GDB - Viewing Data

Learning Outcome

Able to inspect variables (program state) in GDB using the print and info locals commands.

Introduction

In order to check where the program stops behaving as it should, GDB allows you to check that the values of the variables are as you expect. Commands such as print and info locals allow us to do this.

GDB commands allow us to view the state of the program without have to insert printf C code statements and recompile the program itself, which can be much more time consuming. Also, sometimes printf statements alter the behaviour of the program to hide the bug.

Applicable subjects

COMP1521, COMP2521, COMP3231

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print

View simple data including strings, integers, structs, or a statically declared arrays (1D or 2D).

(gdb) print <variable_name>

Printing a dynamically allocated array, linked lists and various other data structures is somewhat more difficult and is covered in the debugging data structures section.

info locals

Print all local variables excluding the arguments passed into the function.

(gdb) info locals

info args

Print the arguments passed into the function.

(gdb) info args

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Example

So far we have used GDB to set breakpoints and run our program, but when debugging, we want to be able to view the state of the program. We can use print and info locals to print out the state of our factorial.c program as it runs.

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.

Recap

In the previous modules we compiled the code for use with GDB, started a gdb session, set a breakpoint on line 15 and ran the code using:

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

(gdb) break 15
(gdb) run

The program will stop on line 15, which means the function hasn’t been called yet.

At this stage of execution, we have 2 variables we can look at: f and n. Let’s print them out.:

(gdb) print f
$1 = 120
(gdb) print n
$2 = 17

Alternatively we could get the same information using info locals

(gdb) info locals
n = 17
f = 120

Since we have already printed these values in the normal execution of the program, this doesn’t help us much. In order to print out any useful values we need to learn how to move through our program in gdb.

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

Date

2020-01-15