The Single UNIX ® Specification, Version 2
Copyright © 1997 The Open Group


recv - receive a message from a connected socket


#include <sys/socket.h>

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


The recv() function receives 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 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 argument.
Specifies the type of message reception. Values of this argument are formed by logically 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 will still return this data.
Requests out-of-band data. The significance and semantics of out-of-band data are protocol-specific.
Requests that the function block until the full amount of data requested can be returned. The function may return a smaller amount of data 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 returns 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 must 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 are discarded. For stream-based sockets such as SOCK_STREAM, message boundaries are ignored. In this case, data is returned to the user as soon as it becomes available, and no data is discarded.

If the MSG_WAITALL flag is not set, data will 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() blocks until a message arrives. If no messages are available at the socket and O_NONBLOCK is set on the socket's file descriptor, recv() fails and sets errno to [EAGAIN] or [EWOULDBLOCK].


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


The recv() function will fail if:

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.
The socket argument is not a valid file descriptor.
A connection was forcibly closed by a peer.
The buffer parameter can not be accessed or written.
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 operation.
Insufficient memory was available to fulfill the request.
There were insufficient STREAMS resources available for the operation to complete.


The recv() function is identical 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 received.


poll(), read(), recvmsg(), recvfrom(), select(), send(), sendmsg(), sendto(), shutdown(), socket(), write(), <sys/socket.h>.

UNIX ® is a registered Trademark of The Open Group.
Copyright © 1997 The Open Group
[ Main Index | XSH | XCU | XBD | XCURSES | XNS ]