Using gdb

Compilation

The program needs to be compiled with debugging information so that gdb can use it. -g flag should be specified.

gcc -g source_file.c -o executable_file

Loading Executables

You can open gdb with the executable as a command line option: gdb executable_file, or you can open gdb first and then specify which executable you want to load using the file command:

• open gdb and then

• file executable_file

General Commands

run - runs the program normally.

print variable_name - to print the value of the variable.

continue - to continue execution wheh gdb halts at a break point.

Break Points

• Add unconditional break point:

  • break line_number
  • break function_name

• Add conditional break point:

  • break line_number if some_valid_c_condition eg. break 7 if i == 0

• Adding condition to existing unconditional break points:

  • condition break_point_number condition eg: condition 7 if value == 10

• Removing condition from a break point:

  • Conditions can be removed from an exsisting breakpoint by condition break_point_number

• Printing Break Points:

  • info break will print all the break points. This will print all the watch points (special break points).

• Enabling / Disabling Break Points:

  • disable break_point_number
  • enable break_point_number

Watch Point

Watch point is a special type of break point. In case of break point we give the line number to break. However in case of watch point you have an expression. When the expression is true the program execution halts.

watch condition eg watch i == 0

You can use both info watch and info break to see the available watch points.

info breakwill show both break points and watch points.

Printing value of variables

info locals

print variable_name

Step / Next

• step command steps into a function.
• next command steps over the function, i.e. next doesn't gets in the function. It will treat the function as a single line and the execution will reach the next line after the function call.

Core Dumps

Load the core dumps with:

gdb executable_file core_file or gdb executable_file -c core_file.

Now you can use the usual commands to debug the matter.

By default core dumps might not be generated in your system. So you will have to enable them manually using ulimit -c unlimited, refer this stack overflow link

Stack Trace

Each function and its variables are assigned frames. The current function will be assigned frame 0 which is also known as the bottom frame.

• bt to print the stack - called as back trace

• frame to check in which frame you are

• frame frame_number to go to frame number say 1.

• up to move up the stack frame

• down to move down the stack frame

You can use all the usual commands that you would use to roam around.

eg. info locals, print, info args, list

Misc

• You can invoke shell commands from gdb. shell command_name eg. shell date
• You can also alter the value of a variable using the set command. set variable_name = value

Quit

To quit from gdb type: quit or just q and hit return.

References

Linux Software Debugging with gdb