The signal() function behaves differently in Windows than it does on Linux/BSD systems. When a signal handler is installed with the signal() function in Windows, the default action is restored for that signal after the signal is triggered. Conversely, Linux/BSD systems leave the signal handler defined by the user in place until it is explicitly removed.

Non-Compliant Code Example (Windows)

This non-complaint code example fails to persist the signal handler on Windows platforms.

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

volatile sig_atomic_t e_flag = 0;

void handler(int signum) {
  e_flag = 1;
}

int main(void) {
  signal(SIGINT, handler);
  while(!e_flag) {}
  puts("Escaped from first while()");
  e_flag = 0;
  while(!e_flag) {}
  puts("Escaped from second while()");
  return 0;
}

When compiled with gcc 3.4.4 and executed under Red Hat Linux, the signal handler is automatically reinstalled upon handler execution.

% ./SIG01-A
^C
Escaped from first while()
^C
Escaped from second while()
%

However, when compiled with Microsoft Visual Studio 2005 version 8.0 and executed under Windows the signal handler is not automatically reinstalled.

> SIG01-A.exe
^C
Escaped from first while()
^C
>

The second interrupt executes the default action for SIGINT, which is to terminate program execution.

Compliant Solution (Windows)

A C99-compliant solution to persist the handler on a Windows system is to rebind the signal to the handler in the first line of the handler itself.

void handler(int signum) {
  signal(signum, handler);
  /* rest of handling code */
}

Risk Analysis

Failure to re-establish a persistent signal handler on Windows platforms can lead to unexpected behavior.

References

\[[ISO/IEC 9899-1999TR2|AA. C References#ISO/IEC 9899-1999]\] "The {{signal}} function"