recvfrom()

Receive a message from the socket at a specified address

Synopsis:

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

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

Arguments:

s

The descriptor for the socket; see socket().

buf

A pointer to a buffer where the function can store the message.

len

The size of the buffer. If len is greater than SSIZE_MAX (see <limits.h>), the function fails and sets errno to EOVERFLOW.

flags

A combination 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. You can't use this flag with protocols that place expedited data at the head of the normal data queue.
MSG_PEEK — peek at the 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.

from

NULL, or a pointer to a sockaddr object where the function can store the source address of the message.

fromlen

A pointer to a socklen_t object that specifies the size of the from buffer. The function stores the actual size of the address in this object.

Library:

libsocket

Use the -l socket option to qcc to link against this library.

Description:

The recvfrom() routine receives a message from the 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 recvfrom() returns -1 is returned and sets the external variable errno 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().

You can use select() to determine when more data is to arrive.

Returns:

The number of bytes received, or -1 if an error occurs (errno is set).

Errors:

EBADF: Invalid descriptor s.
EFAULT: The receive buffer pointer(s) point outside the process's address space.
EINTR: The receive was interrupted by delivery of a signal before any data was available.
ENOTCONN: The socket is associated with a connection-oriented protocol and hasn't been connected; see connect() and accept().
EOVERFLOW: An attempt was made to receive an amount of data that exceeds the allowable limit.
EWOULDBLOCK: Either the socket is marked nonblocking and the receive operation would block, or a receive timeout had been set and the timeout expired before data was received.

Classification:

POSIX 1003.1

Safety:
Cancellation point	Yes
Interrupt handler	No
Signal handler	Yes
Thread	Yes