Invoking debugger from within a C program

Question

I am reading the book - "C Interfaces and Implementations" by David Hanson. This exercise questions seems interesting and am unable to find a solution:

On some systems, a program can invoke a debugger on itself when it has detected an error. This facility is particularly useful during development, when assertion failures may be common.

Can you provide a short example on how to invoke a debugger.

void handle_seg_fault(int arg)
{
    /* how to invoke debugger from within here */
}

int main()
{
    int *ptr = NULL;
    signal(SIGSEGV, handle_seg_fault);
    /* generate segmentation fault */
    *ptr = 1;
}

Answer 1

I have got this:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <string.h>
#include <errno.h>

void exception_handler(int)
{
    int pid = getpid();
    if (fork() != 0) {
        while(true) sleep(1000);
    }
    char buf[20];
    sprintf(buf, "--pid=%u", pid);
    execlp("gdb", "gdb", buf, NULL);
    exit(-1);
}

int init()
{
    if (signal(SIGSEGV, exception_handler) < 0) {
       return errno;
    }
    if (signal(SIGILL, exception_handler) < 0) {
        return errno;
    }
    if (signal(SIGFPE, exception_handler) < 0) {
        return errno;
    }
    return 0;
}

int main()
{
    if (int rc = init()) {
       fprintf(stderr, "init error: %s\n", strerror(rc));
       return 1;
    }
    *((char*)(0)) = 1;
}

Answer 2

Elaborating on Christian.K's comment, fork()ing a debugger in the face of something like a SIGFPE or SIGSEGV might not be the best of ideas, because...

1. your program is potentially corrupt to begin with, making the fork() unsafe;
2. you might want to use something different for debugging than what the program has hardcoded;
3. the average application user does not know how to handle a debugger;
4. anyone competent with a debugger will usually prefer the coredump over the auto-invoked debugger.

A coredump that I can copy to a testbed beats a debugger instance at my customer's workplace any time of the day. A debugger interface makes for a lousy end-user error message.