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.

Prerequisites

A project for the Hi3861 development board has been created as instructed in Creating a Project and Obtaining Source Code.

Procedure

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, to add the my_first_app service to the app directory, where the hello_world.c file stores the service code and BUILD.gn is the compilation script, the directory structure can be planned 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 entry point 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/applications.json file and add the configuration of hello_world_app. The following code snippet is a snippet of the applications.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.)

NOTE

In this example, the OpenHarmony-v3.1-Release version is used, where the component configuration file is build/lite/components/applications.json. In OpenHarmony-v3.2-Beta2 and later versions, the component configuration file is build/lite/components/communication.json.

{
  "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":[] }
        ]
      },
```