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 Input and Output 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 Making a Connection, for
more information about the
Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.
sendto function 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
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
sendto to 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, file descriptors, semaphores or whatever) are freed even if the thread is canceled.