Signals, Part 4: Sigaction

How and why do I use sigaction ?

To change the "signal disposition" of a process - i.e. what happens when a signal is delivered to your process - use sigaction

You can use system call sigaction to set the current handler for a signal or read the current signal handler for a particular signal.

int sigaction(int signum, const struct sigaction *act, struct sigaction *oldact);

The sigaction struct includes two callback functions (we will only look at the 'handler' version), a signal mask and a flags field -

struct sigaction {
               void     (*sa_handler)(int);
               void     (*sa_sigaction)(int, siginfo_t *, void *);
               sigset_t   sa_mask;
               int        sa_flags;
}; 

How do I convert a signal call into the equivalent sigaction call?

Suppose you installed a signal handler for the alarm signal,

signal(SIGALRM, myhander);

The equivalent sigaction code is:

struct sigaction sa; 
sa.sa_handler = myhander;
sigemptyset(&sa.sa_mask);
sa.sa_flags = 0; 
sigaction(SIGINT, &sa, NULL)

However, we typically may also set the mask and the flags field. The mask is a temporary signal mask used during the signal handler execution. The SA_RESTART flag will automatically restart some (but not all) system calls that otherwise would have returned early (with EINTR error). The latter means we can simplify the rest of code somewhat because a restart loop may no longer be required.

sigfillset(&sa.sa_mask);
sa.sa_flags = SA_RESTART; /* Restart functions if  interrupted by handler */     

How do I use sigwait?

Based on http://pubs.opengroup.org/onlinepubs/009695399/functions/pthread_sigmask.html

static sigset_t   signal_mask;  /* signals to block         */

int main (int argc, char *argv[])
{
    pthread_t  sig_thr_id;      /* signal handler thread ID */
    sigemptyset (&signal_mask);
    sigaddset (&signal_mask, SIGINT);
    sigaddset (&signal_mask, SIGTERM);
    pthread_sigmask (SIG_BLOCK, &signal_mask, NULL);

    pthread_create (&sig_thr_id, NULL, signal_thread, NULL);

    /* APPLICATION CODE */
    ...
}

void *signal_thread (void *arg)
{
    int       sig_caught;    /* signal caught       */

    sigwait (&signal_mask, &sig_caught);
    switch (sig_caught)
    {
    case SIGINT:     /* process SIGINT  */
        ...
        break;
    case SIGTERM:    /* process SIGTERM */
        ...
        break;
    default:         /* should normally not happen */
        fprintf (stderr, "\nUnexpected signal %d\n", sig_caught);
        break;
    }
}

(Under construction)