Pause Problems (The GNU C Library)

#### 25.8.2 Problems with `pause` ¶

The simplicity of `pause` can conceal serious timing errors that can make a program hang mysteriously.

It is safe to use `pause` if the real work of your program is done by the signal handlers themselves, and the “main program” does nothing but call `pause`. Each time a signal is delivered, the handler will do the next batch of work that is to be done, and then return, so that the main loop of the program can call `pause` again.

You can’t safely use `pause` to wait until one more signal arrives, and then resume real work. Even if you arrange for the signal handler to cooperate by setting a flag, you still can’t use `pause` reliably. Here is an example of this problem:

    /* usr_interrupt is set by the signal handler.  */
    if (!usr_interrupt)
      pause ();

    /* Do work once the signal arrives.  */
    ...

This has a bug: the signal could arrive after the variable `usr_interrupt` is checked, but before the call to `pause`. If no further signals arrive, the process would never wake up again.

You can put an upper limit on the excess waiting by using `sleep` in a loop, instead of using `pause`. (See Sleeping, for more about `sleep`.) Here is what this looks like:

    /* usr_interrupt is set by the signal handler.
    while (!usr_interrupt)
      sleep (1);

    /* Do work once the signal arrives.  */
    ...

For some purposes, that is good enough. But with a little more complexity, you can wait reliably until a particular signal handler is run, using `sigsuspend`.