Page History

The C99 exit() function is used for normal program termination. Nested calls to exit() result in undefined behavior. This most frequently occurs when multiple functions are registered with atexit().

Non-Compliant Code Example

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 So that it might perform cleanup upon program termination, exit1() is registered by atexit(). If <expr> evaluates to true, exit() will be is called a second time, resulting in undefined behavior.

#include <stdio.h>
#include <stdlib.h>

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

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

Compliant Solution

_Exit() and abort() will both immediately halt program execution, and may be used within functions registered by atexit().

According to C99, \[[ISO/IEC 9899-1999:TC2|AA. C References#ISO/IEC 9899-1999TC2]\]:

The _Exit function causes normal program termination to occur and control to be returned to the host environment. No functions registered by the atexit function or signal handlers registered by the signal function are called. The status returned to the host environment is determined in the same way as for the exit function. Whether open streams with unwritten buffered data are flushed, open streams are closed, or temporary files are removed is implementation-defined. The _Exit function cannot return to its caller.

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;

#include <stdio.h>
#include <stdlib.h>

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

int main(void) {
  if (atexit(/* condition */exit1) != 0) {
      /* ...cleanup code...Handle error */
  }
  if (setjmp(env) == 0) {
   _Exit 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) {
    /* ...program code...Handle error */
  }
  return exit(0);
}

Risk Assessment

Multiple calls Terminating a call to exit() are unlikely, and at worst will only cause denial of service attacks or abnormal program terminationan 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.

Automated Detection

Related Vulnerabilities

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

References

Related Guidelines

Key here (explains table format and definitions)

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-705 and ENV32-C

CWE-705 = Union( ENV32-C, list) where list =

• Improper control flow besides a non-returning exit handler

...

Image Added Image Added Image Added Wiki Markup\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 7.20.4.3, "The {{exit}} function"