Taken from the original at http://www.ecst.csuchico.edu/~beej/guide/net/ ... see end of writeup for Copyright statement.
Blocking. You've heard about it -- now what the hell is it? In a nutshell, "block" is techie jargon for "sleep". You probably noticed that when you run listener, above, it just sits there until a packet arrives. What happened is that it called recvfrom(), there was no data, and so recvfrom() is said to "block" (that is, sleep there) until some data arrives.
Lots of functions block. accept() blocks. All the recv*() functions block. The reason they can do this is because they're allowed to. When you first create the socket descriptor with socket(), the kernel sets it to blocking. If you don't want a socket to be blocking, you have to make a call to fcntl():
sockfd = socket(AF_INET, SOCK_STREAM, 0);
fcntl(sockfd, F_SETFL, O_NONBLOCK);
By setting a socket to non-blocking, you can effectively "poll" the socket for information. If you try to read from a non-blocking socket and there's no data there, it's not allowed to block -- it will return -1 and errno will be set to EWOULDBLOCK.
Generally speaking, however, this type of polling is a bad idea. If you put your program in a busy-wait looking for data on the socket, you'll suck up CPU time like it was going out of style. A more elegant solution for checking to see if there's data waiting to be read comes in the following section on select().
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Copyright © 1995, 1996 by Brian "Beej" Hall. This guide may be reprinted in any medium provided that its content is not altered, it is presented in its entirety, and this copyright notice remains intact. Contact email@example.com for more information.