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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.1‐2008, <sys_socket.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2013 Edition, Standard for Information Technology -- Portable Operating System
Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/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 .