GDB - Debugging Example2

Let us write another program that will cause a core dump due to non-initialized memory.

#include <iostream>  
using namespace std; 

void setint(int*, int); 
int main() 
{ 
   int a; 
   setint(&a, 10); 
   cout << a << endl; 
   
   int* b; 
   setint(b, 10); 
   cout << *b << endl; 
   
   return 0; 
} 

void setint(int* ip, int i)
{
   *ip = i; 
}

To enable debugging, the program must be compiled with the -g option.

$g++ -g crash.cc -o crash 

NOTE: We are using g++ compiler because we have used C++ source code.

When you run this program on your linux machine, it will produce the following result:

segmentation fault (core dumped) 

Now let us debug it using gdb:

$ gdb crash 
(gdb) r 
Starting program: /home/tmp/crash 
10 
10 
Program received signal SIGSEGV, Segmentation fault. 
0x4000b4d9 in _dl_fini () from /lib/ld-linux.so.2 

(gdb) where 
#0  0x4000b4d9 in _dl_fini () from /lib/ld-linux.so.2 
#1  0x40132a12 in exit () from /lib/libc.so.6 
#2  0x4011cdc6 in __libc_start_main () from /lib/libc.so.6 

#3  0x080485f1 in _start () 
(gdb) 

Unfortunately, the program will not crash in either of the user-defined functions, main or setint, so there is no useful trace or local variable information. In this case, it may be more useful to single-step through the program.

(gdb) b main 
# Set a breakpoint at the beginning of the function main 

(gdb) r 
# Run the program, but break immediately due to the breakpoint. 

(gdb) n 
# n = next, runs one line of the program 

(gdb) n 
(gdb) s 
setint(int*, int) (ip=0x400143e0, i=10) at crash2.C:20 
# s = step, is like next, but it will step into functions. 
# In this case the function stepped into is setint. 

(gdb) p ip 
$3 = (int *) 0x400143e0 

(gdb) p *ip 
1073827128 

The value of *ip is the value of the integer pointed to by ip. In this case, it is an unusual value and is strong evidence that there is a problem. The problem in this case is that the pointer was never properly initialized, so it is pointing to some random area in memory (the address 0x40014e0). By pure luck, the process of assigning a value to *ip does not crash the program, but it creates some problem that crashes the program when it finishes.