The normal way of sending data on a datagram socket is by using the
sendto function, declared in sys/socket.h.
You can call connect on a datagram socket, but this only
specifies a default destination for further data transmission on the
socket. When a socket has a default destination you can use
send (see Sending Data) or even write (see I/O Primitives) to send a packet there. You can cancel the default
destination by calling connect using an address format of
AF_UNSPEC in the addr argument. See Connecting, for
more information about the connect function.
The
sendtofunction transmits the data in the buffer through the socket socket to the destination address specified by the addr and length arguments. The size argument specifies the number of bytes to be transmitted.The flags are interpreted the same way as for
send; see Socket Data Options.The return value and error conditions are also the same as for
send, but you cannot rely on the system to detect errors and report them; the most common error is that the packet is lost or there is no-one at the specified address to receive it, and the operating system on your machine usually does not know this.It is also possible for one call to
sendtoto report an error owing to a problem related to a previous call.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.