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void handler(int signum) {
  /* handling code */
}

Compliant Solution (Unix & Windows)

A C99-compliant solution to reset the handler on a Unix system is to rebind the signal to the default handler in the first line of the handler itself. Whereas, Windows automatically resets handlers to default.

void handler(int signum) {
#ifndef WINDOWS
  signal(signum, SIG_DFL);
#endif
  /* handling code */
}

There With the Compliant Solution for Unix, there is no race condition that can be exploited by an attacker in sending a second signal, because . And that is because a second signal sent to the handler before it calls , before the latter calls signal(signum, SIG_DFL), will merely cause it to cause the handler to restart, and call signal() anyway.

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/* Equivalent to signal( SIGUSR1, handler) but make signal non-persistent */
struct sigaction act;
act.sa_handler = handler;
act.sa_flags = SA_RESETHAND;
if (sigemptyset(&act.sa_mask) != 0) {
  /* handle error */
}
if (sigaction(SIGUSR1, &act, NULL) != 0) {
  /* handle error */
}

Compliant Solution (Windows)

Windows automatically resets handlers to default.

void handler(int signum) {
  /* handling code */
}

Risk Assessment

Failure to understand implementation-specific details regarding signal handler persistence can lead to unexpected behavior.

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