CertC++-ENV32

All exit handlers must return normally

Required inputs: IR

The C Standard provides three functions that cause an application to terminate normally:  _Exit(),  exit(), and quick_exit(). These are collectively called exit functions. When the exit() function is called, or control transfers out of the main() entry point function, functions registered with atexit() are called (but not  at_quick_exit()). When the quick_exit() function is called, functions registered with at_quick_exit() (but not atexit()) are called. These functions are collectively called exit handlers.  When the _Exit() function is called, no exit handlers or signal handlers are called.

Exit handlers must terminate by returning. It is important and potentially safety-critical for all exit handlers to be allowed to perform their cleanup actions. This is particularly true because the application programmer does not always know about handlers that may have been installed by support libraries. Two specific issues include nested calls to an exit function and terminating a call to an exit handler by invoking longjmp.

A nested call to an exit function is  undefined behavior. (See undefined behavior 182.) This behavior can occur only when an exit function is invoked from an exit handler or when an exit function is called from within a signal handler. (See SIG30-C. Call only asynchronous-safe functions within signal handlers.)

If a call to the longjmp() function is made that would terminate the call to a function registered with atexit(), the behavior is undefined.

Noncompliant Code Example

In this noncompliant code example, the exit1() and exit2() functions are registered by atexit() to perform required cleanup upon program termination. However, if some_condition evaluates to true, exit() is called a second time, resulting in undefined behavior. 

#include <stdlib.h>

void exit1(void) {
  /* ... Cleanup code ... */
  return;
}
 
void exit2(void) {
  extern int some_condition;
  if (some_condition) {
    /* ... More cleanup code ... */
    exit(0);
  }
  return;
}

int main(void) {
  if (atexit(exit1) != 0) {
    /* Handle error */
  }
  if (atexit(exit2) != 0) {
    /* Handle error */
  }
  /* ... Program code ... */
  return 0;
}

Functions registered by the atexit() function are called in the reverse order from which they were registered. Consequently, if exit2() exits in any way other than by returning, exit1() will not be executed. The same may also be true for atexit() handlers installed by support libraries.

Compliant Solution

A function that is registered as an exit handler by atexit() must exit by returning, as in this compliant solution: 

#include <stdlib.h>

void exit1(void) {
  /* ... Cleanup code ... */
  return;
}
 
void exit2(void) {
  extern int some_condition;
  if (some_condition) {
    /* ... More cleanup code ... */
  }
  return;
}

int main(void) {
  if (atexit(exit1) != 0) {
    /* Handle error */
  }
  if (atexit(exit2) != 0) {
    /* Handle error */
  }
  /* ... Program code ... */
  return 0;
}
Noncompliant Code Example

In this noncompliant code example, exit1() is registered by atexit() so that upon program termination, exit1() is called. The exit1() function jumps back to main() to return, with undefined results. 

#include <stdlib.h>
#include <setjmp.h>

jmp_buf env;
int val;

void exit1(void) {
  longjmp(env, 1);
}

int main(void) {
  if (atexit(exit1) != 0) {
    /* Handle error */
  }
  if (setjmp(env) == 0) {
    exit(0);
  } else {
    return 0;
  }
}
Compliant Solution

This compliant solution does not call longjmp() but instead returns from the exit handler normally: 

#include <stdlib.h>

void exit1(void) {
  return;
}

int main(void) {
  if (atexit(exit1) != 0) {
    /* Handle error */
  }
  return 0;
}
Risk Assessment

Terminating a call to an exit handler in any way other than by returning is undefined behavior and may result in abnormal program termination or other unpredictable behavior. It may also prevent other registered handlers from being invoked.

Rule Severity Likelihood Remediation Cost Priority Level
ENV32-C Medium Likely Medium P12 L1
Related Guidelines
Taxonomy Taxonomy item Relationship
CERT C Secure Coding Standard SIG30-C. Call only asynchronous-safe functions within signal handlers Prior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TR 24772:2013 Structured Programming [EWD] Prior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TR 24772:2013 Termination Strategy [REU] Prior to 2018-01-12: CERT: Unspecified Relationship
CWE 2.11 CWE-705, Incorrect Control Flow Scoping 2017-07-10: CERT: Rule subset of CWE
Excerpt from SEI CERT C++ Coding Standard [https://cmu-sei.github.io/secure-coding-standards/sei-cert-c-coding-standard/rules/environment-env/env32-c], Copyright (C) 1995-2026 Carnegie Mellon University. See section 9.4. "3rd-Party Licenses" in the documentation for full details.

Possible Messages

Key

Text

Severity

Disabled

non_normal_return_exit_handler

All exit handlers must return normally.

None

False

Options

exit_functions

exit_functions : set[bauhaus.analysis.config.QualifiedName] = {'exit', 'longjmp', 'quick_exit'}

Functions which should not be used in exit handlers.