GDB (Step by Step Introduction) - GeeksforGeeks (2024)

GDB stands for GNU Project Debugger and is a powerful debugging tool for C (along with other languages like C++). It helps you to poke around inside your C programs while they are executing and also allows you to see what exactly happens when your program crashes. GDB operates on executable files which are binary files produced by the compilation process.

For demo purposes, the example below is executed on a Linux machine with the below specs.

uname -a

Let’s learn by doing: –

Start GDB

Go to your Linux command prompt and type “gdb”.

gdb

Gdb open prompt lets you know that it is ready for commands. To exit out of gdb, type quit or q.

Compile the code

Below is a program that shows undefined behavior when compiled using C99.

Note: If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate, where the indeterminate value is either an unspecified value or a trap representation.

Now compile the code. (here test.c). g flag means you can see the proper names of variables and functions in your stack frames, get line numbers and see the source as you step around in the executable. -std=C99 flag implies use standard C99 to compile the code. -o flag writes the build output to an output file.

gcc -std=c99 -g -o test test.C

Run GDB with the generated executable

Type the following command to start GDB with the compiled executable.

gdb ./test

Useful GDB commands:

Here are a few useful commands to get started with GDB.

Display the code

Now, type “l” at gdb prompt to display the code.

Set a breakpoint

Let’s introduce a break point, say line 5.

If you want to put breakpoint at different lines, you can type “b line_number“.By default “list or l” display only first 10 lines.

View breakpoints

In order to see the breakpoints, type “info b”.

Disable a breakpoint

Having done the above, let’s say you changed your mind and you want to revert. Type “disable b”.

Re-enable a disabled breakpoint

As marked in the blue circle, Enb becomes n for disabled. 9. To re-enable the recent disabled breakpoint. Type “enable b”.

Run the code

Run the code by typing “run or r”.If you haven’t set any breakpoints, the run command will simply execute the full program.

Print variable values

To see the value of variable, type “print variable_name or p variable_name“.

The above shows the values stored at x at time of execution.

Change variable values

To change the value of variable in gdb and continue execution with changed value, type “set variable_name“.

Debugging output

Below screenshot shows the values of variables from which it’s quite understandable the reason why we got a garbage value as output. At every execution of ./test we will be receiving a different output.

Exercise: Try using set x = 0 in gdb at first run and see the output of c.

GDB offers many more ways to debug and understand your code like examining stack, memory, threads, manipulating the program, etc. I hope the above example helps you get started with gdb.

Conclusion

In this article we have discussed GDB (GNU Debugger) which is a powerful tool in Linux used for debugging C programs. We have discussed some of the following steps so that we can compile your code with debugging information, run GDB, set breakpoint, examine variables, and analyze program behavior. We have also discussed GDB’s features, such as code examination, breakpoint management, variable manipulation, and program execution control which allow us to efficiently debug and issue resolution.

GDB (Step by Step Introduction) – FAQs

How to go step by step in GDB?

To step through a program in GDB, you can use the following commands:

• step (or s): Step into functions.
• next (or n): Step over functions.
• continue (or c): Continue execution until the next breakpoint.

Example:

gdb ./myprogram
(gdb) break main
(gdb) run
(gdb) step
(gdb) next

What is the overview of GDB?

GDB (GNU Debugger) is a powerful debugging tool for C, C++, and other programming languages. It allows developers to see what is happening inside their programs while they are running or what the program was doing at the moment it crashed. GDB provides features like breakpoints, stepping through code, inspecting variables, and changing program execution flow.

How to start a process in GDB?

To start a process in GDB, follow these steps:

1. Compile your program with the -g flag to include debugging information.
2. Start GDB with your program.
3. Run the program inside GDB.

gcc -g -o myprogram myprogram.c

gdb ./myprogram

(gdb) run

How to program with GDB?

To debug a program with GDB:

1. Compile the program with debugging information.
2. Start GDB.
3. Set breakpoints, run the program, and use debugging commands.

gcc -g -o myprogram myprogram.c

gdb ./myprogram

(gdb) break main
(gdb) run
(gdb) step
(gdb) next
(gdb) print variable

How to generate core file in GDB?

To generate a core file in GDB:

1. Enable core dumps in your shell session.
2. Run your program until it crashes.
3. A core file (core or core.<pid>) will be generated in the current directory.
4. Load the core file into GDB for analysis.

ulimit -c unlimited

./myprogram

gdb ./myprogram core