lua-users home
lua-l archive

[Date Prev][Date Next][Thread Prev][Thread Next] [Date Index] [Thread Index]


could somebody tell me a little bit more about the implications of increasing MAXSTACK?
There's no word of warning in llimits.h so it looks as if I could just increase this to 1000
or so if a script is running out of stack space. For example, I have some scripts that need
to pass tables containing lots of values to a function and then Lua exits with a stack overflow

Thus, I tried to increase MAXSTACK but I'm not sure whether that was a good idea. First,
it seems that the maximum number of function arguments is stored in 8bits so it's not
possible to pass more than 255 arguments to a function. I can live with that because I'm
passing all my arguments inside a table anyway. However, I think that this limit should be
mentioned in llimits.h.

Also, there seem to be other implications: With MAXSTACK set to 1000, lua is no
longer able to load bytecode that has been compiled using the default MAXSTACK setting
of 250. Specifically, the checkRK() function in ldebug.c fails then because in bytecode
compiled with the old MAXSTACK of 250 "r" could e.g. be set to 253 (i.e. OLD_MAXSTACK + 3)
but now MAXSTACK is 1000 so that condition fails and leads to a corrupt bytecode error.

So there seems to be no easy way to change the MAXSTACK value without breaking
bytecode compatibility of scripts compiled with a different MAXSTACK setting. Or could
this be worked around somehow without having to hack into the Lua core?

If there is no easy way to increase the stack without having to deal with all kinds of
issues, I'd suggest that a word of warning is put as a comment before the MAXSTACK
definition in llimits.h. Because as it currently is, it looks as if one could just increase
this as desired which apparently is not the case.

So my question is: What could I do to increase stack space for my functions without
breaking compatibility with old bytecode? I do not need to pass more than 255 arguments
to my functions but I need to pass tables with tons of items and they require more stack
space than 250 items. The only thing that comes to my mind is to hack a flag into new
bytecode that signals that the new stack limit is in effect in this bytecode. Whenever
this flag is not there, I know that it is old bytecode and have to adapt all the opcode
arguments accordingly so that the args are relative to the new MAXSTACK then. So
a value of 253 would be adapted into 1003 etc.

But I'd like to hear other suggestions. Maybe it can be achieved in a simpler way?
I'm still using Lua 5.0.2 for what it's worth.