Programming with sockets

Non-blocking sockets

It is occasionally convenient to make use of sockets that do not block; that is, I/O requests that cannot complete immediately and would therefore cause the process to be suspended awaiting completion are not executed, and an error code is returned. Once a socket has been created via the socket call, it may be marked as non-blocking by fcntl(2) as follows:

   #include <fcntl.h>
   #include <sys/file.h>
    ...
   int	s;
    ...
   s = socket(AF_INET, SOCK_STREAM, 0);
    ...
   if (fcntl(s, F_SETFL, O_NDELAY) < 0) {
   	perror("fcntl F_SETFL, O_NDELAY");
   	exit(1);
   }
    ...

When performing non-blocking I/O on sockets, one must be careful to check for the error EWOULDBLOCK (stored in the global variable errno), which occurs when an operation would normally block, but the socket it was performed on is marked as non-blocking. In particular, accept, connect, send, recv, read and write can all return EWOULDBLOCK, and processes should be prepared to deal with such return codes. If an operation such as a send cannot be done in its entirety, but partial writes are sensible (for example, when using a stream socket), the data that can be sent immediately will be processed, and the return value will show the amount actually sent.