I have a function that takes an optional parameter. If it is omitted, I need to get a valid, default version of that parameter. Can I overwrite the parameter passed in without changing the caller's data? Or should I create a LOCAL and mov the value in and then fix it up if needed?
My question is, pThing is an *address* that was passed by *value* on the stack, right? So "mov pThing, eax" won't overwrite the caller's value? I'm pretty sure this is correct, but wanted to get it straight before I commit it to a bazillion lines of code (well, okay, more like 30. But for me, that's still well over my what-was-I-doing threshold for assembly).
Thanks!
-Chalain
MyFunc proc pThing:DWORD
; pThing is optional. If it's NULL, fill it with
; the default Thing obtained from GetThing()
mov eax, pThing
test eax, eax
jnz @F
invoke GetThing ; returns default pThing in eax
;***********************
; DANGER ??? DANGER ???
mov pThing, eax
; ... continue with known-valid pThingMy question is, pThing is an *address* that was passed by *value* on the stack, right? So "mov pThing, eax" won't overwrite the caller's value? I'm pretty sure this is correct, but wanted to get it straight before I commit it to a bazillion lines of code (well, okay, more like 30. But for me, that's still well over my what-was-I-doing threshold for assembly).
Thanks!
-Chalain
Chalain,
If I have your question right, pThing is just a normal parameter passed on the stack which in this instance would be . I cannot see any problem with overwriting this value with whatever you wish as long as you don't try and write a larger piece of data to that address if there were other parameters above it.
Regards,
hutch@movsd.com
If I have your question right, pThing is just a normal parameter passed on the stack which in this instance would be . I cannot see any problem with overwriting this value with whatever you wish as long as you don't try and write a larger piece of data to that address if there were other parameters above it.
Regards,
hutch@movsd.com
Hutch is right, when the function is called, each parameter is pushed on the stack to be used by the called function. The only thing you change is a piece of the stack.
In some cases, even if you want to, you can't change the caller's data, for example:
In this case there is no caller's data, just a value that is passed to the function. The 100 only exists in the stack.
Thomas
In some cases, even if you want to, you can't change the caller's data, for example:
invoke MyFunc, 100
In this case there is no caller's data, just a value that is passed to the function. The 100 only exists in the stack.
Thomas
Perfect! Thank you. That was my question exactly.
I was pretty sure that's how it worked, but assembly keeps biting me just when I think I have it sussed... :eek:
Thanks!
-Chalain
I was pretty sure that's how it worked, but assembly keeps biting me just when I think I have it sussed... :eek:
Thanks!
-Chalain
Chalain,
if you an optional parameter is omitted, feel free to change the value. It is up to the caller to ensure they use the correct method of passing parameters, so the function that takes the optional parameter should have no fear in how it mangles that parameter.
Also, in your example, 'pThing' is not necessarily a pointer, it is only a pointer if specifically built the function to use it as a pointer.
if you an optional parameter is omitted, feel free to change the value. It is up to the caller to ensure they use the correct method of passing parameters, so the function that takes the optional parameter should have no fear in how it mangles that parameter.
Also, in your example, 'pThing' is not necessarily a pointer, it is only a pointer if specifically built the function to use it as a pointer.