How to use gdb for c?

In this article, let us discuss how to debug a c program using gdb debugger in 6 simple steps.

To learn C program debugging, let us create the following C program that calculates and prints the factorial of a number. However this C program contains some errors in it for our debugging purpose.

Let us debug it while reviewing the most useful commands in gdb.

Compile your C program with -g option. This allows the compiler to collect the debugging information.

Note: The above command creates a.out file which will be used for debugging as shown below.

Launch the C debugger (gdb) as shown below.

Other formats:

Places break point in the C program, where you suspect errors. While executing the program, the debugger will stop at the break point, and gives you the prompt to debug.

So before starting up the program, let us place the following break point in our program.

You can start running the program using the run command in the gdb debugger. You can also give command line arguments to the program via run args. The example program we used here does not requires any command line arguments so let us give run, and start the program execution.

Once you executed the C program, it would execute until the first break point, and give you the prompt for debugging.

You can use various gdb commands to debug the C program as explained in the sections below.

As you see above, in the factorial.c, we have not initialized the variable j. So, it gets garbage value resulting in a big numbers as factorial values.

Fix this issue by initializing variable j with 1, compile the C program and execute it again.

Even after this fix there seems to be some problem in the factorial.c program, as it still gives wrong factorial value.

So, place the break point in 10th line, and continue as explained in the next section.

There are three kind of gdb operations you can choose when the program stops at a break point. They are continuing until the next break point, stepping in, or stepping over the next program lines.

By continuing or stepping through you could have found that the issue is because we have not used the <= in the ‘for loop’ condition checking. So changing that from < to <= will solve the issue.

Use following shortcuts for most of the frequent gdb operations.

• Write a sample C program with errors for debugging purpose.
• Compile the C program with debugging option -g.
• Launch gdb.
• Set up a break point inside C program.
• Execute the C program in gdb debugger.
• Printing the variable values inside gdb debugger.