While developing your mod, you might end up wanting to modify the built-in entities or add entirely new ones, add weapons or NPCs, make changes to the HUD, or a number of other things that require using the Half-Life SDK to update your mod's code. This book (once complete) attempts to guide you through most of the common changes you might want to make. With that out of the way, let's get started!

What you will need

First of all, you'll need to get your programming environment set up. In Windows, this is Visual Studio. Don't try and avoid it, you need it. On other platforms, use the editor of your choice, as the approach is quite a bit different to Windows.

Git

If you've done any programming, you probably already know how to use Git, or at least are familiar with it. Make sure Git is installed on your computer before you continue. On Windows, download Git from https://git-scm.com/. On Linux, you probably already have Git installed - otherwise, use your distribution's package manager to install it (e.g. sudo apt-get install git on Debian/Ubuntu).

If you would like to learn more about using Git, visit: https://git-scm.com/book

The Half-Life SDK

You have a few options here. Valve's official SDK is hosted on GitHub - if you want to start from the official base, then this is the code you need. However, the project is configured for Visual Studio 2010 only, and the newer versions are easier to find and use. A lot of people have forked the Valve repository and make various updates to it - such as fixing the code and project files to work properly with newer versions of VS. TWHL member Solokiller has such a fork called halflife-updated which updates the project to support new versions of Visual Studio and GCC, as well as some changes to reduce the number of warnings or potential issues. The repository these guides will be referencing is a fork based on halflife-updated maintained by Penguinboy for the purposes of this guide. The only difference is a small tweak to add convenient batch files to automatically copy the compiled DLLs to your mod folder after each build.

The summary of your options is basically:

• Valve's official repository - https://github.com/ValveSoftware/halflife
  • Pros: It's official
  • Cons: You must use VS2010, or go through the migration process to VS2019 manually, Linux makefiles need a lot of changes before they'll work
• Solokiller's halflife-updated repository - https://github.com/Solokiller/halflife-updated
  • Pros: VS2017 and VS2019 support, modern GCC support
  • Cons: It's not official (but it's trustworthy, you can see the changes that were made)
• Penguinboy's halflife-updated repository - https://github.com/LogicAndTrick/halflife-updated
  • Pros: VS2017 and VS2019 support, modern GCC support, contains branches with code for all the tutorials in this guide
  • Cons: Not official (but is trustworthy)

From here on, these guides will assume you'll use one of the halflife-updated repositories - however, all of the changes will work in any version of Visual Studio.

Linux users: Valve's default repository has a makefile that needs a bunch of changes to work properly. It's recommended that you use one of the halflife-updated repositories as well.
Mac users: Just follow the Linux instructions and hope it works. OSX likes to break things and Valve doesn't like to fix them, so it's possible Half-Life just doesn't run at all any more due to OSX dropping support for 32-bit applications.

Once you've chosen which repository to get, use Git to clone it by running this on the command line (substitute the URL for the repository you want to use):

git clone https://github.com/LogicAndTrick/halflife-updated.git

Visual Studio

Linux/Mac users: Obviously this doesn't apply to you. However, you do need to install some things, if you don't already have them. On Ubuntu, you can install the dependencies with: sudo apt-get install build-essential gcc-multilib g++-multilib.

Based on your decision on which repository and VS version you'll be using, these instructions will be different. However, this guide assumes you're using the latest version of VS2019. 2017 is extremely similar, whereas 2010 is a little different, but mostly only when you're installing - after that, it's much the same.

The free Community edition of VS can be downloaded here: https://visualstudio.microsoft.com/downloads/

When installing, make sure you select Desktop development with C++ in the workloads section. Make sure the following optional components are selected as well, in the Individual components section:

• Windows 10 SDK (only the latest version)
• MSVC v142 - VS 2019 C++ x64/x86 build tools (only the latest version)
• If you want Windows XP support, select:
  • MSVC v141 - VS 2017 C++ x64/x86 build tools
  • C++ Windows XP Support for VS2017 (v141) tools [Deprecated]

Finish the installation and you're ready to go. Open projects\vs2019\projects.sln and it should be good to go.

Your first compile

Now that your environment is set up, let's compile the project and make sure the changes work in your mod. If you've not set up a mod already, do it now. The key is to make a liblist.gam file. Here's mine:

game "Test Mod"
hlversion "1110"
type "singleplayer_only"
startmap "c0a0"
trainmap "t0a0"
allowspectators "0"
mpentity "info_player_deathmatch"
gamedll "dlls/hl.dll"
gamedll_linux "dlls/hl.so"
cldll "1"

Then restart Steam to get it to appear in the game list.

The important things to note in the liblist.gam file are:

• gamedll - this needs to be set to get custom code loading properly. If you like, you can change the name of your DLL to something else.
• gamedll_linux - same as above, for Linux.
• cldll - set this to 1 in order to get custom client-side code to load properly. You can't change the name of the client DLL.

How to compile

How to compile will depend on which operating system you're on. See the section below that's appropriate for you:

Windows (Visual Studio)

1. Before your first compile, go into the settings of each project in the solution. Make sure the Windows SDK Version and Platform Toolset settings are configured correctly for the tools that you have installed. For XP support, the platform tools should be v141_xp. Otherwise, it should be either v141 (VS2017) or v142 (VS2019). The Windows SDK Version doesn't matter, you just need one installed for it to work.
2. If you're using Penguinboy's repository, first update copy-client.bat and copy-server.bat and replace path-to-your-mod-folder with the path to your actual mod folder (for example, C:\Program Files (x86)\Steam\SteamApps\common\Half-Life\MyMod). You won't have to manually copy the DLL files later, which will save you some time.
3. In Visual Studio, press Ctrl+Shift+B or go to Build -> Build Solution. Wait for it to finish.
4. If you're not using Penguinboy's repository, you'll need to copy the DLL files into your mod. The hl.dll file goes into the mod's dlls folder, and client.dll goes into the cl_dlls folder. You'll find them here:
   • halflife-updated\projects\[vs2019]\Debug\hldll\hl.dll
   • halflife-updated\projects\[vs2019]\Debug\hl_cdll\client.dll
5. Run your mod and hope it works!

Linux (make)

These instructions are for the halflife-updated repository - if you're using Valve's official repository, it's going to be tough to get Linux going, because the makefiles require a lot of updates to work with modern tools.

1. If you haven't already, install the dependencies you'll need to build. On Ubuntu, this is what you'll need: sudo apt-get install build-essential gcc-multilib g++-multilib. For other distributions, you'll need to do the equivalent of this using your package manager.
2. In a terminal, navigate to the linux folder of the repository, and run make. Wait for it to finish.
3. The compiled files are in the linux/release folder. Copy hl.so into you mod's dlls folder, and client.so into the cl_dlls folder.
4. Run your mod and hope it works!

Making your first change

Let's make a small change to make sure everything is working properly. Open world.cpp inside the hldll project and find the method void CWorld :: Precache( void ). Let's add a line to the beginning of this: This change will simply print Hello, world! to the console when any map is loaded. Start your build by following the instructions above, and copy the compiled binaries into to the correct place in your mod.

Before you launch the mod, make sure that you go into the launch properties in Steam (right click on mod > Properties... > Set launch options...) and set them to -console -dev. This ensures that you see developer messages in the console. Now launch your mod and start a new game.

Assuming everything is working, you should see the message appear somewhere in the console (there's a lot of messages, so don't miss it!).