Newbie Question: extrn

[TheDaneProgrammer]

Hi OsDevers.

I'm new to OS Developing and Assembler, and I've never got the thing about extrn . What does it do, and what do I have to specify etc?

I couldn't find anything on Google.

Oliver

[cyr1x]

It tells the assembler that the label is within a different object file.
This is necessary when you want to link several object files.

[Combuster]

I've never got the thing about extrn .

It helps if you fix your spelling and search on "extern" instead

[JohnWilson]

It helps if you fix your spelling and search on "extern" instead

I'm guessing it depends on your assembler. I use TASM 5.0 (in MASM mode) and extern gives an error (just tried it) -- all my code uses extrn. But I suppose no one uses TASM any more.

Anyway to answer the OP in a tiny bit of detail, it goes something like this (um, in TASM/MASM, so YMMV if you're using NASM or something):

In one source file:

Code: Select all

foo:    public  foo
... does something ...
        ret

Then in another source file:

Code: Select all

        extrn   foo:near
        call    foo

There's other stuff for calling other languages but that's the basic form. The type can be "near" or "far" if it's a label for a subroutine, or one of the data sizes ("byte", "word", "dword" etc.) if it's the label of a variable, or various other things that you're less likely to need.

John Wilson
D Bit