Hello,
I needed to know how can I write a routine in assembly and call it from my C program and vice versa? Is there any tutorial on the web for this??

Do you need to link statically or dinamically? Win32 or Linux?
Statically, you need to use a linker (I suggest GoLink) and compile your assembly (coff format) exporting the function (see fasm examples for coff)
From C you need to produce a header including the reference of your function and parameters passed to it.

Dinamically, you create a dll with fasm and import your function from C code as if were a system library.
I am going to clean some examples to post here.

Thanks for ur reply, but I don't get a lot of things. Are there any examples I can get to see the interface. I am talking about Win32 examples and it will be good if I can see for both Static and Dynamic linking. As well as a link both ways, assembly to C and C to Assembly. I mainly use Visual C++.

milind,

Here is an example of static linking, you can check the latest version of my inc2inc tool. You can use Hutch's masm32.lib with Fasm:

http://board.flatassembler.net/topic.php?t=588&postdays=0&postorder=asc&start=0

Also, using C run-time library with Fasm:

http://board.win32asmcommunity.net/showthread.php?s=&threadid=9510

just use format COFF, and define extern for all symbol you want to export. Then link created COFF file as .OBJ to your C/C++ project. Just dont forget about extern "C" and _ before names

How do I define extern and global symbols. I checked with FASM Programmers referece document and it did not describe any directive to be extern or global.

Those directives are described in section 2.4. (Formatter directives) of FASM Manual. You can declare public and external symbols for example this way:

remember that C expects all function names in object file to be preceeded with "_" character.

regards

and remember we are talking about C.

If you are using C++, you also have to declare symbols which are defined in fasm code in extern "C" {} block

Hello Vid,
Well I was manage to do something thanks to you. But I don't understand what is the meaning of putting "C" after extern in the C program.

Can't seem to make this work:

VC++ code:

#include<iostream.h>

extern "C" void disp(int, int);
extern "C" int sum;

void main()
{
disp(3, 4);
cout<<sum;
}


FASM code:

format COFF
public disp as "_disp"
extrn "_sum" as sum:word
disp:
push bp
push ax
push bx
mov bp, sp
mov ax, [bp+4]
mov bx, [bp+6]
add ax, bx
mov [sum],ax
pop bx
pop ax
pop bp
ret

Error in VC ++ Build is:

Compiling...
c2assemdispa.cpp
Linking...
c2assemdispa.obj : error LNK2001: unresolved external symbol _sum
dispa.obj : error LNK2001: unresolved external symbol _sum
Debug/Cmixassembly.exe : fatal error LNK1120: 1 unresolved externals
Error executing link.exe.

Cmixassembly.exe - 3 error(s), 0 warning(s)


What's wrong?

That's because you declared "sum" as an external symbol in both C and assembly modules. You have to define this variable in one of them. For example in FASM source:


And when suing MSVC, you should rather use "format MS COFF" than "format COFF"; I'm not sure about that, but those formats are a bit different so this could cause errors in more complec projects.

regards

milind,

Something very important, you can't mix 32-bit VC++ code with 16-bit code:



In 32-bit mode, the stack is dword aligned; statements like mox ax,[bp+4]
are meaningless.

Yes, I found that our, it was my mistake to use the 16 bit code. Now the code is working fine but I want to know how can I use variables declared in C. Right now sum was declared in assembly. How should I declare sum in C and then use it in Assembly.