GDB - Breakpoints

Learning Outcome

Able to set, view and remove breakpoints using the break, info break and delete commands.

Introduction

Breakpoints are used to halt the execution of the program at a specified point in the code. Once execution is halted, one can inspect variables or use it as a starting point to investigate potential problem sections line by line.

Applicable subjects

COMP1521, COMP2521, COMP3231

break

Place a breakpoint at at a specific line number.

(gdb) break <filename>:<line number>

Place a breakpoint at a function.

(gdb) break <filename>:<function>

Note

If you only have one file, you can omit the ‘<filename>:’

info break

List all breakpoints (including their breakpoint number and where they are in the program).

(gdb) info break

delete

Delete a breakpoint so it no longer stops a program. Use info break to find the number that corresponds to a particular breakpoint.

(gdb) delete <breakpoint number>

---

Example

We know that our factorial.c code from the previous module produces incorrect output. Let’s set a breakpoint in our code, so that we can investigate what our code is doing.

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, 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 printed out both the arguments to the function, and the return value of the function (lines 13 and 15). The arguments are correct but the return value is not and therefore we know that something went wrong inside the factorial function. Now we can use breakpoints to stop the program in this function and take a better look at what is happening.

Recap

In the previous module we compiled the code for use with GDB and started a GDB session. Don’t run it just yet.:

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

The program appears correct when 5 is passed in as a parameter to the factorial function, so we should begin our debugging where the function is called with 17 as the parameter. We should place a breakpoint at line 15 which is where this function is called with this value.:

(gdb) break 15

Note

When you break on a line, it doesn’t execute that line yet.

Next run the program:

(gdb) run

Instead of printing the calculated factorials of 5 and 27, as it did without the breakpoint, the program stops at line 15 and only prints the factorial of 5.

The factorial of 5 is 120.
Breakpoint 1, main () at factorial.c:15
15              f = factorial(n);

In order to investigate what went wrong, we need to learn to view the state (variables) of the program.

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

Date

2020-01-15