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The C99 exit() function is used for normal program termination (see ERR04-AC. Choose an appropriate termination strategy). Nested calls to exit() result in undefined behavior. This can only occur when exit() is invoked from a function registered with atexit(), or when exit() 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.

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Noncompliant Code Example

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

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

void exit1(void) {
  /* ...cleanup code... */
  return;
}

void exit2(void) {
  if (/* condition */) {
    /* ...more cleanup code... */
  }
  return;
}

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

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Noncompliant Code Example

The function exit1() is registered by atexit(), so upon program termination, exit1() is called. Execution will jump back to main() and return, with undefined results.

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