Gnu C and MASM 
(for Win32 programming)

Page updated May 13, 2001

Firstly, you need to understand where I'm coming from. I wrote a major Win32 application, goosee.exe, completely in MASM code. Recently, I decided that I wanted to move over to C, that is, to keep working on goosee but mostly in C.
So, I had to keep the huge body of existing MASM code, assemble it with MASM (I use version 6.14), but for the new code I needed a C compiler. Each of these, MASM and the C compiler, will produce an object file, and I needed to link these together to form the final executable.

First, I tried Borland's C/C++ version 5.5, downloadable free off the Web. However, at the link step the two object files could not seem to find each other's symbols. I wondered if this had something to do with the Borland object file being OMF format which Microsoft's link.exe program automatically converted to COFF.
Maybe I should have persisted with trying to get it working, but I opted for another solution, that worked first go ...

Downloading Mingw GCC Dev-C++ and docs

Gnu GCC C/C++ is a very powerful multi-CPU and multi-operating-system compiler and totally free. You can find on-line documentation on the actual compiler here:, or

Anyway, there is a specific version of GCC for Win32 programming, called Mingw, and a complete package of the Mingw compiler with a super-nice IDE/editor is available, called Dev-C++ -- this is what I used. You get it from here:, or

You need to download the complete binary package, early May 2001 it was called
When you install it, it will by default install into C:\dev-c++ \ with various subdirectories including bin\, include\ and lib\.

Note that you can get the Mingw compiler on its own, from the Mingw site, but it is easier if you get the complete Dev-C++ package, as everything gets put into the right directories with minimal fuss. However, you may find it useful to look at the Mingw development site, maybe for extra docs:, or

Getting DEVCPP and MASM together

What I am assuming here is that you have my WINMASM.ZIP package installed. It contains a skeleton Win32 application and a Make file. Microsoft nmake.exe, ml.exe (MASM), rc.exe (resource compiler) and link.exe are including in this package.

To compile a C file and link it with a MASM object file, firstly you need to put this into your AUTOEXEC.BAT file (then reboot):

rem this is for dev-cpp/mingw/gcc c/c++ development...
SET PATH=c:\dev-c_~1\bin;%path%
SET COMPILER_PATH=c:\dev-c_~1\bin\
SET LIBRARY_PATH=c:\dev-c_~1\lib\
SET C_INCLUDE_PATH=c:\dev-c_~1\include\
SET CPP_INCLUDE_PATH=c:\dev-c_~1\include\;c:\dev-c_~1\include\g++\

gcc.exe, the C/C++ compiler, is located in c:\dev-c++\bin, hence the first line sets the DOS search path so it can be found. gcc.exe itself calls other programs, which are located by the COMPILER_PATH environment variable, while the library and include files are located by the other variables.

When you have done that, you can then modify the Make file (let's call it skeleton.mak):

#To run this Make file...
#at the DOS prompt (a DOS box inside Windows).
fc = skeleton_c
fn = skeleton
linksw = /NOD 
masmsw = /c /coff
cflags = -mwindows
#note, -O3 flag adds maximum code-size optimisation to gcc.
$(fc).OBJ : $(fc).C  gcc -c $(fc).c $(cflags) -o $(fc).obj
$(fn).OBJ : $(fn).ASM  ..\masm32\bin\ML $(masmsw) $(fn).ASM
$(fn).RES : $(fn).RC  ..\masm32\bin\RC $(fn).RC
$(fn).EXE : $(fn).OBJ $(fn).RES $(fc).OBJ  ..\masm32\bin\LINK $(linksw) @<<LinkFile /MACHINE:i386 /SUBSYSTEM:WINDOWS,4.0 /ENTRY:start /MAP:$(fn).map /OUT:$(fn).exe $(fn).obj $(fc).obj $(fn).res ..\masm32\lib\USER32.LIB ..\masm32\lib\KERNEL32.LIB ..\masm32\lib\GDI32.LIB ..\masm32\lib\COMDLG32.LIB ..\masm32\lib\COMCTL32.LIB <<NOKEEP

I created a C source file called skeleton_c.c (reproduced at the bottom of this page), and I used the nice IDE/editor provided by the Dev-C++ package -- it has nice C-source color-coding -- pity it can't color-code asm code!
My asm source file was called skeleton.asm.
When you run the makefile, you end up with skeleton.exe, a Win32 application.

Do note that in my case the main Windows application is the asm file, while the C file is just enabling me to add more functions written in C. But, you could quite easily reverse that -- the dev-c++ package has a nice Win32 GUI C skeleton application.

Note also that I placed all the Win32 API library files in ..\masm32\lib\, which is where I placed them, relative to the working directory. You would have to put there whatever path is appropriate. Ditto, I put the Microsoft executables into ..\masm32\bin\. 
Actually, if you have installed my, these files may be in ..\winmasm\bin\ and ..\winmasm\lib\, relative to the skeleton\ example directory.

Exporting symbols between GCC and MASM

This is surprisingly easy. The GCC documentation states that C symbols must have a leading underscore appended when referenced in the asm file (for example, "var1" will be "_var1"), however, MASM does not require this. MASM exports public symbols with an underscore appended, at least when the ".MODEL FLAT,STDCALL" directive is used. Thus, GCC and MASM are able to see each other's symbols without having to append any underscores.

Calling an asm procedure from C

For example, you have a procedure in the asm file that you want to call from the C file:

ASMPROC PROC PUBLIC USES eax ebx ecx edx esi edi ebp


Note that it is declared "PUBLIC". You could also declare it public on a separate line:


Umm, I think also that the way prototypes work, is they make procedures public by default, so instead of the above line you could put this:


Note also that the very beginning of the asm file has these lines:


Which means that all asm procedures will be STDCALL calling convention by default, including ASMPROC. STDCALL is what the Win32 API functions use, so it is practical to standardise on it for all asm functions. So, I guess I don't really need it in the "PROTO" declaration for ASMPROC as shown above, either.

In the C file, you will have to place the prototype, with an "extern" prefix:

extern void STDCALL ASMPROC(void);

Although it is possible to set a commandline switch to make the C compiler default all functions to STDCALL, I decided against it, as I wasn't sure about linking with the standard C function libraries. Therefore, wherever a STDCALL function is to be called, put the "STDCALL" keyword into the prototype, as shown above.

Calling a C function from asm

All you need to do in your asm file is have a prototype, and then call it:

 invoke demo1

Then in your C file, all you need is the actual function code:

void STDCALL demo1(void) {
 //stuff in here

Accessing data in asm from C file

Are you getting the picture here, that this is dead simple? Same goes for data.

For any data that you have defined in the asm file, just declare it as public:

PUBLIC highlightedindex
highlightedindex DWORD 0 

Then over in the C file, define it with the "extern" prefix:

extern unsigned int highlightedindex;

Calling Win32 API functions from C file

Well okay, here is the first test C file that I created, called skeleton_c.c:

#include <windows.h>
extern void STDCALL ASMPROC(void);
extern unsigned int highlightedindex;

void STDCALL demo1(void) {

Each line is discussed in the above sections.
MessageBeep() is a Win32 API function, and you need the prototype for it. For that, you include the file "windows.h".

To wind up, let me explain what the above program does. The asm file calls "demo1()" in the C file, which in turn calls "ASMPROC()" in the asm file, then "MessageBeep()", then accesses a variable "highlightedindex" in the asm file.

Great stuff!

Sharing structures

My situation was that I had defined and instantiated some very large structures, and arrays of structures, in my asm file, and I need to be able to access them from the C file.

It's simple enough to define the same structures in the C file, for example:

struct TERSEDATA {
  U32 nsignature; //nsignature DWORD 0
  U32 nversion; //nversion DWORD 10
  U16 nsizeobject; //nsizeobject WORD (SIZEOF OBJECT)
  S8 szpre7[48]; //szpre7 SBYTE 48 DUP(0)

extern struct TERSEDATA *pTERSEDATA;

Note that "U32" means a 32-bit unsigned number -- I use "typedef" to use these, for example "typedef unsigned int U32;".

The pointer "pTERSEDATA" is already setup in my asm file pointing to the instantiated TERSEDATA structure, and it's easy to use in the C file:


Well, the reason I have used a pointer to access the structure is because I have an array of structures.

There is, however, a problem. GCC by default performs an optimisation on the structure, aligning members of the structure on 32-bit address boundaries. I use lots of 8 and 16-bit members in my structures, and I never bothered to tell MASM to assemble the structures with any kind of member alignment.

So, what you have to do is add the "-fpack-struct" option to GCC, as for example:

gcc -c -mwindows -fpack-struct demo1.c

This causes all the members or fields of the structure to be packed together without any gaps or holes, as per default for MASM. However, the GCC documentation warns that there may be a problem with some C library functions that use structures.

What next?

I never need to use a debugger, but I guess the one referred to in the Dev-C++ package would work (?). Mingw/GCC generates a COFF format object file by default, and MASM does so with the "/coff" switch. But I have no idea about compatible debugging info ... anyone interested in looking into this?

What about an IDE/editor that will nicely integrate the asm, C and makefile?

(c) copyright Barry Kauler 2001