Another C calling question

[Gil]

Hi Guys,

I am stuck with C++ calling ASM.

I got help from the VC guys how to declare it in C, for example:
extern "C" void myfunc(int x);

In the ASM I program myfunc as God intended:
myfunc   proc
. . . .
myfunc   endp

This works fine, however, this calling uses __cdecl, in which the caller has to pop the arguments off the stack, very inconvenient when programming in ASM.

To prevent C++ from popping the arguments one can use stdcall:
extern "C" void __stdcall myfunc(int x);

But now the linker doesn't find myfunc.

When I look at the assembler code that C++ generates when myfunc is called:
with __cdecl  it is called as: call _myfunc
with __stdcall it is called as: call _myfunc@4

I tried to modify the myfunc name in the ASM file but nothing worked.

What's your take?

I can't get answers in a C forum as diehard C programmers get chills when talking about ASM (or any other language) :tdown

Thanks,
Gil

[Vortex]

Hi Gil,

Could you post all your source files to the board? Plus, what are your development tools?

[Gil]

Hi again

Sorry for the omission.
I am using VC++ and MASM that come with visual studio 2003 .NET.
I compile all files from the IDE.
There are no compiling erros.

I didn't post any code because the problem is not related to a specific code, just naming convention and a linker that doesn't agree with me...........

Gil

[Gil]

Eureka!

Sometimes posting a question helps you organize your own thoughts.
Since VC calls myfunc as _myfunc, then if it looks for _myfunc@xx I have to name it myfunc@xx
(xx is the number of bytes pushed on the stack)

End of problem.

[hitchhikr]

It shouldn't be necessary to use such names be sure you assembled the .obj file containing the function with:

{code]
.model flat,stdcall

Code Select Expand




[Robert Collins]

I have a question also about a C program calling a function in MASM.

How do you do it?

My assembly code must have a.....

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start:
  '
  '
end start


If I put a 'invoke ExitProcess ,0' after the start statement then the MASM program exits immediately and my C program has no chance in calling 'myfunct'. If I omit the 'invoke ExitProcess, 0' then the assembly program terminates with an error.

If I put the function within the start.....end start statements like this:

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  '
  '
.code

start:

myfunct proc
     invoke ShowMessageBox
     invoke ExitProcess,0
myfunct endp

END start

....then the assembly execution falls through into the 'myfunct' code.

I don't get it.

[Robert Collins]

.........extern "C" void __stdcall myfunc(int x);

Maybe you should omit the "C" part and just declare your external function as

extern void _stdcall myfunc(int x);

Then in your assembly code do as hitchhikr said... 

.model flat, stdcall

[GregL]

Here's a good article on calling MASM procedures from C/C++:

http://www.developer.com/lang/article.php/3493531

Thanks to 'farrier' for posting the link.

[tenkey]

I have a question also about a C program calling a function in MASM.

How do you do it?

My assembly code must have a.....

Code Select Expand


start:
  '
  '
end start


Not for static linking -- if you're starting from the C main() function. [Or a Windows WinMain() function.]
If you're starting from C or C++ code, eliminate the startup stuff from the ASM code.

For a DLL, the start function is a DllEntry or DllMain function.

[Robert Collins]

Not for static linking -- if you're starting from the C main() function. [Or a Windows WinMain() function.]
If you're starting from C or C++ code, eliminate the startup stuff from the ASM code.

For a DLL, the start function is a DllEntry or DllMain function.

If I do that, like in the following snippit....

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.486
.model flat, stdcall
option casemap :none

.code

TestProc proc dwValue:DWORD

   mov eax, dwValue
   add eax, 100
   ret

TestProc endp

end


I will get the following link errors...

LINK : error LNK2001: unresolved external symbol _WinMainCRTStartup
myasmprog.exe : fatal error LNK1120: 1 unresolved externals

I am not having a C problem, I am having an assembly problem if I omit the 'start: end start' statements.

[GregL]

Are you using LINK /DLL ?

[Robert Collins]

Are you using LINK /DLL ?

No, it is not a DLL I am making in MASM. The snippit I posted was from that
http://www.developer.com/lang/article.php/3493531 link you posted earlier.

I already know how to call a function from an assemble DLL. I was just trying to make a simple function using MASM and then add the .obj to the 'C' program. I am not having any problems with the C program, just with the MASM program if I omit the start: end start statements and that is how it is shown in the sample from that above link.

[GregL]

Robert,

Oh, I thought you were trying to make a DLL now, sorry.  :red

Just assemble the .asm file into an .obj file, don't link it. The error is from the linker looking for an entry point, the entry point is in the C program.

I was able to assemble the .asm file exactly as it is in the article:

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.486
.model flat, stdcall
option casemap :none

.code

TestProc proc dwValue:DWORD

   mov eax, dwValue
   add eax, 100
   ret

TestProc endp

end


I had to make some small changes to the C program as follows (I'm using Pelle's C):

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#include <windows.h>
#include <stdio.h>

extern unsigned int __stdcall TestProc(unsigned int dwValue);

int main(void)
{
   unsigned int dwValue  = 100;
   unsigned int dwReturn = TestProc(dwValue);

   printf("%u\n", dwReturn);
   getchar();

   return 0;
}


It works.  :bg

Sorry for the C code hutch.

[hitchhikr]

I will get the following link errors...

LINK : error LNK2001: unresolved external symbol _WinMainCRTStartup
myasmprog.exe : fatal error LNK1120: 1 unresolved externals

I am not having a C problem, I am having an assembly problem if I omit the 'start: end start' statements.

Use /c option in ml arguments line.
The above example is just perfect, use it as a base but just note that if you're using C++ compiler (like VC) you'll need to use this instead:

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extern "C" unsigned int __stdcall TestProc(unsigned int dwValue);


to tell the compiler that TestProc is the real name of the function and to avoid the name mangling stuff (a c++ compiler would wait for a function called: TestProc@@YGII@Z).

Also in the "end blahblah" stuff "'blahblah" is just a label passed by masm to the linker to be used as an entrypoint (for an exe or a dll).

[hutch--]

 :bg

Greg,

Sorry for the C code hutch

We only get a little short on people who post their homework in C expecting others to translate it for them so they get their assignments done.