Printf is not working in C signal handler

Question

Code in question first (minimized case):

#include 
#include 

int counter = 0;

void react_to_signal(int n) {
    fprintf(stde         


        

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.

Answer 2

You may need to fflush stderr to get the message to write before the program exits.