calling addr on global variable failing to assemble in masm32 but offset works

Question

I literally started learning MASM32 yesterday so bear with me if this is a dumb question.

My understanding of addr and offset is as follows: both return the memory address of a variable but offset only works on global variables (whose memory addresses are know at assemble time) and addr will work on both global and local variables. I know what global and local variables are in higher level languages; my understanding is that variables declared in the .data, .data?, and .const blocks are global and that variable declared with the local keyword inside of procedures are local.

My entire code is:

.386
.model flat, stdcall
option casemap:none

include \masm32\include\windows.inc
include \masm32\include\user32.inc
includelib \masm32\lib\user32.lib
include \masm32\include\kernel32.inc
includelib \masm32\lib\kernel32.lib

.data

szCap db "Hello", 0                   ;caption for the MessageBox
szMsg db "Hello, world!", 0           ;text for the MessageBox

.code

;Procedure for creating a MessaheBox with NULL parent and variable text
TestProcedure proc msg:dword, cap:dword     
    invoke MessageBox, NULL, msg, cap, MB_OK
    ret
TestProcedure endp

main:
    ; error on these lines
    push addr szCap ; C:\masm32\My files\HelloWorld.asm(35) : error A2008: syntax error : addr
    push addr szMsg ; C:\masm32\My files\HelloWorld.asm(36) : error A2008: syntax error : addr

    call TestProcedure

    invoke ExitProcess, 0
end main

However, if I change addr to offset, the code works perfectly. Why does addr fail to assemble in this instance?

Answer 1

In flat mode, all addresses are offsets.

The ADDR operator was added in a fairly recent version of MASM and as far as I understand is used exclusively with the INVOKE keyword. In an INVOKE, ADDR does the same as offset for global identifiers but can calculate the relative address of a local, stack-based variable as well (where it would typically emit an LEA instruction).