Re: Load a shared library using gcc/Cygwin

[Brian Dessent <brian at dessent dot net>]

Yu-Cheng Chou wrote:

> int main(){
>    void *handle;
>    int (*fp)();
>    char *modname = "./module.dll";
>    HMODULE h;
>    void (*init)();
> printf("hello1\n");
>    h = LoadLibrary("cygwin1.dll");
> printf("hello1 h  = %p\n", h);
>    init = ( void (*)())GetProcAddress(h, "cygwin_dll_init");
> printf("init = %p\n", init);
>    init(); // CRASH HERE.......!!!

Of course it crashes, you aren't doing anything to address the fact that
the bottom CYGTLS_PADSIZE bytes of the stack will be clobbered by
Cygwin's tls structure.  This is mentioned in the FAQ and explained
further in how-cygtls-works.txt and the cygload sample.  In a Cygwin
program, this is handled for you automatically by the startup code, but
not so if your program is compiled with VC++ and uses its runtime.

This means you have to add the necessary scratch space to the stack, as
low on the stack as possible after the entry to main().  But the
arguments to main() as well as its return location will already be on
the stack (as well as whatever arbitrary values the MSVCRT chooses to
put there) which means they will be clobbered as well.  You can work
around this by making copies of the arguments and not returning from
main().  Or you can save the state of the stack pre-clobbering and then
restore it before returning from main(), as cygload does.

> I followed the instructions from FAQ to load a shared library.
> But the program main.exe crashed at the line init() highlighted in the
> main.c
> 
> How can I fix the problem?

Go back and read the FAQ again and make sure you understand it all.

Brian

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