Printf is not working in C signal handler

Question

Code in question first (minimized case):

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

int counter = 0;

void react_to_signal(int n) {
    fprintf(stderr, "Caught!\n");
    counter++;
}

int main(int argc, char** argv) {

    signal(SIGINFO, react_to_signal);

    while (1) {
        printf("%d\n", counter);
    }

    return 0;
}

I run the code, it loops as it should, printing out 0. Then in another shell..

kill -s SIGINFO <pid_of_my_process>

Signal is delivered, c is incremented .. but the fprintf doesn't happen.

Why is this so? In what environment/context does handler code run? Where can I read up on this?

Answer 1

In short: you cannot safely use printf within a signal handler.

There's a list of authorized functions in signal handler's man page. There is not fprintf in it.

That's because this function is not reentrant, mainly because it can use malloc and free. See this post for a detailed explanation.