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allocating local variables before entring a function?

 
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Loset
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Joined: 18 Apr 2015
Posts: 32

PostPosted: Wed May 08, 2019 3:32 pm    Post subject: allocating local variables before entring a function? Reply with quote

A function I'm calling from a new thread has 9 local variables.
But as soon as I'm using it, It crashes the game, I only allocated memory for the function arguments but not for the local variables(taken from the stack) because I'm not sure how is that going to work?

Like do you need to do it? If it's a local variable of the function why doesn't it allocate it on it's own?

and if I need to allocate those variables how would that go?
like
Code:
alloc(var1, 8)
alloc(var2, 8)
alloc(var3, 8)
push var1
push var2
push var3


I'm not sure exactly what I'm looking for, Just need a point in the right direction.
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Roogue
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Joined: 31 May 2019
Posts: 14

PostPosted: Sat Jun 01, 2019 6:19 am    Post subject: Reply with quote

I see. The local variables are not "allocated it on it's own", you SHOULD make place on top of the stack for this variables.

Let me get to the point, if you have a "call" instruction to go through a function, the function will look like this :


push ebp // calling conventions (stack frame)
mov ebp, esp // calling conventions (stack frame)
sub esp, 4 // HERE you allocate those local variables. It's giving place on top of
// the stack for the local variables

Assuming every value on the stack is 4 bytes, you will do sub esp, 4 for one local variable ; sub esp, 8 for two ect... it's always +4

Now if you didn't understand this very well, it's normal. I would suggest you reading this links and then you will be ok.

pages . cs . wisc . edu /~remzi/Classes/354/Fall2012/Handouts/Handout-CallReturn . pdf

cs . virginia . edu /~evans/cs216/guides/x86 . html
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