20 replies to this topic

[mebob]

I have been trying to use the C library with NASM, but it isn't working out. I get "Undefined reference to WinMain@16" errors from ld. To link the object file outputted from NASM, I am using gcc -o [filename].exe [filename].o I figure this is a linking error, does anyone know what I should be doing?

[mebob]

I think I figured this out now.

[RhetoricalRuvim]

What was the problem?

[mebob]

I don't really know, there was just some linking error, so I took the easy way out: I made a C file that looked like this:

extern void SomeFuncName(void);


int main()

{

    SomeFuncName();

}

and in the assembly file I made the SomeFuncName function global (using the global keyword), assembled it to an ELF object file, and linked it like this:

gcc [Output from assembler].o [C file].c

[dargueta]

Use the extern keyword in your NASM code where you have your global declarations and stuff.

[mebob]

That is the first thing I tried, and it gave me that undefined reference. I haven't tried making an extern statement for WinMain though, maybe I should.

[RhetoricalRuvim]

Try disassembling the code, if possible, maybe the function name (that you access, referenced by the keyword extern) is different from what it needs to be. I don't know if this is the case, but I sometimes got things like:
C code:

int gcd(int a, int b){ 

.....and so on.....

But when I disassembled the code and the function name, instead of being gcd, was ?gcd@@YAHHH@Z.

Another thing is maybe the function name, in global form (from the C code), might actually be _function_name, instead of function_name, (with leading underscore), but NASM assembles it so it references 'function_name', instead of '_function_name'.

So anyway, I think you could try figuring out the actual object code function label name and type 'extern _whatever_the_function_name_is' and then, if you want to, use '%define whatever_the_function_name_is _whatever_the_function_name_is'.

I'm not sure if this is the problem, though, but you could try this.

[dargueta]

Definitely try to turn name mangling off.

[mebob]

It worked when I defined it as _main! But I didn't clean up the stack after calling _printf, how do I do that?

BTW It appears you were using the Digital Mars compiler for that, RhetoricalRuvim.

[dargueta]

Not sure how Microsoft does it, but the way GCC does it is like this:

myfunc:

    ; create the standard stack frame

    push    ebp

    mov     ebp, esp

    

    ; Save enough space on the stack to pass all arguments to all functions and

    ; keep stack variables as well. If you only call functions with 3 arguments

    ; or less, then subtract 12, as in this example.

    sub     esp, 12

    

    ; ... blah ...

    

    mov     DWORD [esp], format_string

    mov     DWORD [esp + 4], 1234

    mov     DWORD [esp + 8], 5678

    call    printf

    

    ; ... blah ...

    

    add     esp, 12     ; MAKE SURE THIS IS THE SAME AS ABOVE

    pop     ebp

    ret

That way you don't have to clean up after every function call.

[mebob]

It works now, thanks! It is possible to just push the arguments and then add the size of all the arguments put together after the call though, isn't it?

[RhetoricalRuvim]

Maybe something like (not sure if it's the best way, though):

function: 

enter 12, 0 


;; .....  


leave 

ret 

About adding the size of the arguments, you mean something like this?:

function: 

enter 12, 0 


push dword 1234 

push word 5678 

call some_function 

add esp, 6 


leave 

ret 


some_function: 

;; some code .....  

ret 

I usually do it like this, though:

function: 

enter 12, 0 


push dword 1234 

push word 5678 

call some_function 


leave 

ret 


some_function: 

enter 4, 0 


;; some code 


leave 

ret 6 

That way the RET instruction takes care of the adding 6 to the stack pointer.