Page History

Calling the signal() function in a multithreaded program is undefined behavior. (see See undefined behavior 135.).

Noncompliant Code Example

...

#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.

...

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_bool flag = ATOMIC_VAR_INIT(false);

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 */
  atomic_store(&flag,flag = true);

  return 0;
}

Exceptions

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

...

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

Bibliography

...

...

Image Modified Image Modified Image Modified