Page History

Calling the signal() function in a multithreaded program is undefined behavior according to C11 (Section the C Standard, subclause 7.14.1.1, paragraph 7 )[ISO/IEC 9899:2011].

This rule is a specific instance of SIG02-C. Avoid using signals to implement normal functionality.

...

#include <signal.h>
#include <threads.h>
 
volatile sig_atomic_t flag = 0;

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

int func(void *data) {  /* keepRuns running until user sends SIGUSR1. */
int func(void *data) {
  while (!flag) {
    /* ... */
  }
  return 0;
}

int main(void) {
  signal(SIGUSR1, handler); /* Undefined! */
  int result;
  thrd_t tid;
  
  if ((result = thrd_create(&tid, func, NULL))
        != thrd_success) {
    /* Handle Error */
  }

  /* ... */

  return 0;
}

...

#include <stdatomic.h>
#include <threads.h>
 
atomic_flag flag = ATOMIC_VAR_INIT(0);

int func(void *data) {  /* keep running until user sends SIGUSR1 */
  while (!flag) {
    /* ... */
  }
  return 0;
}

int main(void) {
  int result;
  thrd_t tid;
  
  if ((result = thrd_create(&tid, func, NULL))
        != thrd_success) {
    /* Handle Error */
  }

  /* ... */

  /* Set flag when done. */
  while (!atomic_flag_test_and_set( &flag))
   { ; /* tryContinue againattempts. */ }

  return 0;
}

Exceptions

CON37:EX0-EX1: Platforms that provide defined behavior when multithreaded programs use custom signal handlers are exempt from this rule. This would include POSIX, for example.

...

Related Vulnerabilities

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

Bibliography