POSIX Signal

Signals are queued for a process. They can be blocked and remain in pending state until they are unblocked by the process. The size of the queue is implementation dependent. Signal set is a mask for the signals. An example of a 32 bits signal set.

Signal set manipulation:-

/* 
 * initialize the set to unblock all signal 
 */
int sigemptyset(sigset_t *set);

/* 
 * initialize the set to block all signal 
 */
int sigfillset(sigset_t *set);

/* 
 * block a given signal in a set 
 */
int sigaddset(sigset_t *set, int signum);

/* 
 * unblock a given signal in a set 
 */
int sigdelset(sigset_t *set, int signum);

/* 
 * test if a signal is blocked in the set 
 */
int sigismember(const sigset_t *set, int signum);

POSIX Signal Processing:-

The sigaction struct
struct sigaction {
      void (*sa_handler)(int); // address of a signal handler
     sigset_t sa_mask; /* block certain signal during the   execution of the handler */
     int sa_flags;             // signal options
}

Signal handling functions
/* 
install a signal handler for a signal, and return the previous     sigaction struct.
*/
int sigaction(int signum, const struct sigaction *act, struct sigaction *oldact);

/* 
modify the signal mask of the current process. how can be SIG_BLOCK, SIG_UNBLOCK, SIG_SETMASK 
*/
int sigprocmask(int how, const sigset_t *set, sigset_t *oldset);

/* 
returns a set of signals which are blocked 
*/
int sigpending(sigset_t *set);

/* 
suspend the process until the specified signals (in the mask) occur 
*/
int sigsuspend(const sigset_t *mask);

Signal handler will not change after a signal occurs. Signal mask is replaced by the sigaction() during signal handling and restored afterwards.

Posix Signal Example:-

static void signalHandler(int);
int main(int argc, char* argv[]) {
     struct sigaction act;
     struct sigaction oldact;
     act.sa_handler = &signalHandler;
     sigfillset(&act.sa_mask); /*block signal @ signal handler */
     act.sa_flags = 0;
     sigaction(SIGHUP, &act, &oldact);
     while (receivedSignalNo_g != SIGTERM) {
         if (receivedSignal_g) {

             receivedSignal_g = 0; 

             receivedSignalNo_g = 0; 

         }
         sleep(60);
     }
}
static void signalHandler(int signalNumber) {
     receivedSignal_g = 1;
     receivedSignalNo_g = signalNumber;

}