ENV32-C. No atexit handler should terminate in any way other than by returning

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().

No atexit() handler should terminate in any way other than by returning. It is important and potentially safety-critical for all the atexit() handlers to be allowed to perform their cleanup actions. This is particularly true because the main program doesn't always know about handlers that may have been installed by support libraries.

Non-Compliant Code Example

In this non-compliant code example, the exit1() is 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.

#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

A function that is registered as an exit handler by atexit() must exit by returning, and not in any other manner.

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

void exit1(void) {
  if (/* condition */) {
    /* ...cleanup code... */
  }
  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]] Section 7.20.4.3, "The exit function"
[[ISO/IEC PDTR 24772]] "EWD Structured Programming"

      10. Environment (ENV)       ENV33-C. Do not call the longjmp function to terminate a call to a function registered by atexit()