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NASM Forum => Programming with NASM => Topic started by: nobody on August 06, 2008, 09:59:39 AM

Title: problem in using %if
Post by: nobody on August 06, 2008, 09:59:39 AM

HI,I have a problem about %if...
 here is my programme(helloworld.asm):

extern printf
[SECTION .data]
        msg1 db "hello world", 0xa, 0
        msg2 db "wrong",0xa, 0
        num  db 1
[SECTION .text]
        global main
main:
        %if num==1
                push dword msg1
        %else
                push dword msg2
        %endif
        call printf
        add esp, byte 4
return:
        mov eax, 0
        ret

It seems will print "hello world", but in fact , the result is "wrong" .
Is my programme wrong ?

Title: Re: problem in using %if
Post by: Frank Kotler on August 06, 2008, 02:49:26 PM

"num" is a runtime variable, and "%if" tests preprocessor variables. If, instead of "num db 1", you did "num equ 1" or "%define num 1", it would work. If you intend to test the contents of a variable, you'll have to do it at runtime:

cmp byte [num], 1 ; note: "[num]", not "num"!
je it_is_one
...

If you want an "if" macro for that, you'll have to include a macro for it. Nasm64developer's flow-control macros available in the "contributions" section on the download page here should have an "if" macro (I think)...

Best,
Frank

Title: Re: problem in using %if
Post by: nobody on August 07, 2008, 01:27:57 AM

Yeah, got it! Thanks :)