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Calling the signal() function in a multithreaded program is undefined behavior according to C11 (Section 7.14.1.1, paragraph 7).This rule is a specific instance of SIG02-C. Avoid using signals to implement normal functionality.. (See undefined behavior 135.)

Noncompliant Code Example

This code raises a signal within a child thread. This is meant to terminate the program, but results in undefined behavior.noncompliant code example invokes the signal() function from a multithreaded program:

void func(void *data#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  if (thread_should_exitfunc(void *data) {
  while  raise( SIGTERM!flag);  // Undefined! {
  }
  /* ... */
  }
  return 0;
}

int main(void) {
  int result;signal(SIGUSR1, handler); /* Undefined behavior */
  thrd_t threadtid;
 
  int result;
  if ((resultthrd_success != thrd_create(&tid, func, NULL)) != thrd_success) {
    /* Handle Errorerror */
  }
  /* ... */
  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 terminates the child thread rather than raising a signal. This has the same effect as the noncompliant code example, but is well-defined in C11.compliant solution uses an object of type atomic_bool to indicate when the child thread should terminate its loop:

void func(void *data) {
  /* ... */
  if (thread_should_exit#include <stdatomic.h>
#include <stdbool.h>
#include <stddef.h>
#include <threads.h>
 
atomic_bool flag = ATOMIC_VAR_INIT(false);

int func(void *data) {
  while  thrd_exit(0);  // OK(!flag) {
  }
  /* ... */
  }
  return 0;
}

int main(void) {
  int result;
  thrd_t threadtid;
 
  int result;
  if ((resultthrd_success != thrd_create(&tid, func, NULL)) != thrd_success) {
    /* Handle Errorerror */
  }
  /* ... */
  /* Set flag when done */
  flag = true;

  return 0;
}

...

Exceptions

CON37:EX0: Platforms -C-EX1: Implementations such as POSIX that provide defined behavior when multithreaded programs use custom signal handlers are exempt from this rule . This would include POSIX, for example[IEEE Std 1003.1-2013].

Risk Assessment

Mixing signals and threads causes undefined behavior.

Related Vulnerabilities

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

Automated Detection

Bibliography

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Bibliography

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