2. Understanding sigprocmask

The sigprocmask is the function which is used to change the signal mask of the calling process. The signal mask determines which processes are currently blocked from being delivered. It can be understood using the following example:-

#include stdio.h
#include stdlib.h
#include signal.h
#include unistd.h
void handle_signal(int sig)
{
printf("Caught signal %d\n", sig);
}
int main()
{
struct sigaction sa;
sa.sa_handler = handle_signal;
sa.sa_flags = 0;
sigemptyset(&sa.sa_mask);
// Set up the signal handler for SIGINT
if (sigaction(SIGINT, &sa, NULL) == -1)
{
perror("sigaction");
exit(EXIT_FAILURE);
}
sigset_t mask, oldmask, pending;
// Initialize the signal set to empty
sigemptyset(&mask);
// Add SIGINT to the signal set
sigaddset(&mask, SIGINT);
// Block SIGINT and save the old signal mask
if (sigprocmask(SIG_BLOCK, &mask, &oldmask) == -1)
{
perror("sigprocmask");
exit(EXIT_FAILURE);
}
printf("SIGINT blocked. Press Ctrl+C...\n");
sleep(5); // SIGINT is blocked during this sleep
// Check for pending signals
if (sigpending(&pending) == -1)
{
perror("sigpending");
exit(EXIT_FAILURE);
}
// Test if SIGINT is pending
if (sigismember(&pending, SIGINT))
{
printf("SIGINT is pending\n");
}
else
{
printf("SIGINT is not pending\n");
}
// Restore the old signal mask, unblocking SIGINT
if (sigprocmask(SIG_SETMASK, &oldmask, NULL) == -1)
{
perror("sigprocmask");
exit(EXIT_FAILURE);
}
printf("SIGINT unblocked. Press Ctrl+C again...\n");
// Loop indefinitely
while (1)
{
printf("Running...\n");
sleep(1);
}
return 0;
}