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The C99 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 used for normal program termination. If more than one call to exit() is executed by a program, the behavior is undefined. This may occur when functions are registered with atexit(), a function that causes the functions registered to it to be called with when the program exits. If a function called as a result of being registered with atexit() has an exit() call in it there is undefined behavior.

Non-Compliant Code Example

In this code as the main function exits two functions registered with atexit() are called. Given the define statement exitearly=1 the program calls exit within the first function called in the atexit() cycle. The behavior that follows is undefined: some implementations will ignore the second call to exit and just continue calling the atexit functions in the appropriate order. Other implementations will enter an infinite loop with the atexit functions being called repeatedly.

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>

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

#define exitearly 1

void exit1(void) {
  /* ... Cleanup puts("Exit second.\n")code ... */
  return;
}
 
void exit2 (void) {
  extern puts("Exit first.\n")int some_condition;
  if (exitearlysome_condition) {
    /* ... More cleanup code ... */
    exit(10);
  }
  return;
}

int main (void) {
  if (atexit(exit1) != 0) {
    /* Handle error (exit1);
  atexit (exit2);
  puts("In main\n");
  exit(1);
*/
  }
  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

...

To have functionality where the program can quit from within a function registered by at_exit() it is necessary to use a function used for abnormal termination such as _exit() or abort().

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 resultsFrom the man page of _exit():
The function _exit terminates the calling process "immediately". Any open file descriptors belonging to the process are closed; any children of the process are inherited by process 1, init, and the process's parent is sent a SIGCHLD signal.

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

#define exitearly 1jmp_buf env;
int val;

void exit1(void)
 {
  printflongjmp("Exit second.\n"env, 1);

}

voidint exit2 main(void)
 {
  if printf("Exit first.\n");(atexit(exit1) != 0) {
    /* Handle error */
  }
  if (setjmp(exitearlyenv)
 == 0) {
     _exit(10);
  } else {
    return 0;
  }

int main ()
}

Compliant Solution

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

#include <stdlib.h>

void exit1(void) {
  atexit (exit1);return;
}

int main(void) {
  if (atexit (exit2);
  printf("In main\n");
  exit(1);

(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.

References

Automated Detection

Related Vulnerabilities

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

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

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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"