Editing GNU Debugger (GDB)

[[Category:Software]]
The GNU Project Debugger (GDB)

= Installation =
== Ubuntu ==
GDB comes installed by default on most Unix-like systems, but if it's not installed, you can run the following:
<syntaxhighlight lang="Bash">
sudo apt install gdb
</syntaxhighlight>

= 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.

<ol>
<li>Compile <br>
You would compile your file as per usual, but attach the g flag:
<pre>$ cc -g main.c</pre>
</li>
<li>Launch
To launch gdb, run the following, where "a.out" is the output of the compile command above
</li>
<li> Configure Debug
See below for useful debugging commands such as adding breakpoints, stepping through functions or printing the values of variables
</li>
<li>Run
Run the program with your added breakpoints by typing the "run" command.
</li>
<li>Finish
To close gdb, you can enter "q"
</li>
</ol>

= GDB Commands = 
Some useful gdb commands include:
{| class="wikitable" 
|-
!colspan="2"|General
|-
|help
|Shows details about commands
|-
!colspan="2"|Controlling Execution flow
|-
|break <line_number>
|Adds a breakpoint at the given line number. Other options include<br>
break <file_name:line_number><br>
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<br>
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.
|-
!colspan="2"|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 
|-
!colspan="2"|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:
{| class="wikitable" 
|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 [http://sourceware.org/gdb/current/onlinedocs/gdb/Memory.html |this link] for more.
|-
|}