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Do not use signals in multithreaded programs. This Calling the signal() function in a multithreaded program is undefined behavior in C11 (Section 7.14.1.1, paragraph 7). Furthermore, in POSIX, sending an uncaught signal in order to kill a thread causes the signal to kill the entire process, not just the individual thread.

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

Noncompliant Code Example (POSIX)

. (See undefined behavior 135.)

Noncompliant Code Example

This noncompliant code example invokes the signal() function from a multithreaded program:This code uses the pthread_kill() function to send a SIGTERM signal to the created thread. The thread receives the signal, and the entire process is terminated.

Code Block

bgColor	#ffcccc
lang	c

void func(void *foo#include <signal.h>
#include <stddef.h>
#include <threads.h>
 
volatile sig_atomic_t flag = 0;

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

/* Runs Executionuntil user ofsends threadSIGUSR1 */
}

int mainfunc(void *data) {
  while int result;(!flag) {
  pthread_t thread;

 /* if ((result = pthread_create(&thread, NULL, func, 0)) != 0... */
  }
  return 0;
}

int main(void) {
  signal(SIGUSR1, handler); /* HandleUndefined Errorbehavior */
  thrd_t tid;
  }
  if ((resultthrd_success != pthreadthrd_kill(threadcreate(&tid, func, SIGTERMNULL)) != 0) {
    /* Handle Errorerror */
  }

  /* This point is not reached because the process terminates in pthread_kill() */
... */
  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 code uses instead the pthread_cancel() function to terminate the thread. The thread continues to run until it reaches a cancellation point. See The Open Group Base Specifications Issue 6, IEEE Std 1003.1, 2004 Edition [Open Group 2004] for lists of functions that are required and allowed to be cancellation points. If the cancellation type is set to asynchronous, the thread is terminated immediately. However, POSIX requires only the pthread_cancel(), pthread_setcancelstate(), and pthread_setcanceltype() functions to be async-cancel safe. An application that calls other POSIX functions with asynchronous cancellation enabled is nonconforming. Consequently, we recommend disallowing asynchronous cancellation, as explained by POS47-C. Do not use threads that can be canceled asynchronously.solution uses an object of type atomic_bool to indicate when the child thread should terminate its loop:

Code Block

bgColor	#ccccff
lang	c

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

int func(void *foodata) {
  while (!flag) {
    /* Execution of thread */... */
  }
  return 0;
}

int main(void) {
  intthrd_t resulttid;
  pthread_t thread;

  if ((resultthrd_success != pthreadthrd_create(&thread, NULLtid, func, 0NULL)) != 0) {
    /* Handle Errorerror */
  }
  if ((result = pthread_cancel(thread)) != 0) {/* ... */
  /* Set /*flag Handlewhen Errordone */
  }

flag  /* Continue executing */= 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].