Question

These days i found a blog the mentioned abort function in C.

The following is the source code of abort function:
http://cristi.indefero.net/p/uClibc-cristi/source/tree/0_9_14/libc/stdlib/abort.c

I found that it use the hlt instruction (My PC is x86).

But seems that hlt must run in ring 0.
(refer to wiki http://en.wikipedia.org/wiki/HLT)

It seems that the abort is running in the user space. So the usage of the hlt instruction in abort seems illegal.

BTW, I try to run hlt in linux and windows. But i encounter an error.

In linux:

#include <iostream>
using namespace std;

#define HLT_INST asm("hlt")

int main(){
  cout<<"whill run halt"<<endl;

  HLT_INST; //result in SIGSEGV error
  return 0;
}

In Windows:

cout<<"will run hlg"<<endl;
/*Unhandled exception at 0x0040101d in test_learn.exe: 0xC0000096: Privileged instruction.
*/
__asm{
    hlt;
}

Answer 1

The abort function only uses the hlt instruction after sending SIGABRT fails. If you read the source code, the function first tries to:

raise(SIGABRT);

And then calls the invalid instruction :

/* Still here?  Try to suicide with an illegal instruction */
if (been_there_done_that == 2) {
    been_there_done_that++;
    ABORT_INSTRUCTION;
}

So you are right, hlt requires ring 0 privileges. That’s precisely what makes it an invalid instruction. Executing it will call an invalid-instruction handler, which is in your case (I suppose) SIGSEGV.