NASM - define a constant as an EXTERN - nasm

Question

Logically, what I want to do is:
GLOBAL FRED
FRED EQU 5
in one module, and
EXTERN FRED
MOV ECX,FRED
in a second.
I'm aware that
first module
GLOBAL FRED
FRED: DD 5
second module
EXTERN FRED
MOV ECX,[FRED]
works perfectly happily, but what I'd really prefer is the first version.
I've tried
FRED EQU 5
FRED: EQU 5
%define FRED 5
%assign FRED 5
None worked - no error message, but the label is not exported. Only if FRED is an address does it co-operate.
So - is there any way of exporting a value from one module as a constant, not an address?

Answer 1

Nasm treats a dot character (".") as a legal character in symbol names.
You can use this to fake COM (Component Object Model) "Object Properties".
If you have 3 modules, each of which has a "width" variable, you can pull the following stunt:
;ButtonWidget
SECTION .data
global ButtonWidget.width, ButtonWidget.height
ButtonWidget:
.width dd 50
.height dd 10
;SelectWidget
SECTION .data
global SelectWidget.width, SelectWidget.height
SelectWidget:
.width dd 50
.height dd 100
;TextWidget
SECTION .data
global TextWidget.width, TextWidget.height
TextWidget:
.width dd 200
.height dd 10
In your main module,
;GUIapp
extern ButtonWidget.width, ButtonWidget.height
extern SelectWidget.width, SelectWidget.height
extern TextWidget.width, TextWidget.height
%macro MakeChildWindow 4
push %1
push %2
push %3
push %4
call makechildwindow
%endmacro
MakeChildWindow 10, 10, ButtonWidget.width, ButtonWidget.height
MakeChildWindow 10, 30, SelectWidget.width, SelectWidget.height
MakeChildWindow 10, 200, TextWidget.width, TextWidget.height
This technique has the advantage that the external variables can be fixed
or changeable and it is VERY readable.

Answer 2

Short answer: No.
You cannot use equ, define or assign for exporting because they do not show up in the compiled module as separate values. They only affect the preprocessor, so writing
FRED equ 5
mov ecx, FRED
actually compiles to
mov ecx, 5
Common practice is to use C-style header file that has all the constants defined and then just include the header file wherever needed.
fred.asm:
%ifndef FRED_ASM
%define FRED_ASM
%define FRED 5
; ... other constants ...
%endif
module.asm
%include "fred.asm"
mov ecx, FRED

Answer 3

If I understand you correctly, you want FRED to be defined somewhere and used by many modules/source files? Each module/source file, uses FRED and instead of changing the value in each source file and re-assembling each file, just change one instance of FRED and have it use everywhere? If this is the case, you can just plop the equates into a static library and link against it. This does not work with %define, only equ.
Try this:
libequates.asm
global FRED, JOHN
FRED equ 5
JOHN equ 6
makefile for libequates.asm (modify for you needs)
LIBRARY = libequates
all: $(LIBRARY) PostMake
$(LIBRARY): $(LIBRARY).o
ar -cr $(LIBRARY).a $(LIBRARY).o
strip --strip-unneeded $(LIBRARY).a
$(LIBRARY).o: $(LIBRARY).asm
nasm -f elf64 $(LIBRARY).asm -F dwarf
PostMake:
mkdir -p ~/projects/lib/
mv $(LIBRARY).a ~/projects/lib
Now to use and test if it works (I will use printf for simplicity):
externs.asm
extern FRED, JOHN, printf, exit
global start
section .data
fmtint db "%d", 10, 0
section .text
start:
mov rsi, FRED
mov rdi, fmtint
xor rax, rax
call printf
mov rsi, JOHN
mov rdi, fmtint
xor rax, rax
call printf
mov rdi, 0
call exit
makefile for externs.asm
APP = externs
all: $(APP) clean
$(APP): $(APP).o
ld -o $(APP) $(APP).o -entry start -L ~/projects/lib -lequates -lc -dynamic-linker /lib64/ld-linux-x86-64.so.2
$(APP).o: $(APP).asm
nasm -f elf64 $(APP).asm -F dwarf
clean:
rm $(APP).o
and output:
Now, you only need to change the equate value in one place, re-assemble only the static library and re-link your program/modules.