How to redefine c++ pointer function? - c++

Question

I have code from BASS lib.
#ifndef BASSDEF
#define BASSDEF(f) WINAPI f
#else
#define NOBASSOVERLOADS
#endif
HSAMPLE BASSDEF(BASS_SampleLoad)(BOOL mem, const void *file, QWORD offset, DWORD length, DWORD max, DWORD flags);
I need redefine BASSDEF to call dlsym function. How can i do this?
Update:
I using this on Android NDK (Linux) i loaded bass module via function dlopen and i need to make all functions point (here is original header file of bass lib https://pastebin.com/Z2Ty9UsY ) to this loaded module via dlsym function. I need this to call all functions (from JNI inside bass.so) module easily.

Answer 1

Actually, BASSDEF is not a function. It's macro which is known at compile time. So let's unwrap it ourselves:
HSAMPLE WINAPI BASS_SampleLoad(BOOL mem, const void *file, QWORD offset, DWORD length, DWORD max, DWORD flags);
Whoa, just function declaration here. Now "WINAPI" is basicly __stdcall call convention (Microsoft-specific). But, looking to BASS header you provided one can find for non-WIN32 systems:
#define WINAPI
Basicly, under Linux it's just a placeholder which expands to nothing. Now function declaration looks like this:
HSAMPLE BASS_SampleLoad(BOOL mem, const void *file, QWORD offset, DWORD length, DWORD max, DWORD flags);
What's next? You would like to find this function in some shared library via dlsym?
I assume you wants something like this:
// Declare a function pointer in C++11 style
using BASS_SampleLoad_FuncPtr = std::add_pointer<decltype(BASS_SampleLoad)>::type;
// Open library you wants
void* soHandle = dlopen("your_lib_here.so", RTLD_LAZY);
// Error check!
if (nullptr == soHandle) {
// Fail here
}
// Finally, get pointe to function!
BASS_SampleLoad_FuncPtr BASS_SampleLoad = reinterpret_cast<BASS_SampleLoad_FuncPtr>(dlsym(soHandle, "BASS_SampleLoad"));
// Error check!
if (nullptr == BASS_SampleLoad) {
// Fail here
}
// Now only here it's safe to call "BASS_SampleLoad" with required params
auto sample = BASS_SampleLoad(...);
...
// Don't forget to close lib
dlclose(soHandle);
Please, NOTE!
Provided code is not tested and might contain errors. And, it's C++11 standard.
Also, for C++14 and higher replace 'std::add_pointer<...>::type' with 'std::add_pointer_t<...>'
P.S. this code valid because BASS is cross-platform library and all WinAPI-look-a-like stuff (WINAPI, QWORD, BOOL, DWORD, etc.) is defined for Linux in BASS header