GNU Debugger (GDB)

The GNU Project Debugger (GDB)

Installation

Windows

On Windows, gdb is a part of MinGW. See MinGW for more information.

Ubuntu

GDB comes installed by default on most Unix-like systems, but if it's not installed, you can run the following:

sudo apt install gdb

Usage

This section defines simple usage of GDB. It is by no means comprehensive, but it should be sufficient for a fair amount of debugging scenarios.

1. Compile
   You would compile your file as per usual, but attach the g flag:

   $ cc -g main.c

2. Launch To launch gdb, run the following, where "a.out" is the output of the compile command above
3. Configure Debug See below for useful debugging commands such as adding breakpoints, stepping through functions or printing the values of variables
4. Run Run the program with your added breakpoints by typing the "run" command.
5. Finish To close gdb, you can enter "q"

GDB Commands

Some useful gdb commands include:

General
help	Shows details about commands
Controlling Execution flow
break <line_number>	Adds a breakpoint at the given line number. Other options include break <file_name:line_number> break <file_name:function_name>
run	Executes the program in gdb
next	Debugger will execute the next line as single instruction. This includes whole functions Can be abbreviated to n.
step	Runs a single line of code. Will enter into a called function, unlike next. Can be abbreviated to s.
continue	Continues execution until the next breakpoint. Can be abbreviated to c.
finish	If you're in a function which returns at some point, you can use the finish command to execute the program until the end of the function. It's equivalent to setting a breakpoint at the return line of a function, and then letting the program run.
Gathering information
print <variable>	prints the value of a specified variable. Can be abbreviated to p.
info variables	Will give information on all variables, including some that you haven't defined
info locals	Will give information on variables in the current scope
info args	Will return information on the arguments passed
Dealing with Pointers
print *<pointer>	Prints what the pointer is pointing to
print &<variable>	Displays the address of the variable

gdb for openOCD

See the openOCD page for more details on using it. Here are some additional gdb commands that might be useful when debugging on hardware:

monitor <command>	[asses <command> directly to openOCD. Allows openOCD commands to be passed through gdb
info registers	prints the names and values of the CPU registers
info registers rX	Prints the value of the specifed register rX, where X is a number of a register (0-15 in ARM).
set $rX=val	sets register X (O-15 in ARM) to the value val
x/format address	Reads the contents of address and displays it according to format. See this link for more.