Java JNA: GetProcAddress returns null

Question

The following code fails to find LoadLibraryW

private static FOREIGN_THREAD_START_ROUTINE getLoadLibraryWAddress() throws Win32Exception {
    HMODULE module = Kernel32.INSTANCE.GetModuleHandle("KERNEL32");
    if(module == null) {
        Win32Exception.throwWithLastError("Failed to find KERNEL32 module");
    }

    FOREIGN_THREAD_START_ROUTINE address = Kernel32MissingFunctions.INSTANCE.GetProcAddress(module, "LoadLibraryW");
    if(address == null) {
        Win32Exception.throwWithLastError("Failed to find LoadLibraryW in KERNEL32 module");
    }
    return address;
}

where GetProcAddress is declared as following:

public interface Kernel32MissingFunctions extends StdCallLibrary {

    Kernel32MissingFunctions INSTANCE = (Kernel32MissingFunctions) Native.loadLibrary("kernel32",
            Kernel32MissingFunctions.class, W32APIOptions.UNICODE_OPTIONS);

    public static final int MEM_RELEASE = 0x8000;

    public LPVOID VirtualAllocEx(HANDLE hProcess, LPVOID lpAddress, long dwSize, int flAllocationType, int flProtect);

    public int VirtualFreeEx(HANDLE hProcess, LPVOID lpAddress, long dwSize, int dwFreeType);

    public FOREIGN_THREAD_START_ROUTINE GetProcAddress(HMODULE hModule, String lpProcName);
}

Does anyone know why? What is my mistake? Thanks!

What is the last error being reported when GetProcAddress() returns null? And why are you declaring GetProcAddress() to return a FOREIGN_THREAD_START_ROUTINE? That is NOT what the real GetProcAddress() actually returns. It returns a FARPROC, which is essentially just a pointer, and so would be LPVOID in JNA. You would then type-cast the pointer when you need to actually call the function. — Remy Lebeau
– Remy Lebeau, Commented Aug 4, 2016 at 21:33
Are you passing an ANSI string as the seciond argument to the native GetProcAddress call? Unlike other WIndows API funkcions, GetProcAddress has no unicode version since names of exported functions are written using ANSI characters. — Martin Drab
– Martin Drab, Commented Aug 4, 2016 at 23:55

Ethon · Accepted Answer · 2016-08-07 19:41:13Z

0

Martin Drab was right. Using W32APIOptions.UNICODE_OPTIONS passed an Unicode string to the Ansi function. Changing the interfaces as following fixed the problem:

public interface Kernel32MissingFunctions extends StdCallLibrary {

    Kernel32MissingFunctions INSTANCE = (Kernel32MissingFunctions) Native.loadLibrary("kernel32",
            Kernel32MissingFunctions.class, W32APIOptions.ASCII_OPTIONS);

    public static final int MEM_RELEASE = 0x8000;

    public LPVOID VirtualAllocEx(HANDLE hProcess, LPVOID lpAddress, long dwSize, int flAllocationType, int flProtect);

    public int VirtualFreeEx(HANDLE hProcess, LPVOID lpAddress, long dwSize, int dwFreeType);

    public LPVOID GetProcAddress(HMODULE hModule, String lpProcName);
}

answered Aug 7, 2016 at 19:41

Ethon

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