Error compiling Win32 API GUI code with MinGW

Question

Hey guys, this is my first post.

I'm just getting started with win32 API programming in C++ and I'm having trouble compiling the winnie tutorial (http://www.relisoft.com/win32/winnie.html) with MinGW.

My input and output:

C:\Users\Eric\Projects> g++ winnie.cpp -o winnie.exe

/mingw/lib/libmingw32.a(main.o):main.c:(.text+0xd2): undefined reference to 'WinMain@16'

collect2: ld returned 1 exit status

Any help would be very much appreciated.

My source code: http://pastebin.com/tfDTUcBp

Answer 1

The line:

/mingw/lib/libmingw32.a(main.o):main.c:(.text+0xd2):
    undefined reference to 'WinMain@16'

means that the linker has a reference within MinGW's main.o which requires a WinMain function to be provided by the user code. The most likely cause of this is that you haven't provided one.

When you create Win32 applications, main is no longer the entry point you use. The entry point for your user code is now WinMain so you need to provide that rather than main:

int WINAPI WinMain(
    HINSTANCE hInstance,
    HINSTANCE hPrevInstance,
    LPSTR lpCmdLine,
    int nShowCmd);

The parameters are:

• hInstance, a handle to your applications instance, where an instance can be considered to be a single run of your application.
• hPrevInstance is always NULL.
• lpCmdLine, a pointer to a string that holding any command-line arguments.
• nShowCMD, which determines how your application's window will initially be displayed.

---

Now, having looked over your code and seen that you do actually have a WinMain, it's most likely an environment isssue rather than a code issue.

Have you tried compiling with -mwindows as an option?

A good start is to begin with the canonical beginner program and sort out the error there first. Enter the following program:

#include <windows.h>
int WinMain (HINSTANCE p1, HINSTANCE p2, LPSTR p3, int p4) {
    MessageBox (0, "Hello.", "MyProg", MB_OK);
    return 0;
}

and try to compile it the same way as your current program:

g++ -o qq.exe qq.cpp

Then try with:

g++ -mwindows -o qq.exe qq.cpp

and see if that works.

If it works in the former case, there's something wrong with your code. If not, there's probably something wrong with your compiler flags or environment.

And then, if adding -mwindows works, it should solve your problem with your real code as well. And, if not, keep working with the simpler program since that has less variables that can cause problems.

Answer 2

Ugh, sorry. It turns out it was a typo. My bad.

Answer 3

Ok, Here's the switch that works for MinGW

-mwindows

Example

gcc hello.c -o hello.exe -mwindows

the -m switch sets the EUMULATION, so -mwindows sets the EMULATION to Windows.

It's quite different to Borland since Borland uses the -t switch.

e.g. bcc32 -tW (for Windows).

Hope this helps.

Answer 4

I've noticed that this exact same error occurs (mingw / Win XP / gcc in my case) if your source file doesn't have a main() function, even if you are doing nothing Windows related at all.

Answer 5

It is the source code problem. It redefines the WinMain as:

int WINAPI WinMain (
    HINSTANCE hInst, 
    HINSTANCE hPrevInst, 
    char const * cmdParam, 
    int cmdShow)

The third parameter type is "char const *". it is different with required parameter type LPSTR. That's problem!

After change the parameter type back to LPSTR. It can be compiled as:

g++ winnie.cpp -o winnie.exe 

Even without a -mwindows option!