rhaps0dy/howto.md

Ever had those annoying SEGMENTATION FAULT errors? Ever screamed in frustration because you weren't able to find their cause? Don't worry, GDB to the rescue!

#A faulty program We have this C program: (prog1.c)

#include <stdio.h>

int
main()
{
	char buf[16], *c;
	
	puts("Input some string!");
	scanf("%s", buf);
	for(c=buf; *c; c++); /* go to the end of the string */
	puts("Here is your reversed string:");
	while(c!=buf) {
		c--;
		putchar(*c);
	}
	return 0;
}

It's a pretty straightforward case. This program reverses strings, but if we input a string longer than 15 characters (the last position is a null character) we're overstepping the buffer boundaries. If the string is long enough, we overstep the program's boundaries and cause a segmentation fault.

Let's compile this and run it a few times:

$ gcc prog1.c
$ ./a.out
Input some string!
blargh
Here is your reversed string:
hgralb
$ ./a.out
Input some string!
thisstringisverylonghasmorethan16letters
Here is your reversed string:
ÐW¸msahgnolyrevsignirtssiht
Segmentation fault

Oops, what happened? The program received SIGSEGV? Time to use gdb!

#How to find the bug First, we compile the program with debug symbols

$ gcc -g prog1.c

and then run it through gdb:

$ gdb ./a.out
[...]
Reading symbols from ./a.out...done.
(gdb) run
Starting program: $(pwd)/a.out
Input some string!
124567890124567890124567890124567890124567890124567890124567890124567890124567890124567890124567890124567890124567890124567890124567890124567890124567890124567890124567890124567890124567890124567890124567890124567890
Here is your reversed string:
�ÿÿÿÝÈ765421098765421098765421

Program received signal SIGSEGV, Segmentation fault.
0x00000000004006ae in main () at prog1.c:18
18	}

	#include <stdio.h>

	int
	main()
	{
	char buf[16], *c;

	puts("Input some string!");
	scanf("%s", buf);
	for(c=buf; c; c++); / go to the end of the string */
	puts("Here is your reversed string:");
	while(c!=buf) {
	c--;
	putchar(*c);
	}
	return 0;
	}