Debugging Techniques for C Language Code: Using GDB

Debugging is an essential skill for any programmer, and in C language, the GNU Debugger (GDB) is one of the most commonly used debugging tools. This article will introduce you to how to effectively use GDB to debug C language programs.

What is GDB?

The GNU Debugger (GDB) is a powerful source-level debugging tool that allows you to inspect the data inside a running program. Additionally, GDB enables you to pause execution, inspect variables, adjust memory, and control the flow of program execution.

Installing GDB

On most Linux distributions, you can easily install GDB using the package manager, for example:

sudo apt-get install gdb   # For Debian/Ubuntu-based systems
sudo yum install gdb       # For Fedora/RedHat-based systems

For Windows Users

Windows users can install GDB through MinGW or Cygwin. Additionally, you can use WSL (Windows Subsystem for Linux) to obtain it.

Writing Test Code

First, we will write a simple C language example that will be used to demonstrate how to debug with GDB.

#include <stdio.h>
void buggy_function() {
    int a = 10;
    int b = 0; // Intentionally set to 0 to cause an error
    int result = a / b; // Division by zero will cause a runtime error
    printf("Result is: %d\n", result);
}
int main() {
    printf("Starting the program...\n");
    buggy_function();
    return 0;
}

Please save the above code as debug_example.c.

Compiling the Code

When compiling this file, make sure to enable debugging information by using the -g flag as follows:

gcc -g debug_example.c -o debug_example

This will generate an executable file debug_example with symbol information available for GDB debugging.

Starting GDB

Next, start GDB in the terminal and load the executable file we just generated:

gdb ./debug_example

This command will start GDB and attach it to our program. You will see information similar to the following:

(gdb)

Setting Breakpoints

To set a breakpoint so that execution stops at a specific line, you can use the following command. In this example, we will add a breakpoint before the function call:

break main

Then enter the run command to start program execution:

run

When the program runs to the main function, it will pause and display the following message:

Breakpoint 1, main () at debug_example.c:12
12        buggy_function();

Step Debugging Commands

There are several main commands that can help you step through the code:

next: Steps to the next line of code without entering functions.
step: Steps to the next line of code, entering functions if called.
continue: Resumes execution until the next breakpoint or end.

For example, to step into buggy_function(), you can type:

next

Continue stepping through:

Use step to step into buggy_function().
Use step or next to observe the variables a and b.
When you reach the division statement, you will find that it crashes due to b being zero. At this point, you may want to check the status of all variables:

Assuming you want to see the variable values, you can use:

print a
print b

The two print values help confirm the issue, which is that the divisor is invalid due to b=0!

Viewing the Stack Trace

If you need to view the current call stack information, you can enter the following command, but it will only successfully capture after a crash, summarizing the deeper issues that caused the problem:

backtrace

The output will show the current call location, which helps in the debugging process to rectify and improve design and enhance programming skills!

Exiting GDB

After completing all necessary checks, exit the GDB environment by entering the quit command:

quit

The system will ask if you want to exit, confirm again to exit. Of course, you can also choose to close the terminal window directly!

Conclusion

This article has detailed how to use the GNU Debugger (GDB) to debug C language code, including setting breakpoints, stepping through code, and inspecting variables. Mastering these techniques can significantly enhance your debugging capabilities, making software development smoother and more efficient. I hope everyone can practice more to become proficient in using this valuable tool.