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Do not send an uncaught signal to a thread to kill it, because the signal kills the entire process rather than killing just the individual thread. This rule is a specific instance of SIG02-C. Avoid using signals to implement normal functionality.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>
void func(void *foo)#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) {
  int result;while (!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, SIGKILL)) != 0NULL)) {
    /* Handle Errorerror */
  }

  /* Continue executing until the signal kills the process ... */

  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() function to terminate the thread. The thread continues to run until it reaches a cancellation point. See [Open Group 04] for lists of functions that are required to be, and allowed to be, cancellation points. If the cancellation type is set to asynchronous, the thread is terminated immediately. However, POSIX only requires 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 non-conforming. Consequently, we recommend disallowing asynchronous cancellation, as expalined by POS47-C. Do not use threads that can be cancelled asynchronously.compliant solution uses an object of type atomic_bool 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

void func(void *foo) {
    /* ... */
 Execution }
 of threadreturn */0;
}

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

  if ((resultthrd_success != pthreadthrd_create(&threadtid, NULL, func, 0NULL)) != 0) {
    /* Handle Errorerror */
  }
  if ((result = pthread_cancel(thread)) != 0) {
    /* Handle... Error */
  }

  /* ContinueSet executingflag untilwhen thedone signal*/
 kills theflag process= */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

Sending the signal to a process causes it to be abnormally terminatedMixing 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

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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] \[[MKS|AA. References#MKS]\] [{{pthread_cancel()}} Man Page|http://www.mkssoftware.com/docs/man3/pthread_cancel.3.asp] \[[Open Group 04|AA. References#Open Group 04]\] [Threads Overview|http://www.opengroup.org/onlinepubs/009695399/functions/xsh_chap02_09.html]