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void handler(int signum) {
  /* handlingHandle codeSignal */
}

Non-Compliant Code Example

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void handler(int signum) {
  signal(signum, handler);
  /* handlingHandle codeSignal */
}

Unfortunately this solution still contains a race window, starting when the host environment resets the signal and ending when the handler calls signal(). During that time, a second signal sent to the program will trigger the default signal behavior, thereby defeating the persistent behavior.

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

POSIX recommends sigaction() and deprecates signal(). Unfortunately, sigaction() is not defined in C99 and is consequently not as portable a solution.

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void handler(int signum) {
  /* handlingHandle codeSignal */
}

Compliant Solution (Unix & Windows)

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

With the Compliant Solution for Unix, there is no race condition that can be exploited by an attacker in sending a second signal. And that is because a second signal sent to the handler, before the latter calls signal(signum, SIG_DFL), will merely 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) {
  /* handleHandle errorError */
}
if (sigaction(SIGUSR1, &act, NULL) != 0) {
  /* handleHandle errorError */
}

Risk Assessment

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

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