Page History

Calling the signal() function in a multithreaded program is undefined behavior, according to the C Standard. (See also undefined behavior 135 of Annex J.)This rule is a specific instance of SIG02-C. Avoid using signals to implement normal functionality.

Noncompliant Code Example

This noncompliant code example invokes the signal() function from a multithreaded program.:

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

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

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

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

  /* ... */

  return 0;
}

NOTE: The SIGUSR1 signal value is not defined in the C Standard; consequently, this is not a C-compliant code example.

Compliant Solution

This compliant solution uses an object of type atomic_flagbool to indicate when the child thread should terminate its loop.:

#include <stdatomic.h>
#include <stdbool.h>
#include <stddef.h>
#include <threads.h>
 
atomic_flagbool flag = ATOMIC_VAR_INIT(0false);

int func(void *data) {
  while (!flag) {
    /* ... */
  }
  return 0;
}

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

  /* ... */

  /* Set flag when done */
  while (!atomic_flag_test_and_set(&flag))
 =   ; /* Continue attempts */true;

  return 0;
}

Exceptions

CON37-C-EX1: Platforms Implementations such as POSIX that provide defined behavior when multithreaded programs use custom signal handlers are exempt from this rule . This exception includes POSIX, for example[IEEE Std 1003.1-2013].

Risk Assessment

Mixing signals and threads causes undefined behavior.

Related Vulnerabilities

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

Automated Detection

Image Removed Image Removed Image Removed

Bibliography

...

Image Added Image Added Image Added