dlopen/LoadLibrary on same application

Question

I need to call an exported symbol from my own application - therefore I need to know if it's safe to call dlopen / LoadLibrary on "yourself".

An example would be:

LoadLibrary("test.exe");

inside a program called test.exe.

I tested it and it seems to work, but I'm not quite sure if it is actually supported behavior.

Answer 1

From the MSDN documentation here...

The system maintains a per-process reference count on all loaded modules. Calling LoadLibrary increments the reference count. Calling the FreeLibrary or FreeLibraryAndExitThread function decrements the reference count. The system unloads a module when its reference count reaches zero or when the process terminates (regardless of the reference count).

It will work, just remember to clean-up after yourself by calling FreeLibrary, as stated above.

What you may have really wanted instead is GetModuleHandle.

Retrieves a module handle for the specified module. The module must have been loaded by the calling process.

In fact, what you're trying to do is even a special case.

lpModuleName [in, optional]

...

If this parameter is NULL, GetModuleHandle returns a handle to the file used to create the calling process (.exe file).

So, trying this...

#include <stdio.h>
#include <windows.h>

__declspec(dllexport) void print(void) {
  puts("OK");
}

main() {
  HMODULE mod = GetModuleHandle(0);
  FARPROC proc = GetProcAddress(mod, "print");
  proc();
  return 0;
}

... appears to work:

C:\dev\scrap>gcc -oprint print.c

C:\dev\scrap>print
OK

For dlopen, it appears very similar.

If filename is a NULL pointer, then the returned handle is for the main program. When given to dlsym(), this handle causes a search for a symbol in the main program, followed by all shared libraries loaded at program startup, and then all shared libraries loaded by dlopen() with the flag RTLD_GLOBAL.