Question

The following code catches the “SIGINT” signal only once and then interrupts (program exists):

#include <signal.h>
#include <stdio.h>

void IntHandler(int value);

void CatchIntSignal()
{
    struct sigaction intAction;
    intAction.sa_handler = IntHandler;
    sigemptyset(&intAction.sa_mask);
    intAction.sa_flags = 0;
    //if to uncomment any of "0" or "SIG_IGN" - IntHandler will be never called:
    //intAction.sa_sigaction = 0/*SIG_IGN*/;
    if(sigaction(SIGINT, &intAction, NULL) != 0)
    {
        printf("sigaction() failed.\n");
    }
}

void IntHandler(int value)
{
    printf("IntHandler(%d)\n", value);
    //uncommenting this does not help:
    //CatchIntSignal();
}

int main()
{
    CatchIntSignal();
    getchar();
    return 0;
}

How must I modify this code to preserve program’s exit after SIGINT catching?
If to set intAction.sa_sigaction to 0 or to SIG_IGN – IntHandler will never be called – but why? Which undefined value does it have to say system that “it’s necessary to call IntHandler”? If I will set some handler to intAction.sa_sigaction – this handler will be called (but IntHandler will not). How does system know that I did set something to intAction.sa_sigaction?

Answer 1

Your problem is that the sa_handler and sa_sigaction fields of struct sigaction are actually the same field. Quoting the (OSX) manpage for sigaction(2):

 struct  sigaction {
         union __sigaction_u __sigaction_u;  /* signal handler */
         sigset_t sa_mask;               /* signal mask to apply */
         int     sa_flags;               /* see signal options below */
 };

 union __sigaction_u {
         void    (*__sa_handler)(int);
         void    (*__sa_sigaction)(int, struct __siginfo *,
                        void *);
 };

 #define sa_handler      __sigaction_u.__sa_handler
 #define sa_sigaction    __sigaction_u.__sa_sigaction

So your assignment to sa_sigaction is overwriting the handler you already set. You should just set sa_handler and leave sa_sigaction alone. This is how CatchIntSignal should read:

void CatchIntSignal()
{
    struct sigaction intAction;
    intAction.sa_handler = IntHandler;
    sigemptyset(&intAction.sa_mask);
    intAction.sa_flags = SA_RESTART;
    if(sigaction(SIGINT, &intAction, NULL) != 0)
        printf("sigaction() failed: %s\n", strerror(errno));
}

(You may need to add #includes of string.h and errno.h to get the strerror(errno) bit to compile. Always include strerror(errno) in error messages triggered when system calls fail.)

(CORRECTION: added SA_RESTART to flags per ninjalj.)