9.11. fcntl()

Control socket descriptors

#include <sys/unistd.h>
#include <sys/fcntl.h>

int fcntl(int s, int cmd, long arg);
This function is typically used to do file locking and other file-oriented stuff, but it also has a couple socket-related functions that you might see or use from time to time.

Parameter s is the socket descriptor you wish to operate on, cmd should be set to F_SETFL, and arg can be one of the following commands. (Like I said, there's more to fcntl() than I'm letting on here, but I'm trying to stay socket-oriented.)
O_NONBLOCK    Set the socket to be non-blocking. See the section on blocking for more details.

O_ASYNC       Set the socket to do asynchronous I/O. When data is ready to be recv()'d on the socket, the signal SIGIO will be raised. This is rare to see, and beyond the scope of the guide. And I think it's only available on certain systems.
Return Value
Returns zero on success, or -1 on error (and errno will be set accordingly.)

Different uses of the fcntl() system call actually have different return values, but I haven't covered them here because they're not socket-related. See your local fcntl() man page for more information.

int s = socket(PF_INET, SOCK_STREAM, 0);

fcntl(s, F_SETFL, O_NONBLOCK);  // set to non-blocking
fcntl(s, F_SETFL, O_ASYNC);     // set to asynchronous I/O
See Also
Blocking, send()