recvfrom()

receive a message from a socket at a specified address

Synopsis:

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

int recvfrom( int s,
              void *buff,
              int len,
              int flags,
              struct sockaddr *from,
              int *fromlen );

Description:

The recvfrom() routine receives a message from a socket, s, whether or not it's connection-oriented.

If from is nonzero, and the socket is connectionless, the source address of the message is filled in. The parameter 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 stored address.

This routine returns the length of the message on successful completion. If a message is too long for the supplied buffer, buf, excess bytes may be discarded depending on the type of socket that the message is received from-see socket().

If no messages are available at the socket, the receive call waits for a message to arrive, unless the socket is nonblocking-see ioctl()-in which case -1 is returned and the external variable errno is set to EWOULDBLOCK. Normally, the receive calls return any data available, up to the requested amount, rather than wait for the full amount requested; this behavior is affected by the socket-level options SO_RCVLOWAT and SO_RCVTIMEO described in getsockopt().

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

The flags argument is formed by ORing one or more of the values:

MSG_OOB

Process out-of-band data. This flag requests receipt of out-of-band data that wouldn't be received in the normal data stream. This flag can't be used with protocols that place expedited data at the head of the normal data queue.

MSG_PEEK

Peek at incoming message. This 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.

MSG_WAITALL

Wait for full request or error. This flag requests that the operation block until the full request is satisfied. But the call may still return less data than requested if a signal is caught, if an error or disconnect occurs, or if the next data to be received is of a different type than that returned.

Returns:

The number of bytes received.

Classification:

POSIX 1003.1g (draft)

See also:

recv(), recvmsg()