ERR32-C. Do not rely on indeterminate values of errno

A signal handler is allowed to call signal(), and if that fails, signal() returns SIG_ERR and sets errno to a positive value.  However, if the event that caused a signal was external (not as the result of the program calling abort() or raise()), the signal handler must only call _Exit() or abort(), or call signal() on the signal currently being handled, and if signal() fails, the value of errno is indeterminate.

This rule is a special case of SIG31-C. Do not access or modify shared objects in signal handlers. The object designated by errno is of static storage duration and is not a volatile sig_atomic_t.  As a result, performing any action that would require errno to be set would normally cause undefined behavior.  The C standard makes a special exception for errno in this case, saying the only thing that is allowed to go wrong is that errno takes on an indeterminate value.  This makes it possible to call signal() from within a signal handler without risking completely unrestricted undefined behavior, but the handler, and any code executed after the handler returns, must not depend on the value of errno being meaningful.

Non-Compliant Code Example

If the request to set the signal to default can be honored, the signal() function returns the value of the signal handler for the most recent successful call to the signal() function for the specified signal. Otherwise, a value of SIG_ERR is returned and a positive value is stored in errno.

#include <signal.h>
#include <stdlib.h>
#include <string.h>

typedef void (*pfv)(int);

void handler(int signum) {
  pfv old_handler = signal(signum, SIG_DFL);
  if (old_handler == SIG_ERR) {
    perror("SIGINT handler"); /* undefined behavior */
    /* handle error condition */
  }
}

int main(void) {
  pfv old_handler = signal(SIGINT, handler);
  if (old_handler == SIG_ERR) {
    perror("SIGINT handler");
    /* handle error condition */
  }

  /* main code loop */

  return 0;
}

The call to perror() from handler() also violates SIG30-C. Call only asynchronous-safe functions within signal handlers.

Compliant Solution

The compliant solution does not reference errno, and does not return from the signal handler if the signal() call fails.

#include <signal.h>
#include <stdlib.h>
#include <string.h>

typedef void (*pfv)(int);

void handler(int signum) {
  pfv old_handler = signal(signum, SIG_DFL);
  if (old_handler == SIG_ERR) {
    abort();
  }
}

int main(void) {
  pfv old_handler = signal(SIGINT, handler);
  if (old_handler == SIG_ERR) {
    perror("SIGINT handler");
    /* handle error condition */
  }

  /* main code loop */

  return 0;
}

Non-Compliant Code Example (POSIX)

POSIX is less restrictive than C99 about what applications can do in signal handlers. It has a long list of asynchronous-safe functions that can be called. Many of these functions set errno on error. This can lead to a situation where a signal handler is executed in between a call to a failed function and the subsequent inspection of errno, and consequently the value inspected is not the one set by that function but the one set by a function call in the signal handler. POSIX applications can avoid this problem by ensuring that signal handlers which contain code that might alter errno always save the value of errno on entry and restore it before returning.

The signal handler in this non-compliant code example alters the value of errno, and resultantly, could cause incorrect error handling if executed in between a failed function call and the subsequent inspection of errno.

#include <stddef.h>
#include <signal.h>
#include <errno.h>
#include <sys/wait.h>

void reaper(int signum) {
  errno = 0;
  for (;;) {
    int rc = waitpid(-1, NULL, WNOHANG);
    if ( (0 == rc) || (-1 == rc && EINTR != errno) )
      break;
  }
  if (ECHILD != errno) {
    /* handle error */
  }
}

int main(void) {
  struct sigaction act;
  act.sa_handler = reaper;
  act.sa_flags = 0;
  if (sigemptyset(&act.sa_mask) != 0) {
    /* handle error */
  }
  if (sigaction(SIGCHLD, &act, NULL) != 0) {
    /* handle error */
  }

  /* ... */

  return 0;
}

Compliant Solution (POSIX)

The compliant solution saves and restores the value of errno in the signal handler.

#include <stddef.h>
#include <signal.h>
#include <errno.h>
#include <sys/wait.h>

void reaper(int signum) {
  int save_errno = errno;
  errno = 0;
  for (;;) {
    int rc = waitpid(-1, NULL, WNOHANG);
    if ( (0 == rc) || (-1 == rc && EINTR != errno) )
      break;
  }
  if (ECHILD != errno) {
    /* handle error */
  }
  errno = save_errno;
}

int main(void) {
  struct sigaction act;
  act.sa_handler = reaper;
  act.sa_flags = 0;
  if (sigemptyset(&act.sa_mask) != 0) {
    /* handle error */
  }
  if (sigaction(SIGCHLD, &act, NULL) != 0) {
    /* handle error */
  }

  /* ... */

  return 0;
}

Risk Assessment

Referencing indeterminate values can result in undefined behavior.

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

References

[[ISO/IEC 9899-1999]] Section 7.14.1.1, "The signal function"

ERR31-C. Don't redefine errno      13. Error Handling with errno (ERR)       14. Miscellaneous (MSC)