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


void exit1(void) {
if(expr)
       exitif(<expr>); {
       //clean up and ready program for closing* ...cleanup code... */
      exit(0);
     ...}
}

int main (void) {

    atexit(exit1);
    /* ...
program     //program code
 code... */
    exit();
     return 00);
}

Compliant 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().

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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. The value status is returned to the parent process as the process's exit status, and can be collected using one of the wait family of calls. The function _Exit is equivalent to _exit.

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

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

void exit1(void) {
if(expr)
       _Exit();if(<expr>) {
       //clean up and ready program for closing* ...cleanup code... */
      _Exit(0);
     ...}
}

int main (void) {

    atexit(exit1);
    /* ...program    
     //program codecode... */
     exit(0);
     return 0;
}

The call to _Exit() will immediately terminate the program and no undefined behavior will happen like in the non compliant example.

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