ENV32-C. Do not call the exit() function more than once

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

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

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

void exit1(void) {
   if(<expr>) {
      /* ...cleanup code... */
      exit(0);
   }
}

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

Compliant Code

_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]]:

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.

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

void exit1(void) {
   if(<expr>) {
      /* ...cleanup code... */
      _Exit(0);
   }
}

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.

References

[[ISO/IEC 9899-1999]] Section 7.20.4.3, "The exit function"