Posted on 2006-11-20
Medium Priority
Last Modified: 2008-02-07

When using read process memory, here is an example:

dim lngBaseAdd as long
dim lngResult as long
dim lngReadBytes as long

lngBaseAdd = &HF0129F 'or some arbitrary base address

When using the read process memory API you must pass the lngBaseAdd by value, why?

ReadProcessMemory hProcess, lngBaseAdd, lngResult, 4, lngReadBytes '<--- FAILS
ReadProcessMemory hProcess, byVal lngBaseAdd, lngResult, 4, lngReadBytes '<--- WORKS

I dont get why you must pass it by value.


Question by:BrianGEFF719
LVL 28

Accepted Solution

Ark earned 2000 total points
ID: 17983709
Calling by ref is equal to calling by val using pointer to variable instead of variable itself:

ReadProcessMemory hProcess, lngBaseAdd, lngResult, 4, lngReadBytes

is equal to

ReadProcessMemory hProcess, ByVal VarPtr(lngBaseAdd), lngResult, 4, lngReadBytes

lngBaseAddress is a Long type variable, which is actual virtual address in remote process address space. When you pass it ByVal, you tell ReadProcessMemory API to read remote process memory, starting from this actual address, when pass it ByRef, you send to remote process not actual address, but pointer to your variable (VarPtr(lngBaseAddress)). Remote process get a value from this pointer in its own memory space (which can be 0 or any unpredicable value) and start reading from this value - and fail.
LVL 19

Author Comment

ID: 17983809
Thanks Ark.


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