Page History

Sending a uncaught signal to a thread to terminate it should be avoided, as it kills the entire process as opposed to killing just the individual thread.Calling the signal() function in a multithreaded program is undefined behavior. (See undefined behavior 135.)

Noncompliant Code Example

This code uses the pthread_killnoncompliant code example invokes the signal() function to send a SIGKILL signal to the created thread. The thread receives the signal and the entire process is terminated.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) {
    /* ... */
  }int main(int argc, char* argv[]){
  pthread_t thread;

  pthread_create(&thread, NULL, func, 0);
  pthread_kill(thread, SIGKILL);

  /* May run a few more lines until the signal kills the process */

  return 0;
}

int main(void) {
  func(void *foo){

  /* Execution of thread */

}
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 code instead uses the pthread_cancel() to terminate the thread. In the thread function we set the cancel type to asynchronous, so as to ensure an immediate cancel. If desired, the cancel type can be left as the default type of deferred, where the thread will not terminate until it reaches a cancellation point.compliant solution uses an object of type atomic_bool to indicate when the child thread should terminate its loop:

int main(int argc, char* argv[]){
  pthread_t thread;
  pthread_create(&thread, NULL, func, (void*)0);
  sleep(1);
  pthread_cancel(thread);

  /* Continues */#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;
}

voidint funcmain(void) *foo){
  pthread_setcanceltype(PTHREAD_CANCEL_ASYNCHRONOUSthrd_t tid;
  
  if (thrd_success != thrd_create(&tid, func, NULL);
)) {
    /* Handle error */
  }
  /* Execution of thread ... */
  /* Set flag when done */
}
  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].

Risk Assessment

Using signals as described has the simple consequence of terminating the process, which is clearly undesired. However there is no other direct risk.Mixing signals and threads causes undefined behavior.

References

Related Vulnerabilities

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

Automated Detection

Bibliography

...

Image Added Image Added Image Added Wiki Markup\[[OpenBSD|AA. References#OpenBSD]\] [{{signal()}} Man Page|http://www.openbsd.org/cgi-bin/man.cgi?query=signal]