Simple Debugger

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: [!] VersionNotice: The below code pertains to an older Lua version, Lua 4. It does not run as is under Lua 5.

Lua provides tools for debugging, but they are not well advertised. You can use _ERRORMESSAGE to get a stack traceback and you can modify _ALERT to call debug so that you can examine global variables when an error occurs. With a little more code you can even dump out the local variables.[1]

This code[2] has a bug in it.


      1 require'simpledebugger.lua'

      2 

      3 function here( t, str, pat )

      4   assert( type(t,'table'), 'expecting a table' )

      5   assert( type(str,'string'), 'expecting a string' )

      6   pat = pat or '$(%b<>)'

      7   local f = function(w)

      8     local m = t[strsub(w,2,-2)] or w

      9     if type(m,'function') then m = m(t) end

     10     return tostring(m)

     11   end

     12   return( gsub( str, pat, f ) )

     13 end

     14 

     15 temp = [[ 

     16   this is a $<x> for $<y>

     17 ]]

     18 local tab = { x = 'test', y = 'fun' }

     19 write( here( tab, temp ) )

     20 write( here( temp, tab ) )

     21 print"game over"

If we run the code, we see that it dies due to a failed assertion.


$ lua -v testdebugger.lua 

Lua 4.1 (work4)  Copyright (C) 1994-2001 TeCGraf, PUC-Rio

 

  this is a test for fun

error: assertion failed!  expecting a table

stack traceback:

   1:  function `assert' [C]

   2:  function `here' at line 4 [file `testdebugger.lua']

   3:  main of file `testdebugger.lua' at line 20

lua_debug>

We can dump out the locals at stack level 2 and we see that the function was called with the arguments switched.


lua_debug> locals(2)

t = " \

  this is a $<x> for $<y>\

"

str = { -- table: 0x80649b8

  y = "fun",

  x = "test"

}

pat = nil

lua_debug>

You can also dump out the locals further up the stack. Enter cont to quit out of debug.


lua_debug> locals(3)

tab = { -- table: 0x80649b8

  y = "fun",

  x = "test"

}

lua_debug> cont

If we add stop"it" just after line 8, we can inspect the variables in the function here.


$ lua -v testdebugger.lua 

Lua 4.1 (work4)  Copyright (C) 1994-2001 TeCGraf, PUC-Rio

error: it

stack traceback:

   1:  function <7:file `testdebugger.lua'> at line 9

   2:  function `gsub' [C]

   3:  function `here' at line 13 [file `testdebugger.lua']

   4:  main of file `testdebugger.lua' at line 20

lua_debug> locals()

w = "<x>"

m = "test"

lua_debug> cont

error: it

stack traceback:

   1:  function <7:file `testdebugger.lua'> at line 9

   2:  function `gsub' [C]

   3:  function `here' at line 13 [file `testdebugger.lua']

   4:  main of file `testdebugger.lua' at line 20

lua_debug> locals()

w = "<y>"

m = "fun"

lua_debug> cont

 

  this is a test for fun

game over

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Last edited December 30, 2006 11:04 pm GMT (diff)