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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 _exitExit() or abort().

From the man page of _exit<include _Exit() :
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.description>

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

#define exitearly 1

void exit1(void)
{
  printf("Exit second.\n");
}

void exit2 (void)
{
  printf("Exit first.\n");
  if (exitearly) {
     _exitExit(1);
  }
}

int main (void) {

  if (expr) {
    atexit(exit1);
    atexit(exit2);
    exit(1);
  }
  else {
    exit2();
  }

  return 0;
}

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