変な動作

クイズ：なんでこんな動作をするのだろう？ （これはイタズラコード； インテル32ビットCPU + gcc）

/* trick.c */
void bar()

{

  printf("Hi, this is bar.\n");

  printf("... byebye from bar.\n");

  exit(0);

}
unsigned _bar = (unsigned)&bar;
void foo(int arg) {

  unsigned *p = (unsigned *)(&arg - 1); // IA32 + gcc

  *p = _bar;

  printf("This is foo.\n");

}
main()

{

  printf("This is main.\n");

  foo(0);

  printf("Byebye from main.\n");

}

イタズラの悪のり：

/* countdown.c */
void countdown(int *);

void mark(int);
main(int argc)

{

  int c = 10;

  mark(0);

  printf(" %2d\n", c);

  countdown(&argc);

}
unsigned _mark;
void mark(int arg)

{

  _mark = *((unsigned *)(&arg - 1));

}  
void countdown(int *pargc)

{

  unsigned *p = (unsigned *)(&pargc - 1); // IA32 + gcc

  *p = _mark;
  int *pc = pargc - 3;

  if (*pc <= 0) {

    printf("BANG!\n");

    exit(0);

  }

  (*pc)--;

}