How to fix Signal Handler assignment

Question

For Linux C programming, I have this handler and main method:

void handler(int number, signinfo_t, void *ignore){ printf("Signaling %d\n", si->si_pid); } int main(){ struct sigaction sig; sig.sa_flags = SA_SIGINFO; sigemptyset(&sig.sa_mask); sig.sa_handler = handler // This line has error

If I make the handler with just 1 parameter void handler(int num) it works fine; however, I will not be able to use si->si_pid. The warning I am getting is :

 warning: assignment to __sighandler_t from an incompatible pointer type -Wincompatible-pointer-types sig.sa_handler = handler;

Do I make it sig.sa_action instead? I want to fix the warning

Note:you cannot use printf() (and friends) from within a signal handler. — wildplasser
– wildplasser, Commented Mar 7, 2019 at 0:07
You're missing a variable name in the handler function definition. Did you mean to write siginfo_t *si? — Barmar
– Barmar, Commented Mar 7, 2019 at 0:39

Barmar · Accepted Answer · 2019-03-07 00:44:16Z

You're assigning the handler function to the wrong member of sig. The declaration of struct sigaction is:

struct sigaction { void (*sa_handler)(int); void (*sa_sigaction)(int, siginfo_t *, void *); sigset_t sa_mask; int sa_flags; void (*sa_restorer)(void); };

sig.sa_handler is a function with only one argument, the signal number. When you use the SA_SIGINFO flag, you need to assign the 3-argument function to sig.sa_sigaction instead.

int main(){ struct sigaction sig; sig.sa_flags = SA_SIGINFO; sigemptyset(&sig.sa_mask); sig.sa_sigaction = handler;

I did that, now it says struct sigaction has no member named "sa_action"

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