How to specify dll onload function for mingw32? - c

Question

I can compile DLLs properly using mingw and do the exports/imports stuff. What I am looking for is defining the dll onload function properly as you would in MS VC products. Google didn't turn up anything. Anyone have any ideas or a link to a tutorial?

Answer 1

Okay, so after some fiddling...it's working. For anyone else that is having issues here it is. My issues weren't related to compiling in instead of loading dynamically. It was a mash-up of a couple of tutorial/question/how-tos that got me to this point.
dll.c
#include <stdio.h>
#include <windows.h>
#include "dll.h"
//extern "C" BOOL WINAPI DllMain(HINSTANCE hinstDLL, DWORD Reason, LPVOID LPV) {
//This one was only necessary if you were using a C++ compiler
BOOL WINAPI DllMain(HINSTANCE hinstDLL, DWORD fdwReason, LPVOID lpvReserved) {
switch (fdwReason)
{
case DLL_PROCESS_ATTACH:
// Code to run when the DLL is loaded
printf ("Load working...\n");
break;
case DLL_PROCESS_DETACH:
// Code to run when the DLL is freed
printf ("Unload working...\n");
break;
case DLL_THREAD_ATTACH:
// Code to run when a thread is created during the DLL's lifetime
printf ("ThreadLoad working...\n");
break;
case DLL_THREAD_DETACH:
// Code to run when a thread ends normally.
printf ("ThreadUnload working...\n");
break;
}
return TRUE;
}
EXPORT void hello(void) {
printf ("Hello\n");
}
dll.h
#ifndef DLL_H_
#define DLL_H_
#ifdef BUILD_DLL
/* DLL export */
#define EXPORT __declspec(dllexport)
#else
/* EXE import */
#define EXPORT __declspec(dllimport)
#endif
EXPORT void hello(void);
#endif /* DLL_H_ */
hello.c
#include <windows.h>
#include <stdio.h>
int main () {
/*Typedef the hello function*/
typedef void (*pfunc)();
/*Windows handle*/
HANDLE hdll;
/*A pointer to a function*/
pfunc hello;
/*LoadLibrary*/
hdll = LoadLibrary("message.dll");
/*GetProcAddress*/
hello = (pfunc)GetProcAddress(hdll, "hello");
/*Call the function*/
hello();
return 0;
}
when compiled with
gcc -c -DBUILD_DLL dll.c
gcc -shared -o message.dll dll.o -Wl,--out-implib,libmessage.a
gcc -c hello.c
gcc -o hello.exe hello.o message.dll
produces the expected output of
Load working...
Hello
Unload working...

Answer 2

Since mingw is just a windows port of GCC and associated tools, you can use GCC constructor and destructor attributes. These work for both shared and static libraries, and execute code before and after main is run, respectively. Additionally, you can specify multiple constructor and destructor functions per library.
static void __attribute__((constructor))
your_lib_init(void)
{
fprintf(stderr, "library init\n");
}
static void __attribute__((destructor))
your_lib_destroy(void)
{
fprintf(stderr, "library destroy\n");
}