error LNK2001: unresolved external symbol _MessageBox

Question

I am trying to create a helloworld program using only masm and not masm32 libs. Here is the code snippet:

.386
.model flat, stdcall
option casemap :none

extrn MessageBox : PROC
extrn ExitProcess : PROC

.data
        HelloWorld db "Hello There!", 0

.code
start:

        lea eax, HelloWorld
        mov ebx, 0
        push ebx
        push eax
        push eax
        push ebx
        call MessageBox
        push ebx
        call ExitProcess

end start

I am able to assemble this using masm:

c:\masm32\code>ml /c /coff demo.asm
Microsoft (R) Macro Assembler Version 9.00.21022.08
Copyright (C) Microsoft Corporation.  All rights reserved.

 Assembling: demo.asm

However, I am unable to link it:

c:\masm32\code>link /subsystem:windows /defaultlib:kernel32.lib /defaultlib:user
32.lib demo.obj
Microsoft (R) Incremental Linker Version 9.00.21022.08
Copyright (C) Microsoft Corporation.  All rights reserved.

demo.obj : error LNK2001: unresolved external symbol _MessageBox
demo.obj : error LNK2001: unresolved external symbol _ExitProcess
demo.exe : fatal error LNK1120: 2 unresolved externals

I am including the libs during linking, so not sure why it still says unresolved symbols?

UPDATE:

c:\masm32\code>link /subsystem:windows /defaultlib:kernel32.lib /defaultlib:user
32.lib demo.obj
Microsoft (R) Incremental Linker Version 9.00.21022.08
Copyright (C) Microsoft Corporation.  All rights reserved.

demo.obj : error LNK2001: unresolved external symbol _MessageBox@16
demo.exe : fatal error LNK1120: 1 unresolved externals

UPDATE 2: Final working code!

.386
.model flat, stdcall
option casemap :none

extrn MessageBoxA@16 : PROC
extrn ExitProcess@4 : PROC

.data
        HelloWorld db "Hello There!", 0

.code
start:

        lea eax, HelloWorld
        mov ebx, 0
        push ebx
        push eax
        push eax
        push ebx
        call MessageBoxA@16
        push ebx
        call ExitProcess@4

end start

Answer 1

The correct function names are MessageBoxA@16 and ExitProcess@4.

Almost all Win32 API functions are stdcall, so their names are decorated with an @ sign, followed by the number of bytes taken up by their parameters.

Additionally, when a Win32 function takes a string, there are two variants: one that takes an ANSI string (name ends in A) and one that takes a Unicode string (name ends in W). You're supplying an ANSI string, so you want the A version.

When you're not programming in assembly the compiler takes care of these points for you.