GDB (Step by Step Introduction)

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. [caption id="attachment_646191" align="alignnone"]

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.