longjmp.c

void longjmp(jmp_buf buf, int ret_val)
{
  unsigned *esp;

  /* make sure return value is not 0 */
  if(ret_val == 0)
    ret_val++;
  /* EAX is used for return values, so store it in jmp_buf.EAX */
  buf->eax = ret_val;
  /* get ESP for new stack */
  esp = (unsigned *)buf->esp;
  /* push EFLAGS on the new stack */
  esp--;
  *esp = buf->eflags;
  /* push current CS on the new stack */
  esp--;
  __asm__ __volatile__(
      "mov %%cs,%0\n"
      : "=m"(*esp));
  /* push EIP on the new stack */
  esp--;
  *esp = buf->eip;
  /* new ESP is 12 bytes lower; update jmp_buf.ESP */
  buf->esp = (unsigned)esp;
  /* now, briefly, make the jmp_buf struct our stack */
  __asm__ __volatile__(
      "movl %0,%%esp\n"
      /* ESP now points to 8 general-purpose registers stored in jmp_buf
         Pop them */
      "popa\n"
      /* load new stack pointer from jmp_buf */
      "movl -20(%%esp),%%esp\n"
      /* ESP now points to new stack, with the IRET frame (EIP, CS, EFLAGS)
         we created just above. Pop these registers: */
      "iret\n"
      :
      : "m"(buf));
}