Sunday, March 20, 2011

Socket and O_NONBLOCK

It is often stated that a socket is created in blocking mode by default. However, not all implementations of the socket calls adhere to that behavior. Specialized operating systems can be particularly tricky when it comes to blocking and non-blocking mode . One should always test the O_NONBLOCK flag on a socket to be sure if it is in the blocking or nonblocking mode operation before proceeding to send/recv. Here are some example c code to demonstrate how to check and manipulate the O_NONBLOCK flag on a streaming socket.

/* !!! Error checking is omited for brevity !!! */
/* Create a streaming socket */
int sock = socket(AF_INET, SOCK_STREAM, 0);

/* Get the file descriptor flag */
int flags = fcntl(sock, F_GETFL, 0);

/* Set O_NONBLOCK if it is set
 * or unset it if it is not
if ((flags & O_NONBLOCK ) == O_NONBLOCK) {
   fcntl(sock, F_SETFL, flags | O_NONBLOCK);  /* Set */
} else {
   fcntl(sock, F_SETFL, flags & (~O_NONBLOCK));  /* Unset */

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