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(libc.info)Receiving Datagrams

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Receiving Datagrams
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   The `recvfrom' function reads a packet from a datagram socket and
also tells you where it was sent from.  This function is declared in
`sys/socket.h'.

 - Function: int recvfrom (int SOCKET, void *BUFFER, size_t SIZE, int
          FLAGS, struct sockaddr *ADDR, socklen_t *LENGTH-PTR)
     The `recvfrom' function reads one packet from the socket SOCKET
     into the buffer BUFFER.  The SIZE argument specifies the maximum
     number of bytes to be read.

     If the packet is longer than SIZE bytes, then you get the first
     SIZE bytes of the packet and the rest of the packet is lost.
     There's no way to read the rest of the packet.  Thus, when you use
     a packet protocol, you must always know how long a packet to
     expect.

     The ADDR and LENGTH-PTR arguments are used to return the address
     where the packet came from.  Note: Socket Addresses.  For a
     socket in the local domain the address information won't be
     meaningful, since you can't read the address of such a socket
     (Note: Local Namespace).  You can specify a null pointer as the
     ADDR argument if you are not interested in this information.

     The FLAGS are interpreted the same way as for `recv' (Note: Socket
     Data Options).  The return value and error conditions are also
     the same as for `recv'.

     This function is defined as a cancellation point in multi-threaded
     programs, so one has to be prepared for this and make sure that
     allocated resources (like memory, files descriptors, semaphores or
     whatever) are freed even if the thread is canceled.

   You can use plain `recv' (Note: Receiving Data) instead of
`recvfrom' if you don't need to find out who sent the packet (either
because you know where it should come from or because you treat all
possible senders alike).  Even `read' can be used if you don't want to
specify FLAGS (Note: I/O Primitives).