I'm trying to create a program that blocks the signal SIGUSR1 and the it unblocks the signal.
In the middle I want to see that the signal is blocked using sigpending
. But it always says that the signal isn't blocked, and I can use the signal when it's supposed to be blocked.
This is the code that I have.
#include <stdlib.h>
#include <stdio.h>
#include <signal.h>
static void signals(int signaln)
{
switch (signaln) {
case SIGUSR1:
printf("Signal SIGUSR1\n"); break;
}
return;
}
main()
{
sigset_t set,set2;
struct sigaction sigs;
sigs.sa_handler = signals;
sigemptyset(&sigs.sa_mask);
sigs.sa_flags=SA_ONESHOT;
sigaction(SIGUSR1, &sigs,0);
sigemptyset(&set);
sigemptyset(&set2);
sigaddset(&set,SIGUSR1);
if(sigprocmask(SIG_BLOCK, &set, NULL)==0){
printf("Blocking SISGUSR1...\n");
}
sigpending(&set2);
if (sigismember(&set2,SIGUSR1)==1)
{
printf("The signal is blocked\n"); //it should print this
}
wait(2);
kill(getpid(),SIGUSR1); //the signal shouldn't work
wait(2);
if(sigprocmask(SIG_UNBLOCK, &set, NULL)==0){
printf("Unblocking SIGUSR1\n");
}
}
Could anyone help me?
sigpending
doesn't tell you whether a signal is blocked. It tells you whether a signal is waiting to be delivered. (i.e., the signal is blocked and one has been sent.)
Also, blocked doesn't meean that the signal won't be delivered; it means that the signal won't be delivered now. So you can send the signal, and it will be delivered as soon as the signal is unblocked; probably after the call to sigprocmask(SIGUNBLOCKED...)
but before the call to printf
, so you'll probably see the signal received message before you see the "unblocking" message.