GDB - Basic Use

Learning Outcome

Be able to compile programs that are suitable for debugging with GDB, start a GDB session, run the program and quit the session.

Introduction

In order to use any GDB commands covered in this tutorial, the program must first be compiled with the -g flag so that the generated executable contains the debugging information GDB requires to interpret the content.

Once an executable is compiled for debugging, it can be started inside a GDB session, and various GDB debugging commands can then be applied.

Applicable subjects

COMP1521, COMP2521

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Compile for GDB

In order to use GDB, files must be compiled with a -g flag using either the dcc or gcc C compilers:

$ dcc -g -o <file> <file.c>
$ gcc -g <any other flags e.g. -Wall> -o <file> <file.c>

Start a GDB Session

Start a debugging session for a program using GDB:

$ gdb <program_name>

run

Run the program while in GDB.

(gdb) run

If you have command line arguments, instead run with:

(gdb) run <arg1> <arg2> ... <arg n>

If you want to use a file for input, you can run with:

(gdb) run < <data>

quit

Stop the debugging session.

(gdb) quit

Note

GDB commands often have shortcuts. For example, run becomes r and quit becomes q.

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Example

In this example we will explore the basic functionality of GDB by compiling factorial.c, starting a GDB session for the executable produced, running the program and quitting the session. We will be using factorial.c for the next few modules, and by the end of the breakpoints, viewing data, and navigating your program modules, we will have found the bug in the program.

factorial.c

factorial.c

//This program calculates and prints out the factorials of 5 and 17


#include <stdio.h>
#include <stdlib.h>

int factorial(int n);

int main(void) {
	
	int n = 5;
	int f = factorial(n);
	printf("The factorial of %d is %d.\n", n, f);
	n = 17;
	f = factorial(n);
	printf("The factorial of %d is %d.\n", n, f);

	return 0;
		
}
//A factorial is calculated by n! = n * (n - 1) * (n - 2) * ... * 1
//E.g. 5! = 5 * 4 * 3 * 2 * 1 = 120
int factorial(int n) {
	int f = 1;
	int i = 1;
	while (i <= n) {
		f = f * i;
		i++;
	}
	return f;	
}

When we run the above code without gdb, the following output is given:

The factorial of 5 is 120.
The factorial of 17 is -288522240.

We know that the value of 17! should be 355,687,428,096,000 not -288522240, so something has gone amiss! We can use gdb to find out what went wrong.

To compile this code for gdb, you can use either gcc or dcc. In this module, we will be using gcc as dcc has deeper code analysis and diagnostics so that in the case of this program, dcc will tell us exactly where it the problem is (this is why COMP1511 students are encouraged to use dcc).

Compile with gcc:

$ gcc -Wall -g -o factorial factorial.c

Start a gdb session for factorial:

$ gdb factorial

Run the code within gdb:

(gdb) run

The program will run as it usually does and gives output similar to the following:

Starting program: code/factorial
The factorial of 5 is 120.
The factorial of 17 is -288522240.
[Inferior 1 (process 96) exited normally]

Even though the program has finished, we are still in GDB. Exit your debugging session using:

(gdb) quit

Note that GDB has not yet helped to find the problem with the code. To inspect the programs execution more closely, we need to use breakpoints to stop the program before it ends.

Module author: Liz Willer <e.willer@unsw.edu.au>

Date

2020-01-15