Socket timeouts

Lesson

The problem

By default, read or recv on a blocking socket (one that pauses your program until data arrives) can wait forever. If the other side never sends anything, your program hangs.

The fix is to set a deadline. There are three common ways to do this.

Three ways to bound a blocking operation

• setsockopt with SO_RCVTIMEO — Sets a read timeout directly on the socket. There is also SO_SNDTIMEO for send timeouts. This is the simplest option.
• Non-blocking socket plus select or poll — Make the socket non-blocking, then wait for it to become ready using select or poll with a timeout. This gives you the most control.
• SIGALRM with alarm(seconds) — Schedule a signal that interrupts the blocked call after a set number of seconds. This is the older approach and is less flexible.

Code examples

struct timeval tv = { .tv_sec = 5, .tv_usec = 0 };
setsockopt(s, SOL_SOCKET, SO_RCVTIMEO, &tv, sizeof tv);

Practice with these exercises

• Milliseconds to timeval seconds
• Milliseconds to timeval microseconds