Question

I am using Mac OS 10.6.5, g++ 4.2.1. And meet problem with following code:

#include <iostream>
#include <sys/signal.h>
using namespace std;

void segfault_handler(int signum)
{
    cout << "segfault caught!!!\n";
}

int main()
{
    signal(SIGSEGV, segfault_handler);

    int* p = 0;
    *p = 100;

    return 1;
}

It seems the segfault_handler is called infinitely and keep on print:

segfault caught!!!
segfault caught!!!
segfault caught!!!
…

I am new to Mac development, do you have any idea on what happened?

Answer 1

This is because after your signal handler executes, the EIP is back to the instruction which causes the SIGSEGV – so it executes again, and SIGSEGV is raised again.

Usually ignoring SIGSEGV like you do is meaningless anyway – suppose the instruction actually read some value from a pointer to a register, what would you do? You don’t have any ‘correct’ value to put in the register, so the following code will likely SIGSEGV again or, worse, trigger some logic error.

You should either exit the process when SIGSEGV happens, or return to a known safe point – longjmp should work, if you know that this is indeed the safe point (the only possible example that comes to mind is VM interpreters/JITs).