Using sleep and select with signals

Question

I want to use the select() function to wait for 1 second, as my program uses signals to control stuff, so sleep() would return prematurely. The weird thing is that when using select() it also returns prematurely.

I am calling select like this

struct timeval timeout;    
timeout.tv_sec = 10;  
timeout.tv_usec = 1000000; 
select (0 ,NULL, NULL, NULL, &timeout);

but whenever a signal arrives, it returns (I am using a nano second timer for the signal)

Anyone knows why?

Answer 1

Try something like this:

struct timespec timeout;
timeout.tv_sec = 10;
timeout.tv_nsec = 0;
while (nanosleep(&timeout, &timeout) && errno == EINTR);

The "remaining time" pointer to nanosleep will take care of letting you restart the sleep with the necessary amount of remaining time if it gets interrupted.

Answer 2

man 7 signal says:

Interruption of System Calls and Library Functions by Signal Handlers

If a signal handler is invoked while a system call or library function call is blocked, then either:

   * the call is automatically restarted after the signal handler
     returns; or

   * the call fails with the error EINTR.

Which of these two behaviors occurs depends on the interface and whether or not the signal handler was established using the SA_RESTART flag (see sigaction(2)). The details vary across UNIX sys‐ tems; below, the details for Linux.

If a blocked call to one of the following interfaces is interrupted by a signal handler, then the call will be automatically restarted after the signal handler returns if the SA_RESTART flag was used; otherwise the call will fail with the error EINTR

Generally, checking if the return value is -1 and errno == EINTR and then re-calling the function is the right way to correct for this.