

NAME
recv — receive a message from a connected socket

SYNOPSIS

#include <sys/socket.h>

ssize_t recv(int socket, void *buffer, size_t length, int flags);

DESCRIPTION
The recv() function shall receive a message from a connection-mode or connectionless-mode socket. It is normally used with connected sockets because it does not permit the application to retrieve the source address of received data.

The recv() function takes the following arguments:

socket
    Specifies the socket file descriptor.

buffer
    Points to a buffer where the message should be stored.

length
    Specifies the length in bytes of the buffer pointed to by the buffer argument.

flags
    Specifies the type of message reception. Values of this argument are formed by logically OR'ing zero or more of the following values:

    MSG_PEEK
        Peeks at an incoming message. The data is treated as unread and the next recv() or similar function shall still return this data.

    MSG_OOB
        Requests out-of-band data. The significance and semantics of out-of-band data are protocol-specific.

    MSG_WAITALL
        On SOCK_STREAM sockets this requests that the function block until the full amount of data can be returned. The function may return the smaller amount of data if the socket is a message-based socket, if a signal is caught, if the connection is terminated, if MSG_PEEK was specified, or if an error is pending for the socket.

The recv() function shall return the length of the message written to the buffer pointed to by the buffer argument. For message-based sockets, such as SOCK_DGRAM and SOCK_SEQPACKET, the entire message shall be read in a single operation. If a message is too long to fit in the supplied buffer, and MSG_PEEK is not set in the flags argument, the excess bytes shall be discarded. For stream-based sockets, such as SOCK_STREAM, message boundaries shall be ignored. In this case, data shall be returned to the user as soon as it becomes available, and no data shall be discarded.

If the MSG_WAITALL flag is not set, data shall be returned only up to the end of the first message.

If no messages are available at the socket and O_NONBLOCK is not set on the socket's file descriptor, recv() shall block until a message arrives. If no messages are available at the socket and O_NONBLOCK is set on the socket's file descriptor, recv() shall fail and set errno to [EAGAIN] or [EWOULDBLOCK].

RETURN VALUE
Upon successful completion, recv() shall return the length of the message in bytes. If no messages are available to be received and the peer has performed an orderly shutdown, recv() shall return 0. Otherwise, -1 shall be returned and errno set to indicate the error.

ERRORS
The recv() function shall fail if:

EAGAIN or EWOULDBLOCK
    The socket's file descriptor is marked O_NONBLOCK and no data is waiting to be received; or MSG_OOB is set and no out-of-band data is available and either the socket's file descriptor is marked O_NONBLOCK or the socket does not support blocking to await out-of-band data.

EBADF
    The socket argument is not a valid file descriptor.

ECONNRESET
    A connection was forcibly closed by a peer.

EINTR
    The recv() function was interrupted by a signal that was caught, before any data was available.

EINVAL
    The MSG_OOB flag is set and no out-of-band data is available.

ENOTCONN
    A receive is attempted on a connection-mode socket that is not connected.

ENOTSOCK
    The socket argument does not refer to a socket.

EOPNOTSUPP
    The specified flags are not supported for this socket type or protocol.

ETIMEDOUT
    The connection timed out during connection establishment, or due to a transmission timeout on active connection.

The recv() function may fail if:

EIO An I/O error occurred while reading from or writing to the file system.

ENOBUFS
    Insufficient resources were available in the system to perform the operation.

ENOMEM
    Insufficient memory was available to fulfill the request.

The following sections are informative.

EXAMPLES
None.

APPLICATION USAGE
The recv() function is equivalent to recvfrom() with null pointer address and address_len arguments, and to read() if the socket argument refers to a socket and the flags argument is 0.

The select() and poll() functions can be used to determine when data is available to be received.

RATIONALE
None.

FUTURE DIRECTIONS
None.

SEE ALSO
poll(), pselect(), read(), recvmsg(), recvfrom(), send(), sendmsg(), sendto(), shutdown(), socket(), write()

The Base Definitions volume of POSIX.1-2017, <sys_socket.h>

COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .