Question

I was wondering if it is possible to be interrupted by a signal when my program is handling other signal at the same time, I tried to simulate it with:

#include<signal.h>
#include<stdlib.h>
#include<stdio.h>
#include<unistd.h>
#include<sys/wait.h>
#include<string.h>

void sig_output()
{
    sigset_t set;
    sigprocmask(0,NULL,&set);
    printf("currently blocking:");
    if (sigismember(&set,SIGUSR1))
        printf("\nSIGUSR1");
    if(sigismember(&set,SIGUSR2))
        printf("\nSIGUSR2");
    printf("\n");
    return ;
}

void sig_handler(int sig)
{
    raise(SIGUSR1);    
    printf("start\n");
    if (sig==SIGUSR1)
        printf("SIGUSR1\n");
    else if (sig==SIGUSR2)
        printf("SIGUSR2\n");
    printf("end\n");
    return ;
}

void other_sig_handler(int sig)
{  
    printf("start - other\n");
    if (sig==SIGUSR1)
        printf("SIGUSR1\n");
    else if (sig==SIGUSR2)
        printf("SIGUSR2\n");
    printf("end - other\n");
    return ;
}

int main()
{
    sig_output();
    struct sigaction a;
    a.sa_handler=sig_handler;
    a.sa_flags=0;
    sigset_t set,old;
    //blocking SIGUSR1,SIGUSR2
    sigemptyset(&set);
    sigaddset(&set,SIGUSR1);
    sigaddset(&set,SIGUSR2);
    printf("blocking SIGUSR1, SIGUSR2\n");
    sigprocmask(SIG_SETMASK,&set,&old);
    sig_output();
    //adding handles for SIGUSR1,SIGUSR2
    sigemptyset(&(a.sa_mask));
    sigaction(SIGUSR1,&a,NULL);
    a.sa_handler=other_sig_handler;
    sigaction(SIGUSR2,&a,NULL);
    printf("poczatek wysylania \n");
    raise(SIGUSR1);
    raise(SIGUSR2);
    raise(SIGUSR1);
    printf("using sigsuspend\n");
    sigsuspend(&old);
    printf("end of program\n");
    return 0;
}

and everytime I run this program I get

currently blocking:
blocking SIGUSR1, SIGUSR2
currently blocking:
SIGUSR1
SIGUSR2
raising
using sigsuspend
start - other
SIGUSR2
end - other
start
SIGUSR1
end
end of program

is it always like that?

Answer 1

Quoting the sigaction(2) manpage:

Signal routines normally execute with the signal that caused their
invocation blocked, but other signals may yet occur. A global signal mask
defines the set of signals currently blocked from delivery to a process.
The signal mask for a process is initialized from that of its parent
(normally empty). It may be changed with a sigprocmask(2) call, or when
a signal is delivered to the process.

You can control whether the signal is automatically blocked in its signal handler with the SA_NODEFER flag.