Trying to compile simple dll written in C using gcc fails - c

Question

Trying to compile a simple DLL written in C using gcc.
Tried following many tutorials, but can't get it to compile even when I strip the file down to the very basics.
test_dll.c
#include <stdio.h>
__declspec(dllexport) int __stdcall hello() {
printf ("Hello World!\n");
}
Trying to compile this using the command
gcc -c test_dll.c
Failing, getting this output
test_dll.c: In function '__declspec':
test_dll.c:3:37: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'hello'
__declspec(dllexport) int __stdcall hello() {
^
test_dll.c:5:1: error: expected '{' at end of input
}
^
gcc version
gcc version 4.8.4 (Ubuntu 4.8.4-2ubuntu1~14.04.3)

Answer 1

It depends on what you're trying to do:
1. build a library for linux on linux
then remove the __declspec(dllexport) and __stdcall. On linux, you need nothing special in the source code for building a library. Note that libraries aren't DLLs on linux, they're named *.so (shared object). You'll have to compile with -fPIC and link with -shared to create your .so file. Please use google for more details on this.
2. build a windows DLL on linux
Install mingw packages (search for them in your package manager). Then, instead of just gcc, invoke the cross compiler targeting windows/mingw, e.g. i686-w64-mingw32-gcc.
3. allow to build libraries cross-platform, including windows
If you want to be able to build a library from the same code on windows and linux, you'll need some preprocessor magic for this, so __declespec() is only used when targeting windows. I normally use something like this:
#undef my___cdecl
#undef SOEXPORT
#undef SOLOCAL
#undef DECLEXPORT
#ifdef __cplusplus
# define my___cdecl extern "C"
#else
# define my___cdecl
#endif
#ifndef __GNUC__
# define __attribute__(x)
#endif
#ifdef _WIN32
# define SOEXPORT my___cdecl __declspec(dllexport)
# define SOLOCAL
#else
# define DECLEXPORT my___cdecl
# if __GNUC__ >= 4
# define SOEXPORT my___cdecl __attribute__((visibility("default")))
# define SOLOCAL __attribute__((visibility("hidden")))
# else
# define SOEXPORT my___cdecl
# define SOLOCAL
# endif
#endif
#ifdef _WIN32
# undef DECLEXPORT
# ifdef BUILDING_MYLIB
# define DECLEXPORT __declspec(dllexport)
# else
# ifdef MYLIB_STATIC
# define DECLEXPORT my___cdecl
# else
# define DECLEXPORT my___cdecl __declspec(dllimport)
# endif
# endif
#endif
Then put a DECLEXPORT in front of each declaration to be exported by the lib and SOEXPORT in front of each definition. That's just a quick example.

Answer 2

Since you are compiling on Linux, the gcc takes Linux as the target.
What you want to do is cross compile for Windows. Which means you will need a cross compiler. The one available for Ubuntu Linux is mingw.
You can install it using
apt-get install gcc-mingw32
Then you can compile with
gcc-mingw32 -c test_dll.c
Further compiling into dll would need
gcc-mingw32 --shared test_dll.o -o test_dll.dll
This dll can then be used on Windows.