LoadLibrary failed at 2nd time

You can use GetLastError() and check against the error code table to understand what goes wrong.

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/debug/base/system_error_codes.asp

I've tried as the following:

try
{
hd = LoadLibrary("hello.dll");
if (hd)
{
(..)aa = GetProcAddress(...);
aa(); //successful
int a = FreeLibrary(hd); //a is not-zero
}
}
catch(...)
{
int a = GetLastError();
}

//but at the 2nd time calling loadlibrary(), the program crashed, never come into catch()

The LoadLibrary() function doesn't throw any exceptions when an error occurs.
If its return value is NULL, then you should call GetLastError() to get an error code.

LoadLibrary() the 2nd time, then the program will crash.
GetLastError() right after LoadLibrary() will useless.

then I can't get any error code

the program and the dll are both simple

CXXX::OnBtnAA()
{
LoadLibrary();
...
FreeLibrary();
}

when I clicked the button the 2nd time, the programe crashed after LoadLibrary()

Notice: the dll is created under Cygwin/gcc;
gcc -shared -o hello.dll hello.o

LoadLibrary() the 2nd time, then the program will crash.
GetLastError() right after LoadLibrary() will useless.

then I can't get any error code

the program and the dll are both simple

CXXX::OnBtnAA()
{
LoadLibrary();
...
FreeLibrary();
}

when I clicked the button the 2nd time, the programe crashed after LoadLibrary()

Notice: the dll is created under Cygwin/gcc;
gcc -shared -o hello.dll hello.o

You're not doing any of this loading within InitInstance or ExitInstance are you? If so, this could be the problem (can't load or unload DLLs there)...

why you can't load/freelibrary in Init/exitInstance of your CWinApp/Thread derived classes?
it if for example a normal (and only suitable) way fpr loading/freeing resource dlls... so why yo you think one can't load/freelibrary dlls in init/exitinstance?

@astar:
why do you want to load/freelibrary the dll in a OnXXX Handler? why you don't load it on OnCreate and free it in OnDestroy or in the constructor/destructor?
why such overhead?
(LoadLibrary maps the desired dll (or at least tries it) in the processes adress space and FreeLibrary unmaps it from the processes adress space, so every time the handler is called windows has to map and unmap the dll)

But when I called LoadLibrary() the second time, the program crashed.

I don't konw why and how to solve the problem,
Thanks for any direction!
So you are saying that this fails?

#include <windows.h>

int main()
{
HMODULE hMod;
for ( int i = 0; i < 2; ++i )
{
hMod = LoadLibrary("hello.dll");
if ( hMod )
FreeLibrary( hMod );
}
}

Does this code work (it calls LoadLibrary/FreeLibrary 2 times)? If it does, the problem is *not* just calling "FreeLibrary" a second time. So you should confirm with a very simple program first to make sure the problem is what you think it is. If it does work, then your program is causing some other bug that is affecting the call to FreeLibrary. In other words, FreeLibrary crashing is a symptom, and not the cause of the problem.

Regards,

Paul McKenzie

hm.. maybe i don't really get your point...
just for making clear what you want to achieve:
- you have a dll
- the dll exports a function
- which returns a value of a global variable in the dll (??)

what kind of variable or value does the function return? (can you post the code?)

but if you load the dll everytime the handler is called(and in case that you only have one of this button in your app, the dll gets mapped and unmapped every time the handler is called... hm... no... it does get mapped/unmaped every time, also if you have more of these buttons (because your app isn't multithreaded, is it?)......

back to the variable and the value... every time your handler gets called, the dll gets mapped, and so the variable with its initial value. i really don't know what you do in your function... but if you just return this variable, it will have the same value (it also will, if you doesn't modify its contents...) every time you call the function.
also if you modify the variable, right after the modify your dll (and so the variable) gets unmapped from adress space. and the next time your handler gets called the dll gets mapped again (with the variable, with its initial value......)

hm... no idea.

please post the code of this function and the mentioned variables.