# How to Create Executables that Use MulleObjC: A Complete Guide > Learn to create MulleObjC executables by linking to the MulleObjC-startup library. Initialize the runtime without a full Foundation stack for efficient C-based Objective-C development. - Repository: [mulle-objc/mulleobjc-startup](https://github.com/mulle-objc/mulleobjc-startup) - Tags: how-to-guide - Published: 2026-03-07 --- **Link your executable against the MulleObjC-startup static library to automatically register the `__register_mulle_objc_universe` function and initialize the runtime without pulling in the full Foundation stack.** The [mulle-objc/mulleobjc-startup](https://github.com/mulle-objc/mulleobjc-startup) repository provides a lightweight bootstrap mechanism for building standalone executables that leverage the Mulle Objective-C runtime. By including this static library in your build, you obtain the minimal runtime initialization code required to boot the MulleObjC universe, along with automatic handling of `mulle-atinit` and `mulle-atexit` helpers. ## What Is MulleObjC-startup? MulleObjC-startup is a static library that supplies the mandatory `__register_mulle_objc_universe` function referenced by the MulleObjC runtime. When linked into an executable, this library ensures the Objective-C universe is properly initialized before your `main()` function executes. The core implementation resides in **`src/MulleObjC-startup.m`**, where the private registration symbol is defined and the `bang` routine invokes `MulleObjCBang` to perform the actual universe setup. Unlike the full Foundation framework, this library provides only the essential startup mechanics, making it ideal for lightweight command-line tools and embedded applications. ## Prerequisites and Project Setup The recommended workflow uses the `mulle-sde` command-line tool to scaffold a CMake-based project preconfigured for MulleObjC development. Initialize a new executable project: ```bash mulle-sde init -d my-project -m mulle-objc/objc-developer executable cd my-project ``` Enable colorful build output (optional): ```bash mulle-sde vibecoding on ``` Add the startup library dependency: ```bash mulle-sde add github:mulle-objc/MulleObjC-startup ``` If you require the complete Foundation stack later, you can additionally add `github:mulle-objc/MulleFoundation-startup`, though the basic MulleObjC-startup is sufficient for runtime-only executables. ## Writing Your Executable Code Include the public header in your source files to ensure proper registration: ```c #import ``` A minimal executable looks like this: ```c #import #include int main( int argc, const char * argv[] ) { // The universe is automatically registered by the library's constructor printf( "Hello from MulleObjC!\n"); return 0; } ``` The runtime registration happens automatically through library constructors, so you do not need to manually invoke initialization functions in your `main()` routine. ## CMake Configuration and Linker Flags When using `mulle-sde`, the build system automatically configures the necessary linker flags through the templates in **`cmake/share/ExecutableObjC.cmake`** and **`cmake/share/InstallExecutable.cmake`**. The generated [`CMakeLists.txt`](https://github.com/mulle-objc/mulleobjc-startup/blob/main/CMakeLists.txt) links your executable against the startup library: ```cmake cmake_minimum_required(VERSION 3.13) project(my_project C) add_executable( my_exe src/main.c ) target_link_libraries( my_exe PUBLIC MulleObjC-startup) ``` The CMake scripts automatically inject platform-specific linker directives: - **Linux**: `--export-dynamic` to expose symbols for runtime registration - **macOS**: `-exported_symbol,__register_mulle_objc_universe` to explicitly export the required registration symbol These flags ensure the `__register_mulle_objc_universe` symbol remains visible to the dynamic linker during universe initialization. ## Build and Run Compile and link your executable using the standard mulle-sde workflow: ```bash mulle-sde run ``` Alternatively, invoke CMake directly: ```bash cmake --build . && ./my_exe ``` During the build process, the system: 1. Pulls in the MulleObjC-startup static library from your dependencies 2. Applies the linker flags defined in `ExecutableObjC.cmake` 3. Produces an executable with the runtime pre-registered and ready for Objective-C code ## Summary - **MulleObjC-startup** is a static library providing the `__register_mulle_objc_universe` symbol required to boot the MulleObjC runtime - Use **`mulle-sde init`** to scaffold projects with proper CMake configuration already in place - Include **[`MulleObjC-startup/MulleObjC-startup.h`](https://github.com/mulle-objc/mulleobjc-startup/blob/main/MulleObjC-startup/MulleObjC-startup.h)** to access the runtime registration interface - The **`cmake/share/ExecutableObjC.cmake`** template automatically handles platform-specific linker flags for symbol export - The initialization sequence in **`src/MulleObjC-startup.m`** invokes `MulleObjCBang` to configure the universe before `main()` executes ## Frequently Asked Questions ### Do I need to call any initialization functions in my main() function? No. The MulleObjC-startup library uses constructor attributes to automatically register the universe during dynamic library loading. By including the header and linking the library, the `__register_mulle_objc_universe` function defined in `src/MulleObjC-startup.m` executes before your `main()` function, ensuring the runtime is ready when your code begins executing. ### Why does my executable need special linker flags? The MulleObjC runtime dynamically discovers and registers universes through the `__register_mulle_objc_universe` symbol. On Linux, `--export-dynamic` ensures this symbol is visible to the dynamic linker, while macOS requires `-exported_symbol,__register_mulle_objc_universe` to prevent symbol stripping. The `ExecutableObjC.cmake` file in the repository provides these flags automatically when using the recommended build system. ### Can I use MulleObjC-startup without mulle-sde? Yes, though you must manually configure your build system to link against the static library and include the appropriate linker flags. Clone the repository, build the library using the provided [`CMakeLists.txt`](https://github.com/mulle-objc/mulleobjc-startup/blob/main/CMakeLists.txt), then add the library path to your executable's link step. You must also manually apply the platform-specific export flags found in `cmake/share/ExecutableObjC.cmake` to ensure proper symbol visibility. ### What is the difference between MulleObjC-startup and MulleFoundation-startup? **MulleObjC-startup** provides only the minimal runtime bootstrap code—the universe registration and basic initialization hooks. **MulleFoundation-startup** extends this with the full Foundation framework initialization, including standard library classes and utilities. For command-line tools that only need the Objective-C runtime without Foundation classes, MulleObjC-startup is the lighter and faster option.