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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.

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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 6:04 pm GMT (diff)