# safwk<a name="en-us_TOPIC_0000001115588558"></a>
## Introduction<a name="section11660541593"></a>

The **safwk** module of the System Ability Management subsystem defines how to implement a system ability in OpenHarmony and provides APIs to start and register system abilities.

## Directory Structure<a name="section161941989596"></a>

```
/foundation/systemabilitymgr
│── safwk                # Directory for the safwk module
│  ├── bundle.json      # Description and build file of safwk
│  ├── etc              # Configuration files
│  ├── interfaces       # APIs exposed externally
│  ├── services         # Service implementation
│  ├── test             # Test cases
```

## Usage<a name="section1312121216216"></a>

### Available APIs<a name="section1551164914237"></a>

<a name="table775715438253"></a>
<table><thead align="left"><tr id="row12757154342519"><th class="cellrowborder" valign="top" width="43.19%" id="mcps1.1.3.1.1"><p id="p1075794372512"><a name="p1075794372512"></a><a name="p1075794372512"></a>API</p>
</th>
<th class="cellrowborder" valign="top" width="56.81%" id="mcps1.1.3.1.2"><p id="p375844342518"><a name="p375844342518"></a><a name="p375844342518"></a>Description</p>
</th>
</tr>
</thead>
<tbody><tr id="row1975804332517"><td class="cellrowborder" valign="top" width="43.19%" headers="mcps1.1.3.1.1 "><p id="p5758174313255"><a name="p5758174313255"></a><a name="p5758174313255"></a>sptr&lt;IRemoteObject&gt; GetSystemAbility(int32_t systemAbilityId);</p>
</td>
<td class="cellrowborder" valign="top" width="56.81%" headers="mcps1.1.3.1.2 "><p id="p14758743192519"><a name="p14758743192519"></a><a name="p14758743192519"></a>Obtains the Remote Procedure Call (RPC) object of a system ability.</p>
</td>
</tr>
<tr id="row2758943102514"><td class="cellrowborder" valign="top" width="43.19%" headers="mcps1.1.3.1.1 "><p id="p107581438250"><a name="p107581438250"></a><a name="p107581438250"></a>bool Publish(sptr&lt;IRemoteObject&gt; systemAbility);</p>
</td>
<td class="cellrowborder" valign="top" width="56.81%" headers="mcps1.1.3.1.2 "><p id="p8758743202512"><a name="p8758743202512"></a><a name="p8758743202512"></a>Publishes a system ability.</p>
</td>
</tr>
<tr id="row09311240175710"><td class="cellrowborder" valign="top" width="43.19%" headers="mcps1.1.3.1.1 "><p id="p159328405571"><a name="p159328405571"></a><a name="p159328405571"></a>virtual void DoStartSAProcess(const std::string&amp; profilePath) = 0;</p>
</td>
<td class="cellrowborder" valign="top" width="56.81%" headers="mcps1.1.3.1.2 "><p id="p493294018574"><a name="p493294018574"></a><a name="p493294018574"></a>Starts the system ability based on the system ability profile.</p>
</td>
</tr>
</tbody>
</table>

### How to Use<a name="section129654513264"></a>

A system ability is implemented by using a XXX.cfg, a profile.xml, and a libXXX.z.so. The init process starts the SystemAbility process by executing the corresponding XXX.cfg file.

**Implementing a System Ability in C++**

The sample code is as follows:

**1. Define the *IXXX* class for IPC.**

The *IXXX* class is used to define the functions for the system ability to provide specific capabilities. To define this class, inherit from the **IRemoteBroker** class provided by OpenHarmony for Inter-Process Communication (IPC) and implement the **DECLARE\_INTERFACE\_DESCRIPTOR\(*XXX*)** that uniquely identifies this class. The identifier is used for IPC verification.

```
namespace OHOS {
class IListenAbility : public IRemoteBroker {
public:
    virtual int AddVolume(int volume) = 0;

public:
    enum {
        ADD_VOLUME = 1,
    };
public:
    DECLARE_INTERFACE_DESCRIPTOR(u"OHOS.test.IListenAbility");
};
}
```

**2. Define the *XXX*Proxy class for client communication.**

```
namespace OHOS {
class ListenAbilityProxy : public IRemoteProxy<IListenAbility> {
public:
    int AddVolume(int volume);

    explicit ListenAbilityProxy(const sptr<IRemoteObject>& impl)
        : IRemoteProxy<IListenAbility>(impl)
    {
    }

private:
    static inline BrokerDelegator<ListenAbilityProxy> delegator_;
};
} // namespace OHOS
```

**3. Define the *XXX*Stub class for server communication.**

```
namespace OHOS {
int32_t ListenAbilityStub::OnRemoteRequest(uint32_t code,
    MessageParcel& data, MessageParcel &reply, MessageOption &option)
{
    switch (code) {
        case ADD_VOLUME: {
            return reply.WriteInt32(AddVolume(data.ReadInt32()));
        }

        default:
            return IPCObjectStub::OnRemoteRequest(code, data, reply, option);
    }
}
}
```

**4. Implement a system ability.**

```
namespace {
constexpr OHOS::HiviewDFX::HiLogLabel LABEL = {LOG_CORE, 0xD001800, "SA_TST"};
}

REGISTER_SYSTEM_ABILITY_BY_ID(ListenAbility, DISTRIBUTED_SCHED_TEST_LISTEN_ID, true);

ListenAbility::ListenAbility(int32_t saId, bool runOnCreate) : SystemAbility(saId, runOnCreate)
{
    HiLog::Info(LABEL, ":%s called", __func__);
    HiLog::Info(LABEL, "ListenAbility()");
}

ListenAbility::~ListenAbility()
{
    HiLog::Info(LABEL, "~ListenAbility()");
}

int ListenAbility::AddVolume(int volume)
{
    pid_t current = getpid();
    HiLog::Info(LABEL, "ListenAbility::AddVolume volume = %d, pid = %d.", volume, current);
    return (volume + 1);
}

void ListenAbility::OnDump()
{
}

void ListenAbility::OnStart()
{
    HiLog::Info(LABEL, "ListenAbility::OnStart()");
    HiLog::Info(LABEL, "ListenAbility:%s called:-----Publish------", __func__);
    bool res = Publish(this);
    if (res) {
        HiLog::Error(LABEL, "ListenAbility: res == false");
    }
    HiLog::Info(LABEL, "ListenAbility:%s called:AddAbilityListener_OS_TST----beg-----", __func__);
    AddSystemAbilityListener(DISTRIBUTED_SCHED_TEST_OS_ID);
    HiLog::Info(LABEL, "ListenAbility:%s called:AddAbilityListener_OS_TST----end-----", __func__);

    HiLog::Info(LABEL, "ListenAbility:%s called:StopAbility_OS_TST----beg-----", __func__);
    StopAbility(DISTRIBUTED_SCHED_TEST_OS_ID);
    HiLog::Info(LABEL, "ListenAbility:%s called:StopAbility_OS_TST----end-----", __func__);
    return;
}

void ListenAbility::OnStop()
{
}
```

**5. Configure the system ability.**

Configure the profile of the system ability so that the system ability can be loaded and registered. The configuration procedure is as follows:

Create a folder named **sa_profile** in the root directory of the subsystem. Then, create two files in this folder, including an XML file prefixed with the service ID of the system ability and a **BUILD.gn** file.

Sample *serviceid*.xml file:

```


    <process>listen_test</process>
    
    <name>serviceid</name>
    <libpath>/system/lib64/liblistentest.z.so</libpath>
    <run-on-create>true</run-on-create>
    <distributed>false</distributed>
    <dump-level>1</dump-level>


```

Sample **BUILD.gn** file:

```
import("//build/ohos/sa_profile/sa_profile.gni")
ohos_sa_profile("xxx_sa_profile") {
    sources = [
        "serviceid.xml"
    ]
    subsystem_name = "systemabilitymgr"
}
```

>**NOTE**<br/>
>- Set **process** to the name of the process where the system ability will run. This parameter is mandatory.
>- The *serviceid*.xml file can contain only one **systemability** node. Multiple **systemability** nodes will cause a build failure.
>- Set **name** to the service ID registered in the code for the system ability. This parameter is mandatory.
>- Set **libpath** to the path for loading the system ability. This parameter is mandatory.
>- Set **run-on-create** to **true** if you want to register this system ability with the **samgr** module immediately after the process is started. Set it to **false** if you want the system ability to start only when it is accessed. This parameter is mandatory.
>- Set **distributed** to **true** if this system ability allows cross-device access. Set it to **false** if it allows IPC only on the local device.
>- **bootphase** specifies the startup priority of the system ability. The value can be **BootStartPhase** (highest), **CoreStartPhase**, or **OtherStartPhase** (lowest). In the same process, system abilities of a lower priority can be started and registered only after those of a higher priority have all been started and registered. This parameter is optional. The default value is **OtherStartPhase**.
>- **dump-level** specifies the level supported by the system dumper. The default value is **1**.
>- In the **BUILD.gn** file, set **subsystem_name** to the subsystem name, and add the list of system abilities to be configured for the subsystem in **sources**. Multiple system abilities can be configured.

After the preceding steps are complete, an XML file named by the process will be generated in the **out**, for example, **out\...\system\profile\listen_test.xml**.

**6. Configure the .cfg file.**

The .cfg file contains the native process startup policy provided by Linux. During the system startup process, the init process parses the .cfg file to start the native process.

```
{
    "jobs" : [{
            "name" : "post-fs-data",
            "cmds" : [
                "start listen_test"
            ]
        }
    ],
	"services" : [{
            "name" : "listen_test",
            "path" : ["/system/bin/sa_main", "/system/profile/listen_test.xml"],
            "uid" : "system",
            "gid" : ["system", "shell"]
        }
    ]
}
```

>**NOTE**<br/>
>For details about the implementation of listen_ability, see **test/services/safwk/unittest/common/listen_ability**.

## Repositories Involved<a name="section1371113476307"></a>

**System Ability Management Subsystem**

[**systemabilitymgr\_safwk**](https://gitee.com/openharmony/systemabilitymgr_safwk)

[systemabilitymgr\_samgr](https://gitee.com/openharmony/systemabilitymgr_samgr)

[systemabilitymgr\_safwk\_lite](https://gitee.com/openharmony/systemabilitymgr_safwk_lite)

[systemabilitymgr\_samgr\_lite](https://gitee.com/openharmony/systemabilitymgr_samgr_lite)