The signal() function has implementation-defined behavior and behaves differently, for example, on Windows than it does on Unix systems.

The following example code illustrates this behavior:

#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;
}

Unix (and Unix-like) systems automatically reinstall signal handlers upon handler execution, meaning that the signal handler defined by the user is left in place until it is explicitly removed. For example, when this code is compiled with gcc 3.4.4 and executed under Red Hat Linux, the SIGINT is captured both times by handler.

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

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. This means that signal handlers are not automatically reinstalled. For example, when this code is compiled with Microsoft Visual Studio 2005 version 8.0, only the first SIGINT is captured by handler.

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

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

Different actions must be taken depending on whether or not the application requires signal handlers to be persistent.

Persistent Handlers

Errors and potential vulnerabilities exist when the actual signal handler persistence behavior is inconsistent with the developer's expectations, for example, the developer expects the signal handler to persist but it does not.

Non-Compliant Code Example (Windows)

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

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

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) {
#ifdef WINDOWS
  signal(signum, handler);
#endif
  /* handling code */
}

Non-Persistent Handlers

Errors may also occur when the developer expects the default action to be restored for a signal, but instead, the signal handler persists.

Non-Compliant Code Example (Unix)

This non-compliant code example fails to reset the signal handler to its default behavior on Unix systems.

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

Compliant Solution (Unix)

A C99-compliant solution to reset the handler on a Unix system is to rebind the signal to the implementation-defined default handler in the first line of the handler itself.

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

Windows automatically resets handlers to default.

Risk Assessment

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

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

\[[ISO/IEC 9899-1999TR2|AA. C References#ISO/IEC 9899-1999]\] Section 7.14.1.1, "The {{signal}} function"

SIG00-A. Avoid using the same handler for multiple signals      12. Signals (SIG)       SIG02-A. Avoid using signals to implement normal functionality