I have two include files, both of them need the exact same structure. I don't always use both include files at the same time, so I decided to conditionally define both structures in both include files with the if ~defined operator. When I use both include files (which happens rarely) I get an error "Symbol already defined.

I have it in both include files and it gives me an "Already defined error" when assembling.

But if I use this, it works:


When I test it against a data element inside that structure, it works. How can I define the same structure in multiple include files, conditionally, without any assembly errors?

I guess there is nothing wrong with checking if an element of a structure is defined, but it's an ugly way of doing it, I'm looking for a better way.

I can not check your example - you didn't use the struc operator (is not named struc<t>) - must be a macro. If so, which one ?

I would refer to the manual, "2.3.4 structures" in detail and in general the section "2.2.2 conditional assembly". There is existing a "used" operator as well as an "eqtype" operator which may be useful in this context. What is working with defined operator is check of constants and labels, there maybe restriction on complex data types. This is different here to C language.

I use labels in my programs to avoid double definitions in conjunction with defined.

All include files that come with fasm uses struct.



That one worked. (Everything compiles normally again)

The "if defined" is ignored by the preprocessor and both structures get defined regardless. For conditional preprocessing you will need to use something like match.

Maybe - but not every programmer uses FASM's include files. Could have been something own defined, could have been renamed struc to struct with "fix" directive or something other own defined structure. So at least it would be helpful a piece of code which runs itself in FASM - not just a portion of your code.

So struct is a macro and a macro is handled by the preprocessor and for the preprocessor you need conditional preprocessing which is quite different to conditional assembly. It is very useful to go through FASM documentation in a silent hour or so to explore the sometimes hidden features - you could learn much unexpected stuff.

I myself detected yesterday that there is a "used" directive - maybe newly added feature in the last years. I am still using version 1.71.01 while I mostly studied the documentation when using the version 1.69.35 or so.

Thanks for the great tips, I appreciate it. (1.71.01 )

I use FASM for 8 bit developments (Z80 version) - so support of newer Intel and AMD instructions, 64bit support and some other directives are not so important for me. But I like the general structure and easyness of FASM.

This thread is already a bit cold, but I would like to add my input. People already said this, but let me make it more clear: things like "define", "equ", "macro", "struc", "irps", match", etc. are managed by the preprocessor, they are not affected by "if/end if", this doesn't exist for the preprocessor.
So for the code:


the preprocessor only sees:


So the structure will be defined and then redefined, no matter the if/end if

Then, comes the assembler, it doesn't see preprocessing stuf, because it has already been preprocessed, defines, equ's, macros, etc replaced by their values. So (I'm not entirely clear on this), the assembler will see:



It will not understand "MyStructure" in that position, because structures can't be placed there. "defined" only works on assembly time variables and constants, meaning labels and numeric variables:


So in this case, "const", "here" and "nothing" would be valid for "defined".
What I do when I need some conditional preprocessing, is, for example:


this code will display:


If you comment out the define, nothing will happen.

deleted this post, I found something by searching