Code Debugging with GDB - part 2: Custom Commands

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Abstract

This article describes custom commands in GDB and how to write it.

Customize a gdb command

Syntax

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define <command-name>
	<body>
end
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define <defined-command-name><sub-command-name>
	<body>
end

Debug

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ethanol@ethanol:~/Desktop/pic$ gdb -q ./main_mix
GEF for linux ready, type `gef' to start, `gef config' to configure
96 commands loaded for GDB 9.2 using Python engine 3.8
Reading symbols from ./main_mix...
gef➤  define hello
Type commands for definition of "hello".
End with a line saying just "end".
>echo hello, world\n
>end
gef➤  hello
hello, world

View custom gdb command

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gef➤  show user
User command "hello":
  echo hello, world\n

gef➤  show user hello
User command "hello":
  echo hello, world\n

Get parameters

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echo $arg0
echo $arg1
echo $arg<n-1>
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gef➤  define log_info
Type commands for definition of "log_info".
End with a line saying just "end".
>echo argc=$argc\n
>echo arg1=$arg0\n
>echo arg2=$arg1\n
>end
gef➤  log_info hello world
argc=2
arg1=hello
arg2=world

Use conditional branching

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gef➤  define log_info
Type commands for definition of "log_info".
End with a line saying just "end".
>if $argc==2
>echo argc=$argc\n
>echo arg1=$arg0\n
>echo arg2=$arg1\n
>else
>echo args error\n
>end
>end
gef➤  log_info hello world
argc=2
arg1=hello
arg2=world

Use loop

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define log_info
	if $argc < 1
		echo args error\n
		return
	end
	
	set $i = 0
	while $1 < argc
		printf "%dth arg\n", $i
		set $i = $i + 1
	end
end

Execute custom gdb commands in files

In gdb:

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source cmd.gdb
show user

Code debugging with GDB Series

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