QUOTE
The purpose of a debugger such as GDB is to allow you to see what is
going on ‘‘inside’’ another program while it executes—or what another
program was doing at the moment it crashed.
GDB can do four main kinds of things (plus other things in support of
these) to help you catch bugs in the act:
· Start your program, specifying anything that might affect its
behavior.
· Make your program stop on specified conditions.
· Examine what has happened, when your program has stopped.
· Change things in your program, so you can experiment with cor-
recting the effects of one bug and go on to learn about
another.
--GDB Manpage
The first thing you need to do to start debugging your program is to compile it with debugging symbols, this is accomplished with the -g flag:
gcc filename.c -g -o filename
g++ filename.cpp -g -o filename
Lets start with a simple program that gets a line of text from the user, and prints it out backwards to the screen:
CODE
#include <stdio.h>
int main(){
char input[50];
int i=0;
scanf("%s",input);
for(i=strlen(input);i>=0;i--)
{ printf("%c",input[i]);}
printf("\n");
return 0;
}
compile and start the debugger with:
gcc -g debug.c
gdb ./a.out
You should now be in the debugger.
There are 8 main commands that you will mostly be using in your debugging session
1.) break
2.) run
3.) print
4.) next
5.) step
6.) continue
7.) display
8.) where
1.) The Break Command:
gdb will remeber the line numbers of your source file. This will let us easily set up break points in the program. A break point, is a line in the code where you want execution to pause. Once you pause execution you will be able to examine variables, and walk through the program, and other things of that nature.
Continueing with our example lets set up a break point at line 6, just before we declare int i=0;
QUOTE
(gdb) break 6
Breakpoint 1 at 0x80483ec: file debug.c, line 6.
(gdb)
2.) The Run Command
Run will begin inital execution of your program. This will run your program as you normally would outside of the debugger, until it reaches a break point line.
At this moment, you will have been returned to the gdb command prompt.
(Using run again after your prgram has been started, will ask to terminate the current execution and start over)
From our example:
QUOTE
(gdb) run
Starting program: /u/khan/tmp/a.out
Breakpoint 1, main () at debug.c:6
6 int i=0;
(gdb)
3.)The Print Command
Print will let you see the values of data in your program. It takes an argument of the variable name.
In our example, we are paused right before we declare and intitalize i. Lets look what the value of i is now:
QUOTE
(gdb) print i
$1 = -1075457232
(gdb)
i contains junk, we havent put anything into it yet.
4. & 5.) Next and Step
Next and Step do basically the same thing, step line by line through the program. The difference is that next steps over a function call, and step will step into it.
Now in our example, we will step to the beginning of the next instruction
QUOTE
(gdb) step
8 scanf("%s",input);
(gdb)
before we execute the scanf, lets check the value of i again:
QUOTE
(gdb) print i
$2 = 0
(gdb)
i is now equal to 0, like it should be.
Now lets use next to move into the scanf statement:
QUOTE
(gdb) next
What happened here? We werent returned to the gdb prompt. Well the program is inside scanf, waiting for us to input something.
Input string here, and press enter:
6.) The Continue Command
Continue will pick up execution of the program after it has reached a break point.
Lets continue to the end of the program now:
QUOTE
(gdb) continue
Continuing.
tupni
Program exited normally.
(gdb)
Here we've reached the end of our program, you can see that it printed in resevese "input" which was what I fed it in scanf.
7.) The Display Command
display will show a vaiables contents each step of the way in your program. Lets start over in our example. Delete the breakpoint at line 6
QUOTE
(gdb) del break 1
This deletes our first breakpoint at line 6.
Now lets set a new breakpoint at line 11, the printf statement inside the for loop
QUOTE
(gdb) break 11
Breakpoint 3 at 0x8048421: file debug.c, line 11.
(gdb)
Run the program again, and enter the input, when it returns to the gdb command prompt we will display input[i] and watch it through the for loop
QUOTE
Breakpoint 3, main () at debug.c:11
11 { printf("%c",input[i]);}
(gdb) display input[i]
1: input[i] = 0 '\0'
(gdb) next
10 for(i=strlen(input);i>=0;i--)
1: input[i] = 0 '\0'
(gdb) next
Breakpoint 3, main () at debug.c:11
11 { printf("%c",input[i]);}
1: input[i] = 116 't'
(gdb) next
10 for(i=strlen(input);i>=0;i--)
1: input[i] = 116 't'
(gdb) next
Breakpoint 3, main () at debug.c:11
11 { printf("%c",input[i]);}
1: input[i] = 117 'u'
(gdb) next
10 for(i=strlen(input);i>=0;i--)
1: input[i] = 117 'u'
(gdb) next
Breakpoint 3, main () at debug.c:11
11 { printf("%c",input[i]);}
1: input[i] = 112 'p'
(gdb) next
10 for(i=strlen(input);i>=0;i--)
1: input[i] = 112 'p'
(gdb) next
Breakpoint 3, main () at debug.c:11
11 { printf("%c",input[i]);}
1: input[i] = 110 'n'
(gdb) next
10 for(i=strlen(input);i>=0;i--)
1: input[i] = 110 'n'
(gdb) next
Breakpoint 3, main () at debug.c:11
11 { printf("%c",input[i]);}
1: input[i] = 105 'i'
(gdb) next
10 for(i=strlen(input);i>=0;i--)
1: input[i] = 105 'i'
(gdb) next
12 printf("\n");
1: input[i] = -1 '\uffff'
(gdb)
Here we stepped through the loop, always looking at what input[i] was equal to.
9.)The Where Command
The where command prints a backtrace of all stack frames. This may not make much since but it is useful in seeing where our program crashes.
Lets modify our program just a little so that it will crash:
CODE
#include <stdio.h>
int main(){
char *input;
int i=0;
scanf("%s",input);
for(i=strlen(input);i>=0;i--)
{ printf("%c",input[i]);}
printf("\n");
return 0;
}
Here we've changed input to be a pointer to a char.
Recompile and run gdb on it again.
and see what happens when it crashes
QUOTE
(gdb) r
Starting program: /u/khan/tmp/a.out
AAA
Program received signal SIGSEGV, Segmentation fault.
0x009ef267 in _IO_vfscanf_internal () from /lib/tls/libc.so.6
(gdb) where
#0 0x009ef267 in _IO_vfscanf_internal () from /lib/tls/libc.so.6
#1 0x009f3808 in scanf () from /lib/tls/libc.so.6
#2 0x08048403 in main () at debug.c:8
(gdb)
We see at the bottom, three frames,
#2, the top most shows where we crashed at.
use the up command to move up the stack
QUOTE
(gdb) up
#1 0x009f3808 in scanf () from /lib/tls/libc.so.6
(gdb) up
#2 0x08048403 in main () at debug.c:8
8 scanf("%s",input);
(gdb)
Here we see line #8
8 scanf("%s",input);
The line we crashed at.
These are the basics to gdb, you can now find where a program crashed, watch variables, and step through line by line of your program.
For more info check out the info page
info gdb