Page History

The C99 exit() function is used for normal program termination. Nested calls to exit() result in undefined behavior. This can only occur when exit() is invoked from a function registered with atexit().

Non-Compliant Code Example

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

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

void exit1(void) {
   if (/* condition */) {
      /* ...cleanup code... */
      exit(0);
   }
}

int main (void) {
    atexit(exit1);
    /* ...program code... */
    exit(0);
}

Compliant Solution

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

...

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

void exit1(void) {
   if (/* condition */) {
      /* ...cleanup code... */
  }
    _Exit(0)return;
   }
}

int main (void) {
    atexit(exit1);
    /* ...program code... */
    exit(0);
}

Risk Assessment

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

Related Vulnerabilities

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

References

\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\]  Section 7.20.4.3, "The {{exit}} function"
\[[ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\] "EWD Structured Programming"

...