RECV(P) POSIX Programmer's Manual RECV(P)
recv - receive a message from a connected socket
ssize_t recv(int socket, void *buffer, size_t length, int flags);
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 applica-
tion 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 log-
ically OR'ing zero or more of the following values:
Peeks at an incoming message. The data is treated as unread and the next recv() or
similar function shall still return this data.
Requests out-of-band data. The significance and semantics of out-of-band data are
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 ter-
minated, 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_SEQ-
PACKET, 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 bound-
aries 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
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].
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
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 block-
ing to await out-of-band data.
EBADF The socket argument is not a valid file descriptor.
A connection was forcibly closed by a peer.
EINTR The recv() function was interrupted by a signal that was caught, before any data
EINVAL The MSG_OOB flag is set and no out-of-band data is available.
A receive is attempted on a connection-mode socket that is not connected.
The socket argument does not refer to a socket.
The specified flags are not supported for this socket type or protocol.
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.
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.
The recv() function is equivalent to recvfrom() with a zero address_len argument, and to
read() if no flags are used.
The select() and poll() functions can be used to determine when data is available to be
poll() , read() , recvmsg() , recvfrom() , select() , send() , sendmsg() , sendto() ,
shutdown() , socket() , write() , the Base Definitions volume of IEEE Std 1003.1-2001,
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System
Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 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 orig-
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 RECV(P)