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.

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
```
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);
```
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.

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"
      ],

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":[] }
        ]
      },
```