CERT C: Rule CON37-C

Do not call signal() in a multithreaded program

Description

This checker is deactivated in a default Polyspace^® as You Code analysis. See Checkers Deactivated in Polyspace as You Code Analysis (Polyspace Access).

Rule Definition

Do not call signal() in a multithreaded program.¹

Polyspace Implementation

The rule checker checks for Signal call in multithreaded program.

Examples

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Signal call in multithreaded program

Issue

Signal call in multithreaded program occurs when you use the signal() function in a program with multiple threads.

Risk

According to the C11 standard (Section 7.14.1.1), use of the signal() function in a multithreaded program is undefined behavior.

Fix

Depending on your intent, use other ways to perform an asynchronous action on a specific thread.

Example - Use of signal() Function to Terminate Loop in Thread

#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(SIGINT, handler); /* Undefined behavior */ //Noncompliant
  thrd_t tid;

  if (thrd_success != thrd_create(&tid, func, NULL)) {
    /* Handle error */
  }
  /* ... */
  return 0;
}

In this example, the signal function is used to terminate a while loop in the thread created with thrd_create.

Correction — Use atomic_bool Variable to Terminate Loop

One possible correction is to use an atomic_bool variable that multiple threads can access. In the corrected example, the child thread evaluates this variable before every loop iteration. After completing the program, you can modify this variable so that the child thread exits the 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) {
  thrd_t tid;
   
  if (thrd_success != thrd_create(&tid, func, NULL)) {
    /* Handle error */
  }
  /* ... */
  /* Set flag when done */
  flag = true;
 
  return 0;
}

Check Information

Group: Rule 14. Concurrency (CON)

Version History

Introduced in R2019a

¹ This software has been created by MathWorks incorporating portions of: the “SEI CERT-C Website,” © 2017 Carnegie Mellon University, the SEI CERT-C++ Web site © 2017 Carnegie Mellon University, ”SEI CERT C Coding Standard – Rules for Developing safe, Reliable and Secure systems – 2016 Edition,” © 2016 Carnegie Mellon University, and “SEI CERT C++ Coding Standard – Rules for Developing safe, Reliable and Secure systems in C++ – 2016 Edition” © 2016 Carnegie Mellon University, with special permission from its Software Engineering Institute.

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