The clean and reliable way to wait for a signal to arrive is to block it
and then use sigsuspend. By using sigsuspend in a loop,
you can wait for certain kinds of signals, while letting other kinds of
signals be handled by their handlers.
— Function: int sigsuspend (const sigset_t *set)
This function replaces the process's signal mask with set and then
suspends the process until a signal is delivered whose action is either
to terminate the process or invoke a signal handling function. In other
words, the program is effectively suspended until one of the signals that
is not a member of set arrives.
If the process is woken up by delivery of a signal that invokes a handler
function, and the handler function returns, then sigsuspend also
returns.
The mask remains set only as long as sigsuspend is waiting.
The function sigsuspend always restores the previous signal mask
when it returns.
The return value and error conditions are the same as for pause.
With sigsuspend, you can replace the pause or sleep
loop in the previous section with something completely reliable:
sigset_t mask, oldmask;
...
/* Set up the mask of signals to temporarily block. */
sigemptyset (&mask);
sigaddset (&mask, SIGUSR1);
...
/* Wait for a signal to arrive. */
sigprocmask (SIG_BLOCK, &mask, &oldmask);
while (!usr_interrupt)
sigsuspend (&oldmask);
sigprocmask (SIG_UNBLOCK, &mask, NULL);
This last piece of code is a little tricky. The key point to remember
here is that when sigsuspend returns, it resets the process's
signal mask to the original value, the value from before the call to
sigsuspend—in this case, the SIGUSR1 signal is once
again blocked. The second call to sigprocmask is
necessary to explicitly unblock this signal.
One other point: you may be wondering why the while loop is
necessary at all, since the program is apparently only waiting for one
SIGUSR1 signal. The answer is that the mask passed to
sigsuspend permits the process to be woken up by the delivery of
other kinds of signals, as well—for example, job control signals. If
the process is woken up by a signal that doesn't set
usr_interrupt, it just suspends itself again until the “right”
kind of signal eventually arrives.
This technique takes a few more lines of preparation, but that is needed
just once for each kind of wait criterion you want to use. The code
that actually waits is just four lines.
Published under the terms of the GNU General Public License