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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 sets a signal handler while also setting a child thread to do work, which results in undefined behavior.code example invokes the signal() function from a multithreaded program:

Code Block

bgColor	#ffcccc
lang	c

#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 dispenses with the signal handler and uses an object of type atomic_flagbool to indicate when the child thread should terminate its loop:

Code Block

bgColor	#ccccff
lang	c

#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 would include POSIX, for example[IEEE Std 1003.1-2013].