Trying to compile simple dll written in C using gcc fails

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.