undefined reference to `WinMain@16'

Question

When I try to build a program using Eclipse CDT, I get the following:

/mingw/lib/libmingw32.a(main.o):main.c:(.text+0x106): undefined reference to `WinMain@16

Why is that? And, how can I solve this issue?

Answer 1

You're missing the main() function (or WinMain()).

If you do have one, you probably forgot to save your source code, and ended up compiling an empty file (without main()).

You can use either main or WinMain. The error will always say WinMain, which is a hed herring, you can always use either one on MinGW.

On MinGW, the choice of main vs WinMain is unrelated to the "subsystem" setting ("windows" vs "console") that other answer talks about. The subsystem only matters for this on the MSVC compiler.

If you're using SDL2:

Then the above doesn't apply to you, because SDL2 provides its own main function. It renames yours to SDL_main (via #define main SDL_main) and calls it from theirs.

Then the error means that you forgot the -lSDL2main linker flag.

If you do have, it, make sure that the flag order is correct: .c/.cpp/.o files, then -lmingw32 -lSDL2main -lSDL2.

If that still doesn't work, the version of SDL2 you're using is likely incompatible with your compiler. If you're using MSYS2, consider installing their version of SDL2 (which is guaranteed to be compatible). Otherwise, you're likely using x32 SDL2 with x64 compiler, or vice versa. The SDL2 files you've downloaded come in two directries: i686... (x32) and x86_64 (x64), try both.

Defining SDL_MAIN_HANDLED is never necessary if you do everything correctly.

Answer 2

Had same problem. To fix it I clicked save to save my .c file before building. I believe my computer was referencing a path to a file with no information inside of it.

Answer 3

Try saving your .c file before building. I believe your computer is referencing a path to a file with no information inside of it.

Answer 4

This error occurs when the linker can't find WinMain function, so it is probably missing. In your case, you are probably missing main too.

Consider the following Windows API-level program:

#define NOMINMAX
#include <windows.h>

int main()
{
    MessageBox( 0, "Blah blah...", "My Windows app!", MB_SETFOREGROUND );
}

Now let's build it using GNU toolchain (i.e. g++), no special options. Here gnuc is just a batch file that I use for that. It only supplies options to make g++ more standard:

C:\test> gnuc x.cpp

C:\test> objdump -x a.exe | findstr /i "^subsystem"
Subsystem               00000003        (Windows CUI)

C:\test> _

This means that the linker by default produced a console subsystem executable. The subsystem value in the file header tells Windows what services the program requires. In this case, with console system, that the program requires a console window.

This also causes the command interpreter to wait for the program to complete.

Now let's build it with GUI subsystem, which just means that the program does not require a console window:

C:\test> gnuc x.cpp -mwindows

C:\test> objdump -x a.exe | findstr /i "^subsystem"
Subsystem               00000002        (Windows GUI)

C:\test> _

Hopefully that's OK so far, although the -mwindows flag is just semi-documented.

Building without that semi-documented flag one would have to more specifically tell the linker which subsystem value one desires, and some Windows API import libraries will then in general have to be specified explicitly:

C:\test> gnuc x.cpp -Wl,-subsystem,windows

C:\test> objdump -x a.exe | findstr /i "^subsystem"
Subsystem               00000002        (Windows GUI)

C:\test> _

That worked fine, with the GNU toolchain.

But what about the Microsoft toolchain, i.e. Visual C++?

Well, building as a console subsystem executable works fine:

C:\test> msvc x.cpp user32.lib
x.cpp

C:\test> dumpbin /headers x.exe | find /i "subsystem" | find /i "Windows"
               3 subsystem (Windows CUI)

C:\test> _

However, with Microsoft's toolchain building as GUI subsystem does not work by default:

C:\test> msvc x.cpp user32.lib /link /subsystem:windows
x.cpp
LIBCMT.lib(wincrt0.obj) : error LNK2019: unresolved external symbol _WinMain@16 referenced in function ___tmainCRTStartu
p
x.exe : fatal error LNK1120: 1 unresolved externals

C:\test> _

Technically this is because Microsoft’s linker is non-standard by default for GUI subsystem. By default, when the subsystem is GUI, then Microsoft's linker uses a runtime library entry point, the function where the machine code execution starts, called winMainCRTStartup, that calls Microsoft's non-standard WinMain instead of standard main.

No big deal to fix that, though.

All you have to do is to tell Microsoft's linker which entry point to use, namely mainCRTStartup, which calls standard main:

C:\test> msvc x.cpp user32.lib /link /subsystem:windows /entry:mainCRTStartup
x.cpp

C:\test> dumpbin /headers x.exe | find /i "subsystem" | find /i "Windows"
               2 subsystem (Windows GUI)

C:\test> _

No problem, but very tedious. And so arcane and hidden that most Windows programmers, who mostly only use Microsoft’s non-standard-by-default tools, do not even know about it, and mistakenly think that a Windows GUI subsystem program “must” have non-standard WinMain instead of standard main. In passing, with C++0x Microsoft will have a problem with this, since the compiler must then advertize whether it's free-standing or hosted (when hosted it must support standard main).

Anyway, that's the reason why g++ can complain about WinMain missing: it's a silly non-standard startup function that Microsoft's tools require by default for GUI subsystem programs.

But as you can see above, g++ has no problem with standard main even for a GUI subsystem program.

So what could be the problem?

Well, you are probably missing a main. And you probably have no (proper) WinMain either! And then g++, after having searched for main (no such), and for Microsoft's non-standard WinMain (no such), reports that the latter is missing.

Testing with an empty source:

C:\test> type nul >y.cpp

C:\test> gnuc y.cpp -mwindows
c:/program files/mingw/bin/../lib/gcc/mingw32/4.4.1/../../../libmingw32.a(main.o):main.c:(.text+0xd2): undefined referen
ce to `WinMain@16'
collect2: ld returned 1 exit status

C:\test> _

Answer 5

My situation was that I did not have a main function.

Answer 6

Check that All Files are Included in Your Project:

I had this same error pop up after I updated cLion. After hours of tinkering, I noticed one of my files was not included in the project target. After I added it back to the active project, I stopped getting the undefined reference to winmain16, and the code compiled.

Edit: It's also worthwhile to check the build settings within your IDE.

(Not sure if this error is related to having recently updated the IDE - could be causal or simply correlative. Feel free to comment with any insight on that factor!)

Answer 7

I was encountering this error while compiling my application with SDL. This was caused by SDL defining it's own main function in SDL_main.h. To prevent SDL define the main function an SDL_MAIN_HANDLED macro has to be defined before the SDL.h header is included.

Answer 8

To summarize the above post by Cheers and hth. - Alf, Make sure you have main() or WinMain() defined and g++ should do the right thing.

My problem was that main() was defined inside of a namespace by accident.