Capítulo 2.4: Your First Mod -- Minimum Viable

Inicio | << Anterior: mod.cpp & Workshop | Minimum Viable Mod | Siguiente: File Organization >>

Resumen: This chapter walks you through creating the smallest possible DayZ mod from scratch. By the end, you will have a working mod that prints a message to the script log when the game starts. Three files, zero dependencies, under five minutes.

What You Need

DayZ game installed (retail or DayZ Tools/Diag)
A text editor (VS Code, Notepad++, or any plain text editor)
DayZ Tools installed (for PBO packing) -- OR you can test without packing (see Step 5)

The Goal

We will create a mod called HelloMod that:

Loads into DayZ without errors
Prints "[HelloMod] Mission started!" to the script log
Uses the correct standard structure

This is the DayZ equivalent of "Hello World."

Step 1: Create the Directory Structure

Create the following folders and files. You need exactly 3 files:

HelloMod/
  mod.cpp
  Scripts/
    config.cpp
    5_Mission/
      HelloMod/
        HelloMission.c

That is the complete structure. Let us create each file.

Step 2: Create mod.cpp

Create HelloMod/mod.cpp with this content:

cpp

name = "Hello Mod";
author = "YourName";
version = "1.0";
overview = "My first DayZ mod - prints a message on mission start.";

This is the minimum metadata. The DayZ launcher will show "Hello Mod" in the mod list.

Step 3: Create config.cpp

Create HelloMod/Scripts/config.cpp with this content:

cpp

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

class CfgMods
{
    class HelloMod
    {
        dir = "HelloMod";
        name = "Hello Mod";
        author = "YourName";
        type = "mod";

        dependencies[] = { "Mission" };

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

Let us break down what each part does:

CfgPatches declares the mod to the engine. requiredAddons says we depend on DZ_Data (vanilla DayZ data), which ensures we load after the base game.
CfgMods tells the engine where our scripts live. We only use 5_Mission because that is where mission lifecycle hooks are available.
dependencies lists "Mission" because our code hooks into the mission script module.

Step 4: Create Your First Script

Create HelloMod/Scripts/5_Mission/HelloMod/HelloMission.c with this content:

modded class MissionServer
{
    override void OnInit()
    {
        super.OnInit();
        Print("[HelloMod] Mission started! Server is running.");
    }
};

modded class MissionGameplay
{
    override void OnInit()
    {
        super.OnInit();
        Print("[HelloMod] Mission started! Client is running.");
    }
};

What this does:

modded class MissionServer extends the vanilla server mission class. When the server starts a mission, OnInit() fires and our message prints.
modded class MissionGameplay does the same for the client side.
super.OnInit() calls the original (vanilla) implementation first -- this is critical. Never skip it.
Print() writes to the DayZ script log file.

Step 5: Pack and Test

You have two options for testing:

Option A: File Patching (No PBO Required -- Development Only)

DayZ supports loading unpacked mods during development. This is the fastest way to iterate.

Place your HelloMod/ folder inside your DayZ installation directory (or use the P: drive with workbench)
Launch DayZ with the -filePatching parameter and load your mod:

DayZDiag_x64.exe -mod=HelloMod -filePatching

This loads scripts directly from the folder without PBO packing.

Option B: PBO Packing (Required for Distribution)

For Workshop publishing or server deployment, you need to pack into a PBO:

Open DayZ Tools (from Steam)
Open Addon Builder
Set the source directory to HelloMod/Scripts/
Set the output to @HelloMod/Addons/HelloMod_Scripts.pbo
Click Pack

Or use a command-line packer like PBOConsole:

PBOConsole.exe -pack HelloMod/Scripts @HelloMod/Addons/HelloMod_Scripts.pbo

Place the mod.cpp next to the Addons/ folder:

@HelloMod/
  mod.cpp
  Addons/
    HelloMod_Scripts.pbo

Then launch DayZ:

DayZDiag_x64.exe -mod=@HelloMod

Step 6: Verify It Works

Finding the Script Log

DayZ writes script output to log files in your profile directory:

Windows: C:\Users\YourName\AppData\Local\DayZ\

Look for the most recent .RPT or .log file. The script log is typically named:

script_<date>_<time>.log

What to Look For

Open the log file and search for [HelloMod]. You should see:

[HelloMod] Mission started! Server is running.

or (if you joined as a client):

[HelloMod] Mission started! Client is running.

If you see this message, congratulations -- your mod is working.

If You See Errors

If the log contains lines starting with SCRIPT (E):, something went wrong. See the Troubleshooting section below.

Understanding What Happened

Here is the sequence of events when DayZ loaded your mod:

1. Engine starts, reads config.cpp files from all PBOs
2. CfgPatches "HelloMod_Scripts" is registered
   --> requiredAddons ensures it loads after DZ_Data
3. CfgMods "HelloMod" is registered
   --> Engine knows about the missionScriptModule path
4. Engine compiles all mods' 5_Mission scripts
   --> HelloMission.c is compiled
   --> "modded class MissionServer" patches the vanilla class
5. Server starts a mission
   --> MissionServer.OnInit() is called
   --> Your override runs, calling super.OnInit() first
   --> Print() writes to the script log
6. Client connects and loads
   --> MissionGameplay.OnInit() is called
   --> Your override runs
   --> Print() writes to the client log

The modded keyword is the key mechanism. It tells the engine "take the existing class and add my changes on top." This is how every DayZ mod integrates with vanilla code.

Next Steps

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

Add a 3_Game Layer

Add configuration data or constants that do not depend on world entities:

HelloMod/
  Scripts/
    config.cpp              <-- Add gameScriptModule entry
    3_Game/
      HelloMod/
        HelloConfig.c       <-- Configuration class
    5_Mission/
      HelloMod/
        HelloMission.c      <-- Existing file

Update config.cpp to include the new layer:

cpp

dependencies[] = { "Game", "Mission" };

class defs
{
    class gameScriptModule
    {
        value = "";
        files[] = { "HelloMod/Scripts/3_Game" };
    };
    class missionScriptModule
    {
        value = "";
        files[] = { "HelloMod/Scripts/5_Mission" };
    };
};

Add a 4_World Layer

Create custom items, extend players, or add world managers:

HelloMod/
  Scripts/
    config.cpp              <-- Add worldScriptModule entry
    3_Game/
      HelloMod/
        HelloConfig.c
    4_World/
      HelloMod/
        HelloManager.c      <-- World-aware logic
    5_Mission/
      HelloMod/
        HelloMission.c

Add UI

Create a simple in-game panel (covered in Part 3 of this guide):

HelloMod/
  GUI/
    layouts/
      hello_panel.layout    <-- UI layout file
  Scripts/
    5_Mission/
      HelloMod/
        HelloPanel.c        <-- UI script

Add a Custom Item

Define an item in Data/config.cpp and create its script behavior in 4_World:

HelloMod/
  Data/
    config.cpp              <-- CfgVehicles with item definition
    Models/
      hello_item.p3d        <-- 3D model
  Scripts/
    4_World/
      HelloMod/
        HelloItem.c         <-- Item behavior script

Depend on a Framework

If you want to use Community Framework (CF) features, add the dependency:

cpp

// In config.cpp
requiredAddons[] = { "DZ_Data", "JM_CF_Scripts" };

Solución de Problemas

"Addon HelloMod_Scripts requires addon DZ_Data which is not loaded"

Your requiredAddons references an addon that is not present. Make sure DZ_Data is spelled correctly and DayZ base game is loaded.

No Log Output (Mod Seems to Not Load)

Check these in order:

Is the mod in the launch parameter? Verify -mod=HelloMod or -mod=@HelloMod is in your launch command.
Is config.cpp in the right place? It must be at the root of the PBO (or the root of the Scripts/ folder when file-patching).
Are the script paths correct? The files[] paths in config.cpp must match the actual directory structure. "HelloMod/Scripts/5_Mission" means the engine looks for that exact path.
Is there a CfgPatches class? Without it, the PBO is ignored.

SCRIPT (E): Undefined variable / Undefined type

Your code references something that does not exist at that layer. Common causes:

Referencing PlayerBase from 3_Game (it is defined in 4_World)
Typo in a class or variable name
Missing super.OnInit() call (causes cascade failures)

SCRIPT (E): Member not found

The method or property you are calling does not exist on that class. Double-check the vanilla API. Common mistake: calling methods from a newer DayZ version when running an older one.

Mod Loads But Script Does Not Run

Check that your .c file is inside the directory listed in files[]
Ensure the file has a .c extension (not .txt or .cs)
Verify the modded class name matches the vanilla class exactly (case-sensitive)

PBO Packing Errors

Ensure config.cpp is at the root level inside the PBO
File paths inside PBOs use forward slashes (/), not backslashes
Make sure there are no binary files in the Scripts folder (only .c and .cpp)

Mejores Prácticas

Always call super.OnInit() before your custom code in modded mission classes -- skipping it breaks other mods' initialization.
Use a unique prefix in your Print() messages (e.g., [HelloMod]) so you can grep log files quickly.
Start with 5_Mission only. Add 3_Game and 4_World layers incrementally as your mod grows.
Use -filePatching during development to avoid re-packing PBOs on every change.
Keep your first mod under 3 files until it works, then expand. Debugging a minimal structure is far easier.

Teoría vs Práctica

Concepto	Teoría	Realidad
`Print()` outputs to log	Messages appear in script log	Output goes to the `.RPT` file, not a separate script log. On dedicated servers, check the server RPT in the profile folder
`-filePatching` loads loose files	Unpacked mods work instantly	Some assets (models, textures) still require PBO packing; scripts work loose, but `.layout` files may not load from unpacked folders on all setups
`modded class` patches vanilla	Your override replaces the original	Multiple mods can `modded class` the same class; they chain in load order. If one skips `super.OnInit()`, all later mods break
`DZ_Data` is the only needed dependency	Minimal `requiredAddons`	Works for pure script mods, but if you reference any vanilla weapon/item class, you also need `DZ_Scripts` or the specific vanilla PBO
Three files is enough	Mod loads with mod.cpp + config.cpp + one .c file	True for a script-only mod, but adding items or UI requires additional PBOs (Data, GUI)

Complete File Listing

For reference, here are all three files in their entirety:

HelloMod/mod.cpp

cpp

name = "Hello Mod";
author = "YourName";
version = "1.0";
overview = "My first DayZ mod - prints a message on mission start.";

HelloMod/Scripts/config.cpp

cpp

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

class CfgMods
{
    class HelloMod
    {
        dir = "HelloMod";
        name = "Hello Mod";
        author = "YourName";
        type = "mod";

        dependencies[] = { "Mission" };

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

HelloMod/Scripts/5_Mission/HelloMod/HelloMission.c

modded class MissionServer
{
    override void OnInit()
    {
        super.OnInit();
        Print("[HelloMod] Mission started! Server is running.");
    }
};

modded class MissionGameplay
{
    override void OnInit()
    {
        super.OnInit();
        Print("[HelloMod] Mission started! Client is running.");
    }
};

Anterior: Chapter 2.3: mod.cpp & WorkshopSiguiente: Chapter 2.5: File Organization Best Practices

Capítulo 2.4: Your First Mod -- Minimum Viable ​

Tabla de Contenidos ​

What You Need ​

The Goal ​

Step 1: Create the Directory Structure ​

Step 2: Create mod.cpp ​

Step 3: Create config.cpp ​

Step 4: Create Your First Script ​

Step 5: Pack and Test ​

Option A: File Patching (No PBO Required -- Development Only) ​

Option B: PBO Packing (Required for Distribution) ​

Step 6: Verify It Works ​

Finding the Script Log ​

What to Look For ​

If You See Errors ​

Understanding What Happened ​

Next Steps ​

Add a 3_Game Layer ​

Add a 4_World Layer ​

Add UI ​

Add a Custom Item ​

Depend on a Framework ​

Solución de Problemas ​

"Addon HelloMod_Scripts requires addon DZ_Data which is not loaded" ​

No Log Output (Mod Seems to Not Load) ​

SCRIPT (E): Undefined variable / Undefined type ​

SCRIPT (E): Member not found ​

Mod Loads But Script Does Not Run ​

PBO Packing Errors ​

Mejores Prácticas ​

Teoría vs Práctica ​

Complete File Listing ​

HelloMod/mod.cpp ​

HelloMod/Scripts/config.cpp ​

HelloMod/Scripts/5_Mission/HelloMod/HelloMission.c ​