# Writing a Hello World Program


The following exemplifies how to create a program by modifying the source code. The created program outputs the message "Hello world." Perform the steps below in the source code directory.


1. Determine the directory structure.
   
   Before writing service code, you must create a directory (or a directory structure) in **./applications/sample/wifi-iot/app** to store source code files.

   For example, add the **my_first_app** service to the **app** directory, where **hello_world.c** is the service code and **BUILD.gn** is the compilation script. The directory structure is shown as follows:

   
   ```
   .
   └── applications
       └── sample
           └── wifi-iot
               └── app
                   └── my_first_app
                     │── hello_world.c
                     └── BUILD.gn
   ```

2. Write the service code.
   
   Create the **hello_world.c** file in **./applications/sample/wifi-iot/app/my_first_app**. Then, create the service entry function **HelloWorld** in **hello_world.c** and implement service logic. Call **SYS_RUN()** of OpenHarmony to start the service. (**SYS_RUN** is defined in the **ohos_init.h** file.)
   
   ```
   #include <stdio.h>
   #include "ohos_init.h"
   #include "ohos_types.h"
   
   void HelloWorld(void)
   {
       printf("[DEMO] Hello world.\n");
   }
   SYS_RUN(HelloWorld);
   ```

3. Compile the **BUILD.gn** file for building services into a static library.
   
   Create the **BUILD.gn** file in **./applications/sample/wifi-iot/app/my_first_app** and configure the file as follows:

   The **BUILD.gn** file consists of three parts, including target, source file, and header file path. You need to fill in all of these parts.

   
   ```
   static_library("myapp") {
       sources = [
           "hello_world.c"
       ]
       include_dirs = [
           "//utils/native/lite/include"
       ]
   }
   ```

   - Specify the compilation result named **libmyapp.a** in **static_library**. You can fill in this part based on your need.
   - Specify the .c file on which a file depends and its path in **sources**. The path that contains **//** represents an absolute path (the code root path). The path that does not contain **//** is a relative path.
   - Specify the path of .h file on which **sources** depends in **include_dirs**.

4. Add a component.
   
   Modify the **build/lite/components/communication.json** file and add the configuration of **hello_world_app**. The following code snippet is a snippet of the **communication.json** file, where the configuration between **\#\#start\#\#** and **\#\#end\#\#** is the new entry. (The **\#\#start\#\#** and **\#\#end\#\#** lines are only used to identify the location. After the configuration is complete, delete these lines.)

   
   ```
   {
     "components": [
       {
         "component": "camera_sample_communication",
         "description": "Communication related samples.",
         "optional": "true",
         "dirs": [
           "applications/sample/camera/communication"
         ],
         "targets": [
           "//applications/sample/camera/communication:sample"
         ],
         "rom": "",
         "ram": "",
         "output": [],
         "adapted_kernel": [ "liteos_a" ],
         "features": [],
         "deps": {
           "components": [],
           "third_party": []
         }
       },
   ##start##
       {
         "component": "hello_world_app",
         "description": "hello world samples.",
         "optional": "true",
         "dirs": [
           "applications/sample/wifi-iot/app/my_first_app"
         ],
         "targets": [
           "//applications/sample/wifi-iot/app/my_first_app:myapp"
         ],
         "rom": "",
         "ram": "",
         "output": [],
         "adapted_kernel": [ "liteos_m" ],
         "features": [],
         "deps": {
           "components": [],
           "third_party": []
         }
       },
   ##end##
       {
         "component": "camera_sample_app",
         "description": "Camera related samples.",
         "optional": "true",
         "dirs": [
           "applications/sample/camera/launcher",
           "applications/sample/camera/cameraApp",
           "applications/sample/camera/setting",
           "applications/sample/camera/gallery",
           "applications/sample/camera/media"
         ],
   ```

5. Modify the board configuration file.
   
   Modify the **vendor/hisilicon/hispark_pegasus/config.json** file and add an entry for the **hello_world_app** component. The following code snippet is the configuration of the **applications** subsystem, where the configuration between **\#\#start\#\#** and **\#\#end\#\#** is the new entry. (The **\#\#start\#\#** and **\#\#end\#\#** lines are only used to identify the location. After the configuration is complete, delete these lines.)

   
   ```
         {
           "subsystem": "applications",
           "components": [
   ##start##
             { "component": "hello_world_app", "features":[] },
   ##end##
             { "component": "wifi_iot_sample_app", "features":[] }
           ]
         },
   ```