Question

I have a Visual Studio 2008 C++ project for Windows Mobile 6 with two processes. Both of which I would like to have access to the same function which is contained in process1.

That function is Buzz:

struct TEST_STRUCT
{
    int bar;
    WCHAR foo[ 20 ];
};

typedef int( *pfn_Buzz )( TEST_STRUCT* );

int Buzz( TEST_STRUCT* info );

Process1 contains the definition for Buzz and creates a function pointer for it in a memory-mapped file:

int Buzz( TEST_STRUCT* info )
{
    info->bar = 1;
    wsprintf( info->foo, L"Hello!" );
    return 100;
}

int _tmain( int argc, _TCHAR* argv[] )
{
    // create a memory-mapped file shared memory space that can be read by any process
    HANDLE mapping = ::CreateFileMapping( INVALID_HANDLE_VALUE, NULL, PAGE_READWRITE, 0, sizeof( pfn_Buzz ) , NULL );
    LPVOID buzz_addr = ::MapViewOfFile( mapping, FILE_MAP_ALL_ACCESS, 0, 0, sizeof( pfn_Buzz ) );

    // find our process' memory offset
    DWORD offset = ::GetCurrentProcessIndex() + 0x02000000;

    // copy the complete function address to the shared memory space
    buzz_addr = ( LPVOID )( ( DWORD )&Buzz + offset );

    // convert the function address to a string to send to process2.exe
    WCHAR address[ 9 ] = { 0 };
    wsprintf( address, L"%x", ( ( DWORD )buzz_addr ) );

    // start process2.exe and wait for it to finish
    PROCESS_INFORMATION pi = { 0 };
    ::CreateProcess( L"\\test_user.exe", address, NULL, NULL, FALSE, 0, NULL, NULL, NULL, &pi );
    ::WaitForSingleObject( pi.hProcess, INFINITE );

    ::UnmapViewOfFile( buzz_addr );
    ::CloseHandle( mapping );

    return 0;
}

Process2 receives the address for Buzz from Process1, casts it to the pfn_Buzz function pointer, and executes it.

// process2.exe
int _tmain( int argc, _TCHAR* argv[] )
{
    // get the address of the Buzz() function pointer
    WCHAR* wszAddr = argv[ 1 ];
    WCHAR* wszAddrEnd = &argv[ 1 ][ 8 ];
    DWORD address = wcstol( wszAddr, &wszAddrEnd, 16 );
    pfn_Buzz PFNBuzz = ( pfn_Buzz )address;

    // execute buzz
    TEST_STRUCT test = { 0 };
    PFNBuzz( &test );

    return 0;
}

Unfortunately, I get an Illegal Instruction exception in process2 when I try to execute the PFNBuzz function.

Can anybody suggest what I could change to get the functionality I’m after?

Thanks,
PaulH

Answer 1

If you want both processes to use the same function, put the function into a DLL and link that DLL to both processes. Each process has its own address space, so an address in one process may not work at all in another process, and even if it “works” it’ll be pointing to something completely different.

Although you’re taking/passing the offset into the memory mapped region on the parent side, I don’t see any matching code to map the same region and add its base to the pointer when you try to access it on the client side.

Visual C++ has a __based pointer type that can simplify this a bit.