How do you get a Windows Environment Variable in assembly language?

Question

I have an assembly language program that attempts to print out the value of a Window Environment Variable, namely "%AppData%". Unfortunately, my program only crashes. I suspect the problem is caused by an incorrect stack reservation. Here is my code:

includelib \Masm64\Lib\Kernel32.lib
includelib \Masm64\Lib\User32.lib
extern GetEnvironmentVariableA : proc
extern MessageBoxA : proc
extern ExitProcess : proc

dseg        segment para 'DATA'
capt        db      'Debug', 0
msg         db      0 dup(30h)
evar        db      '%AppData%', 0
dseg        ends

cseg        segment para 'CODE'
start       proc
            sub     rsp, 18h
            mov     r8d, 30h
            lea     rdx, [msg]
            lea     rcx, [evar]
            call    GetEnvironmentVariableA

            sub     rsp, 28h
            xor     r9d, r9d
            lea     r8, [capt]
            lea     rdx, [msg]
            xor     rcx, rcx
            call    MessageBoxA

fini:       call    ExitProcess
start       endp
cseg        ends
            end

Any suggestions?

Answer 1

MSDN specifies: "The caller is responsible for allocating space for parameters to the callee, and must always allocate sufficient space for the 4 register parameters, even if the callee doesn't have that many parameters."

You are only allocating 18h bytes before the first call - maybe you should allocate 20h? Visual C seems to always allocate 28h bytes. You don't need to allocate it again before each function call, though.

Additionally, GetEnvironmentVariable only takes in the name of the variable, without the percent characters.

The following modified code seems to work:

includelib \Masm64\Lib\Kernel32.lib
includelib \Masm64\Lib\User32.lib
extern GetEnvironmentVariableA : proc
extern MessageBoxA : proc
extern ExitProcess : proc

dseg        segment para 'DATA'
capt        db      'Debug', 0
msg         db      0 dup(30h)
evar        db      'AppData', 0
dseg        ends

cseg        segment para 'CODE'
start       proc
            sub     rsp, 28h
            mov     r8d, 30h
            lea     rdx, [msg]
            lea     rcx, [evar]
            call    GetEnvironmentVariableA

            xor     r9d, r9d
            lea     r8, [capt]
            lea     rdx, [msg]
            xor     rcx, rcx
            call    MessageBoxA

fini:       call    ExitProcess
start       endp
cseg        ends
            end