Question

If we use sigaction to define a signal handler, then why don’t we need to reset the handler? If we do the use signal(sig_no,handler_func) then we must reset it. Why is this?

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

void func(int sig)
{
 printf("caught signal:%d\n",sig);
 // Not needed to reset handler. Why? 
}

int main()
{
 struct sigaction sa;

 sa.sa_handler=(void*)func;
 sigaction(SIGRTMIN,&sa,NULL);
 kill(0,SIGRTMIN);
 kill(0,SIGRTMIN);
 kill(0,SIGRTMIN);
}
Output:
[root@dhcppc0 signals]# ./a.out
     caught signal:34
     caught signal:34
     caught signal:34 (3 times signal caught by same handler without resetting handler)

Answer 1

Unless you specify SA_RESETHAND in the flags, the disposition isn’t changed (and therefore doesn’t need to be reset)

If you were to set sa.sa_flags = SA_RESETHAND You’d need to reset it because the disposition would be reset to SIG_DFL (which is what happens with signal())

Basically, the answer to your question is “Because that’s how sigaction works. Its behavior is different than signal”.