Chapter 8.1: Your First Mod (Hello World)

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Prerequisites
Step 1: Install DayZ Tools
Step 2: Set Up the P: Drive (Workdrive)
Step 3: Create the Mod Directory Structure
Step 4: Write mod.cpp
Step 5: Write config.cpp
Step 6: Write Your First Script
Step 7: Pack the PBO with Addon Builder
Step 8: Load the Mod in DayZ
Step 9: Verify in the Script Log
Step 10: Troubleshooting Common Issues
Complete File Reference
Next Steps

Prerequisites

Before you begin, make sure you have:

Steam installed and logged in
DayZ game installed (retail version from Steam)
A text editor (VS Code, Notepad++, or even Notepad)
About 15 GB of free disk space for DayZ Tools

That is everything.

Step 1: Install DayZ Tools

DayZ Tools is a free application on Steam that includes everything you need to build mods: the Workbench script editor, Addon Builder for PBO packing, Terrain Builder, and Object Builder.

How to Install

Open Steam
Go to Library
In the dropdown filter at the top, change Games to Tools
Search for DayZ Tools
Click Install
Wait for the download to complete (it is roughly 12-15 GB)

Once installed, you will find DayZ Tools in your Steam library under Tools. The default installation path is:

C:\Program Files (x86)\Steam\steamapps\common\DayZ Tools\

What Gets Installed

Tool	Purpose
Addon Builder	Packs your mod files into `.pbo` archives
Workbench	Script editor with syntax highlighting
Object Builder	3D model viewer and editor for `.p3d` files
Terrain Builder	Map/terrain editor
TexView2	Texture viewer/converter (`.paa`, `.edds`)

For this tutorial, you only need Addon Builder. The others are useful later.

Step 2: Set Up the P: Drive (Workdrive)

DayZ modding uses a virtual drive letter P: as a shared workspace. All mods and game data reference paths starting from P:, which keeps paths consistent across different machines.

Creating the P: Drive

Open DayZ Tools from Steam
In the main DayZ Tools window, click P: Drive Management (or look for a button labeled "Mount P drive" / "Setup P drive")
Click Create/Mount P: Drive
Choose a location for the P: drive data (default is fine, or pick a drive with enough space)
Wait for the process to complete

Verify It Works

Open File Explorer and navigate to P:\. You should see a directory that contains DayZ game data. If the P: drive exists and you can browse it, you are ready to proceed.

Alternative: Manual P: Drive

If the DayZ Tools GUI does not work, you can create a P: drive manually using a Windows command prompt (run as Administrator):

batch

subst P: "C:\DayZWorkdrive"

Replace C:\DayZWorkdrive with any folder you want. This creates a temporary drive mapping that lasts until you reboot. For a permanent mapping, use net use or the DayZ Tools GUI.

What If I Do Not Want to Use P: Drive?

You can develop without the P: drive by placing your mod folder directly in the DayZ game directory and using -filePatching mode. However, the P: drive is the standard workflow and all official documentation assumes it. We strongly recommend setting it up.

Step 3: Create the Mod Directory Structure

Every DayZ mod follows a specific folder structure. Create the following directories and files on your P: drive (or in your DayZ game directory if not using P:):

P:\MyFirstMod\
    mod.cpp
    Scripts\
        config.cpp
        5_Mission\
            MyFirstMod\
                MissionHello.c

Create the Folders

Open File Explorer
Navigate to P:\
Create a new folder called MyFirstMod
Inside MyFirstMod, create a folder called Scripts
Inside Scripts, create a folder called 5_Mission
Inside 5_Mission, create a folder called MyFirstMod

Understanding the Structure

Path	Purpose
`MyFirstMod/`	Root of your mod
`mod.cpp`	Metadata (name, author) shown in the DayZ launcher
`Scripts/config.cpp`	Tells the engine what your mod depends on and where scripts live
`Scripts/5_Mission/`	The mission script layer (UI, startup hooks)
`Scripts/5_Mission/MyFirstMod/`	Subfolder for your mod's mission scripts
`Scripts/5_Mission/MyFirstMod/MissionHello.c`	Your actual script file

You need exactly 3 files. Let us create them one by one.

Step 4: Write mod.cpp

Create the file P:\MyFirstMod\mod.cpp in your text editor and paste this content:

cpp

name = "My First Mod";
author = "YourName";
version = "1.0";
overview = "My very first DayZ mod. Prints Hello World to the script log.";

What Each Line Does

name -- The display name shown in the DayZ launcher mod list. Players see this when selecting mods.
author -- Your name or team name.
version -- Any version string you like. The engine does not parse it.
overview -- A description shown when expanding the mod details.

Save the file. That is your mod's identity card.

Step 5: Write config.cpp

Create the file P:\MyFirstMod\Scripts\config.cpp and paste this content:

cpp

class CfgPatches
{
    class MyFirstMod_Scripts
    {
        units[] = {};
        weapons[] = {};
        requiredVersion = 0.1;
        requiredAddons[] =
        {
            "DZ_Data"
        };
    };
};

class CfgMods
{
    class MyFirstMod
    {
        dir = "MyFirstMod";
        name = "My First Mod";
        author = "YourName";
        type = "mod";

        dependencies[] = { "Mission" };

        class defs
        {
            class missionScriptModule
            {
                value = "";
                files[] = { "MyFirstMod/Scripts/5_Mission" };
            };
        };
    };
};

What Each Section Does

CfgPatches declares your mod to the DayZ engine:

class MyFirstMod_Scripts -- A unique identifier for your mod's script package. Must not collide with any other mod.
units[] = {}; weapons[] = {}; -- Lists of entities and weapons your mod adds. Empty for now.
requiredVersion = 0.1; -- Minimum game version. Always 0.1.
requiredAddons[] = { "DZ_Data" }; -- Dependencies. DZ_Data is the base game data. This ensures your mod loads after the base game.

CfgMods tells the engine where your scripts live:

dir = "MyFirstMod"; -- Root directory of the mod.
type = "mod"; -- This is a client+server mod (as opposed to "servermod" for server-only).
dependencies[] = { "Mission" }; -- Your code hooks into the Mission script module.
class missionScriptModule -- Tells the engine to compile all .c files found in MyFirstMod/Scripts/5_Mission/.

Why only 5_Mission? Because our Hello World script hooks into the mission startup event, which lives in the mission layer. Most simple mods start here.

Step 6: Write Your First Script

Create the file P:\MyFirstMod\Scripts\5_Mission\MyFirstMod\MissionHello.c and paste this content:

modded class MissionServer
{
    override void OnInit()
    {
        super.OnInit();
        Print("[MyFirstMod] Hello World! The SERVER mission has started.");
    }
};

modded class MissionGameplay
{
    override void OnInit()
    {
        super.OnInit();
        Print("[MyFirstMod] Hello World! The CLIENT mission has started.");
    }
};

Line-by-Line Explanation

modded class MissionServer

The modded keyword is the heart of DayZ modding. It says: "Take the existing MissionServer class from the vanilla game and add my changes on top." You are not creating a new class -- you are extending the existing one.

    override void OnInit()

OnInit() is called by the engine when a mission starts. override tells the compiler that this method already exists in the parent class and we are replacing it with our version.

        super.OnInit();

This line is critical. super.OnInit() calls the original vanilla implementation. If you skip this, the vanilla mission initialization code never runs and the game breaks. Always call super first.

        Print("[MyFirstMod] Hello World! The SERVER mission has started.");

Print() writes a message to the DayZ script log file. The [MyFirstMod] prefix makes it easy to find your messages in the log.

modded class MissionGameplay

MissionGameplay is the client-side equivalent of MissionServer. When a player joins a server, MissionGameplay.OnInit() fires on their machine. By modding both classes, your message appears in both server and client logs.

About `.c` Files

DayZ scripts use the .c file extension. Despite looking like C, this is Enforce Script, DayZ's own scripting language. It has classes, inheritance, arrays, and maps, but it is not C, C++, or C#. Your IDE may show syntax errors -- that is normal and expected.

Step 7: Pack the PBO with Addon Builder

DayZ loads mods from .pbo archive files (similar to .zip but in a format the engine understands). You need to pack your Scripts folder into a PBO.

Using Addon Builder (GUI)

Open DayZ Tools from Steam
Click Addon Builder to launch it
Set Source directory to: P:\MyFirstMod\Scripts\
Set Output/Destination directory to a new folder: P:\@MyFirstMod\Addons\
Create the @MyFirstMod\Addons\ folder first if it does not exist.
In Addon Builder Options:
- Set Prefix to: MyFirstMod\Scripts
- Leave other options at defaults
Click Pack

If successful, you will see a file at:

P:\@MyFirstMod\Addons\Scripts.pbo

Set Up the Final Mod Structure

Now copy your mod.cpp next to the Addons folder:

P:\@MyFirstMod\
    mod.cpp                         <-- Copy from P:\MyFirstMod\mod.cpp
    Addons\
        Scripts.pbo                 <-- Created by Addon Builder

The @ prefix on the folder name is a convention for distributable mods. It signals to server administrators and the launcher that this is a mod package.

Alternative: Test Without Packing (File Patching)

During development, you can skip PBO packing entirely using file patching mode. This loads scripts directly from your source folders:

DayZDiag_x64.exe -mod=P:\MyFirstMod -filePatching

File patching is faster for iteration because you edit a .c file, restart the game, and see the changes immediately. No packing step needed. However, file patching only works with the diagnostic executable (DayZDiag_x64.exe) and is not suitable for distribution.

Step 8: Load the Mod in DayZ

There are two ways to load your mod: through the launcher or via command-line parameters.

Option A: DayZ Launcher

Open the DayZ Launcher from Steam
Go to the Mods tab
Click Add local mod (or "Add mod from local storage")
Browse to P:\@MyFirstMod\
Enable the mod by checking its checkbox
Click Play (make sure you are connecting to a local/offline server, or launching single-player)

Option B: Command Line (Recommended for Development)

For faster iteration, launch DayZ directly with command-line parameters. Create a shortcut or batch file:

Using the Diagnostic Executable (with file patching, no PBO needed):

batch

"C:\Program Files (x86)\Steam\steamapps\common\DayZ\DayZDiag_x64.exe" -mod=P:\MyFirstMod -filePatching -server -config=serverDZ.cfg -port=2302

Using the packed PBO:

batch

"C:\Program Files (x86)\Steam\steamapps\common\DayZ\DayZDiag_x64.exe" -mod=P:\@MyFirstMod -server -config=serverDZ.cfg -port=2302

The -server flag launches a local listen server. The -filePatching flag allows loading scripts from unpacked folders.

Quick Test: Offline Mode

The fastest way to test is to launch DayZ in offline mode:

batch

DayZDiag_x64.exe -mod=P:\MyFirstMod -filePatching

Then in the main menu, click Play and select Offline Mode (or Community Offline). This starts a local single-player session without needing a server.

Step 9: Verify in the Script Log

After launching DayZ with your mod, the engine writes all Print() output to log files.

Finding the Log Files

DayZ stores logs in your local AppData directory:

C:\Users\<YourWindowsUsername>\AppData\Local\DayZ\

To get there quickly:

Press Win + R to open the Run dialog
Type %localappdata%\DayZ and press Enter

Look for the most recent file named like:

script_<date>_<time>.log

For example: script_2025-01-15_14-30-22.log

What to Search For

Open the log file in your text editor and search for [MyFirstMod]. You should see one of these messages:

[MyFirstMod] Hello World! The SERVER mission has started.

or (if you loaded as a client):

[MyFirstMod] Hello World! The CLIENT mission has started.

If you see your message: congratulations. Your first DayZ mod is working. You have successfully:

Created a mod directory structure
Written a config that the engine reads
Hooked into vanilla game code with modded class
Printed output to the script log

What If You See Errors?

If the log contains lines starting with SCRIPT (E):, something went wrong. Read the next section.

Step 10: Troubleshooting Common Issues

Problem: No Log Output At All (Mod Does Not Seem to Load)

Check your launch parameters. The -mod= path must point to the correct folder. If using file patching, verify the path points to the folder containing Scripts/config.cpp directly (not the @ folder).

Check that config.cpp exists at the right level. It must be at Scripts/config.cpp inside your mod root. If it is in the wrong folder, the engine silently ignores your mod.

Check the CfgPatches class name. If there is no CfgPatches block, or its syntax is wrong, the entire PBO is skipped.

Look at the main DayZ log (not just the script log). Check:

C:\Users\<YourName>\AppData\Local\DayZ\DayZ_<date>_<time>.RPT

Search for your mod name. You may see messages like "Addon MyFirstMod_Scripts requires addon DZ_Data which is not loaded."

Problem: `SCRIPT (E): Undefined variable` or `Undefined type`

This means your code references something the engine does not recognize. Common causes:

Typo in a class name. MisionServer instead of MissionServer (note the double 's').
Wrong script layer. If you reference PlayerBase from 5_Mission, it should work. But if you accidentally placed your file in 3_Game and reference mission types, you will get this error.
Missing super.OnInit() call. Omitting it can cause cascading failures.

Problem: `SCRIPT (E): Member not found`

The method you are calling does not exist on the class. Double-check the method name and make sure you are overriding a real vanilla method. OnInit exists on MissionServer and MissionGameplay -- but not on every class.

Problem: Mod Loads But Script Never Executes

File extension: Make sure your script file ends in .c (not .c.txt or .cs). Windows may hide extensions by default.
Script path mismatch: The files[] path in config.cpp must match your actual directory. "MyFirstMod/Scripts/5_Mission" means the engine looks for a folder at that exact path relative to the mod root.
Class name: modded class MissionServer is case-sensitive. It must match the vanilla class name exactly.

Problem: PBO Packing Errors

Ensure config.cpp is at the root level of what you are packing (the Scripts/ folder).
Check that the prefix in Addon Builder matches your mod path.
Make sure there are no non-text files mixed into the Scripts folder (no .exe, .dll, or binary files).

Problem: Game Crashes on Startup

Check for syntax errors in config.cpp. A missing semicolon, brace, or quote mark can crash the config parser.
Verify that requiredAddons lists valid addon names. A misspelled addon name causes a hard failure.
Remove your mod from the launch parameters and confirm the game starts without it. Then add it back to isolate the issue.

Complete File Reference

Here are all three files in their complete form, for easy copy-paste:

File 1: `MyFirstMod/mod.cpp`

cpp

name = "My First Mod";
author = "YourName";
version = "1.0";
overview = "My very first DayZ mod. Prints Hello World to the script log.";

File 2: `MyFirstMod/Scripts/config.cpp`

cpp

class CfgPatches
{
    class MyFirstMod_Scripts
    {
        units[] = {};
        weapons[] = {};
        requiredVersion = 0.1;
        requiredAddons[] =
        {
            "DZ_Data"
        };
    };
};

class CfgMods
{
    class MyFirstMod
    {
        dir = "MyFirstMod";
        name = "My First Mod";
        author = "YourName";
        type = "mod";

        dependencies[] = { "Mission" };

        class defs
        {
            class missionScriptModule
            {
                value = "";
                files[] = { "MyFirstMod/Scripts/5_Mission" };
            };
        };
    };
};

File 3: `MyFirstMod/Scripts/5_Mission/MyFirstMod/MissionHello.c`

modded class MissionServer
{
    override void OnInit()
    {
        super.OnInit();
        Print("[MyFirstMod] Hello World! The SERVER mission has started.");
    }
};

modded class MissionGameplay
{
    override void OnInit()
    {
        super.OnInit();
        Print("[MyFirstMod] Hello World! The CLIENT mission has started.");
    }
};

Next Steps

Now that you have a working mod, here are the natural progressions:

Chapter 8.2: Creating a Custom Item -- Define a new in-game item with textures and spawning.
Add more script layers -- Create 3_Game and 4_World folders to organize configuration, data classes, and entity logic. See Chapter 2.1: The 5-Layer Script Hierarchy.
Add keybindings -- Create an Inputs.xml file and register custom key actions.
Create UI -- Build in-game panels using layout files and ScriptedWidgetEventHandler. See Chapter 3: GUI System.
Use a framework -- Integrate with Community Framework (CF) or MyMod Core for advanced features like RPC, config management, and admin panels.

Tips

Test with -filePatching before building PBOs. It cuts iteration time from minutes to seconds.
Start with the 5_Mission layer. Only add 3_Game and 4_World when you actually need them.
Always call super first in overridden methods. Omitting it silently breaks vanilla behavior and every other mod hooking the same method.
Use a unique prefix in Print() output (e.g., [MyFirstMod]). Logs contain thousands of lines -- a prefix is the only way to find yours.
A missing semicolon or brace in config.cpp causes a hard crash or silent mod skip with no clear error.

Gotchas

The version field in mod.cpp is purely cosmetic. The engine does not parse it for dependency resolution.
If you misspell an addon name in requiredAddons, the entire PBO is silently skipped with no error in the script log. Check the .RPT file instead.
config.cpp changes and newly added .c files are not covered by file patching. You still need a PBO rebuild for those.
Some APIs (like GetGame().GetPlayer().GetIdentity()) return NULL in offline mode, causing crashes that do not happen on a real server.

Next: Chapter 8.2: Creating a Custom Item

Chapter 8.1: Your First Mod (Hello World) ​

Table of Contents ​

Prerequisites ​

Step 1: Install DayZ Tools ​

How to Install ​

What Gets Installed ​

Step 2: Set Up the P: Drive (Workdrive) ​

Creating the P: Drive ​

Verify It Works ​

Alternative: Manual P: Drive ​

What If I Do Not Want to Use P: Drive? ​

Step 3: Create the Mod Directory Structure ​

Create the Folders ​

Understanding the Structure ​

Step 4: Write mod.cpp ​

What Each Line Does ​

Step 5: Write config.cpp ​

What Each Section Does ​

Step 6: Write Your First Script ​

Line-by-Line Explanation ​

About .c Files ​

Step 7: Pack the PBO with Addon Builder ​

Using Addon Builder (GUI) ​

Set Up the Final Mod Structure ​

Alternative: Test Without Packing (File Patching) ​

Step 8: Load the Mod in DayZ ​

Option A: DayZ Launcher ​

Option B: Command Line (Recommended for Development) ​

Quick Test: Offline Mode ​

Step 9: Verify in the Script Log ​

Finding the Log Files ​

What to Search For ​

What If You See Errors? ​

Step 10: Troubleshooting Common Issues ​

Problem: No Log Output At All (Mod Does Not Seem to Load) ​

Problem: SCRIPT (E): Undefined variable or Undefined type ​

Problem: SCRIPT (E): Member not found ​

Problem: Mod Loads But Script Never Executes ​

Problem: PBO Packing Errors ​

Problem: Game Crashes on Startup ​

Complete File Reference ​

File 1: MyFirstMod/mod.cpp ​

File 2: MyFirstMod/Scripts/config.cpp ​

File 3: MyFirstMod/Scripts/5_Mission/MyFirstMod/MissionHello.c ​

Next Steps ​

Tips ​

Gotchas ​

Chapter 8.1: Your First Mod (Hello World)

Table of Contents

Prerequisites

Step 1: Install DayZ Tools

How to Install

What Gets Installed

Step 2: Set Up the P: Drive (Workdrive)

Creating the P: Drive

Verify It Works

Alternative: Manual P: Drive

What If I Do Not Want to Use P: Drive?

Step 3: Create the Mod Directory Structure

Create the Folders

Understanding the Structure

Step 4: Write mod.cpp

What Each Line Does

Step 5: Write config.cpp

What Each Section Does

Step 6: Write Your First Script

Line-by-Line Explanation

About `.c` Files

Step 7: Pack the PBO with Addon Builder

Using Addon Builder (GUI)

Set Up the Final Mod Structure

Alternative: Test Without Packing (File Patching)

Step 8: Load the Mod in DayZ

Option A: DayZ Launcher

Option B: Command Line (Recommended for Development)

Quick Test: Offline Mode

Step 9: Verify in the Script Log

Finding the Log Files

What to Search For

What If You See Errors?

Step 10: Troubleshooting Common Issues

Problem: No Log Output At All (Mod Does Not Seem to Load)

Problem: `SCRIPT (E): Undefined variable` or `Undefined type`

Problem: `SCRIPT (E): Member not found`

Problem: Mod Loads But Script Never Executes

Problem: PBO Packing Errors

Problem: Game Crashes on Startup

Complete File Reference

File 1: `MyFirstMod/mod.cpp`

File 2: `MyFirstMod/Scripts/config.cpp`

File 3: `MyFirstMod/Scripts/5_Mission/MyFirstMod/MissionHello.c`

Next Steps

Tips

Gotchas