recv()

NAME

recv(), recvfrom() - receive a message from a socket

SYNOPSIS

#include <sys/types.h>
#include <sys/socket.h>


ssize_t recv (int s, void *buf, size_t len, int flags)
ssize_t recvfrom (int s, void *buf, size_t len, int flags,
				struct sockaddr *from, int *fromlen)

DESCRIPTION

The recvfrom(2) function is used to receive messages from a socket, and may be used to receive data on a socket whether or not it is connection-oriented.

If from is non-nil, and the socket is not connection-oriented, the source address of the message is filled in. Fromlen is a value-result parameter, initialized to the size of the buffer associated with from, and modified on return to indicate the actual size of the address stored there.

The recv(2) call is normally used only on a connected socket (see connect(2)) and is identical to recvfrom(2) with a nil from parameter. As it is redundant, it may not be supported in future releases.

Both routines return the length of the message on successful completion. If a message is too long to fit in the supplied buffer, excess bytes may be discarded depending on the type of socket the message is received from (see socket(2)).

If no messages are available at the socket, the receive call waits for a message to arrive, unless the socket is nonblocking (see fcntl(2)) in which case the value -1 is returned and the external variable errno set to [EAGAIN]. The receive calls normally return any data available, up to the requested amount, rather than waiting for receipt of the full amount requested; this behavior is affected by the socket-level options SO_RCVLOWAT and SO_RCVTIMEO described in getsockopt(2).

The select(2) call may be used to determine when more data arrive.

The flags argument to a recv call is formed by OR'ing one or more of the values:

process out-of-band data
peek at incoming message

The MSG_OOB flag requests receipt of out-of-band data that would not be received in the normal data stream. Some protocols place expedited data at the head of the normal data queue, and thus this flag cannot be used with such protocols. The MSG_PEEK flag causes the receive operation to return data from the beginning of the receive queue without removing that data from the queue. Thus, a subsequent receive call will return the same data.

RETURN VALUES

These calls return the number of bytes received, or -1 if an error occurred.

ERRORS

The calls fail if:

[EBADF]: The argument s is an invalid descriptor.
[ENOTCONN]: The socket is associated with a connection-oriented protocol and has not been connected (see connect(2) and accept(2)).
[ENOTSOCK]: The argument s does not refer to a socket.
[EAGAIN]: The socket is marked non-blocking, and the receive operation would block, or a receive time-out had been set, and the time-out expired before data were received.
[EINTR]: The receive was interrupted by delivery of a signal before any data were available.
[EFAULT]: The receive buffer pointer(s) point outside the process's address space.