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:
Specifies the socket file descriptor.
Points to a buffer where the message should be stored.
Specifies the length in bytes of the buffer pointed to by the buffer
Specifies the type of message reception. Values of this argument are
formed by logically OR'ing zero or more of the following
Peeks at an incoming message. The data is treated as unread and the
next recv() or similar function shall still return
Requests out-of-band data. The significance and semantics of out-of-band
data are protocol-specific.
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].
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.
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
The socket argument is not a valid file descriptor.
A connection was forcibly closed by a peer.
The recv() function was interrupted by a signal that was caught,
before any data was available.
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:
An I/O error occurred while reading from or writing to the file system.
Insufficient resources were available in the system to perform the
Insufficient memory was available to fulfill the request.
The following sections are informative.
The recv() function is equivalent to recvfrom() with a
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 received.
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 original Standard can be obtained online at