Your First Hook

INFO

You can find the reference code for this section here.

Now that you have your mod working, you probably want it to do something instead of crashing the game.

In this section, we will hook Dimension::getTimeOfDay to make time appear to pass faster. We'll need to scan a signature to find this function, and then hook the function. To do this, we'll need to add some dependency libraries.

To scan signatures, we will use libhat, a memory hacking library that includes a fast signature scanner.

For hooking, we will use MinHook.

CPM

CPM is a CMake library that makes it easier to add other libraries.

To use it, download CPM.cmake from the latest release and save it at cmake/CPM.cmake in your project's directory.

Now, in your CMakeLists.txt, below the CMAKE_MSVC_RUNTIME_LIBRARY line, add the following:

include(cmake/CPM.cmake)

You now have CPM added to your project.

Adding the Libraries

Below the CPM line, add the following:

CPMAddPackage("gh:BasedInc/libhat@0.9.0")
CPMAddPackage("gh:TsudaKageyu/minhook@1.3.4")

This will make CPM automatically download the libraries. To use them in your mod, you must link against them. Add the following at the bottom of your CMakeLists.txt:

target_link_libraries(<project_name_here> PRIVATE 
        libhat::libhat 
        minhook)

Adding Your First Hook

Now, we will use libhat to scan for Dimension::getTimeOfDay. As of 1.21.120, the signature for getTimeOfDay is 44 8B C2 B8 F1 19 76 05 F7 EA.

#include <libhat/scanner.hpp>
#include <libhat/signature.hpp>

static DWORD WINAPI startup(LPVOID dll) {
    constexpr auto signature = hat::compile_signature<"44 8B C2 B8 F1 19 76 05 F7 EA">();

    hat::scan_result scanResult = hat::find_pattern(signature, ".text");
    std::byte* getTimeOfDay = scanResult.get();
}

Next, write the detour function which MinHook will redirect getTimeOfDay to.

// MinHook will redirect Dimension::getTimeOfDay to this function, replacing its behavior with our own code.
static float hk_getTimeOfDay(void* _this, int32_t time, float alpha) {
    static float value = 0.f;
    value += 0.00001f;
    value = std::fmodf(value, 1.f);

    return value;
}

Finally, we'll initialize MinHook after scanning for getTimeOfDay, create a hook that redirects it to hk_getTimeOfDay, and enable that hook. Then we will exit startup. Because of the way DLL injection works, this will cause our client to remain injected until the game is closed.

#include <MinHook.h>

static DWORD WINAPI startup(LPVOID dll) {
    constexpr auto signature = hat::compile_signature<"44 8B C2 B8 F1 19 76 05 F7 EA">();

    hat::scan_result scanResult = hat::find_pattern(signature, ".text");
    std::byte* getTimeOfDay = scanResult.get();

    // Initialize MinHook.
    MH_Initialize();

    // Hook getTimeOfDay.
    LPVOID original;
    MH_CreateHook(getTimeOfDay, &hk_getTimeOfDay, &original);
    MH_EnableHook(getTimeOfDay);

    // Exiting this thread without calling FreeLibraryAndExitThread on `dll` will cause the DLL to remain injected forever.
    return 0;
}

Now build your mod again, and inject the DLL. If you see the sun and moon spinning around you, it worked!

You can try changing the constant in hk_getTimeOfDay to make the time go faster or slower.

TIP

You can't build your mod while the DLL is injected. Close the game before building your mod again.