All Products
Search

This Product

    IoT Platform:C Link SDK

    Document Center

    IoT Platform:C Link SDK

    Last Updated:Jun 20, 2026

    This topic shows how to use the C Link SDK to connect a simulated device (in this example, a night light switch) to IoT Platform over MQTT and publish data to a custom topic.

    Prerequisites

    IoT Platform is activated.

    Create a product and a device

    1. Log on to the IoT Platform console and click the target instance card.

    2. In the left-side navigation pane, choose Devices > Products, and click Create Product.

    3. On the Create Product page, set the Product Name parameter to NightLightSwitch and the Category parameter to Custom Category. Use the default values for other parameters. Then, click OK.

    4. On the Create Product page, click Add in the Add Device section.

    5. On the Devices page, click Add Device.

    6. In the Add Device dialog box, enter a device name. Example: LightSwitch. Then, click OK.

      For Nickname, enter Quick Start.

    7. In the The devices have been added. dialog box, click Copy Device Certificate. Then, save the device certificate information to an on-premises server. For more information, see Device certificates.

    Prepare the development environment

    In this example, the development environment is Ubuntu 16.04, and the development tools are gcc and make. For more information, see Project configurations.

    1. Install Ubuntu 16.04.

      For more information, see Ubuntu.

    2. Run the following command to install gcc and make: 

      sudo apt-get -y install gcc make

    Configure the SDK

    1. Download a demo package from Demo file to the development environment and then decompress the package.

      IoT Platform allows you to develop custom features for Link SDKs. For more information, see Obtain Link SDK for C.

    2. Open the ./LinkSDK/demos/mqtt_basic_demo.c file and configure the relevant parameters.

      • Configure the parameters that are used to specify device connection information to connect the device to IoT Platform. The following table describes the parameters. Sample code:

        char *product_key       = "a18wP******";
char *device_name       = "LightSwitch";
char *device_secret     = "uwMTmVAMnGGHaAkqmeDY6cHxxB******";
char  *mqtt_host = "a18wP******.iot-as-mqtt.cn-shanghai.aliyuncs.com";

        Parameter

        Example

        Description

        mqtt_host

        a18wP******.iot-as-mqtt.cn-shanghai.aliyuncs.com

        The endpoint to which you want to connect the device.

        • For Enterprise Edition instances and new public instances: Find the endpoint on the Development Configuration panel of the Instance Details page.

        • If you use a public instance of the previous version, the endpoint of the public instance is in the ${YourProductKey}.iot-as-mqtt.${YourRegionId}.aliyuncs.com format.

        For more information about instances and endpoints, see View instance endpoints.

        product_key

        a18wP******

        The device certificate information. The device certificate is saved on your on-premises server after the device is created.

        You can view the device certificate on the Device Details page in the IoT Platform console. For more information, see Obtain device verification information.

        device_name

        LightSwitch

        device_secret

        uwMTmVAMnGGHaAkqmeDY6cHxxB******

      • Specify a topic to which you want the device to subscribe to receive messages from IoT Platform. Example: /a18wP******/LightSwitch/user/get. Sample code:

        Note

        If you want to use the feature, uncomment the code that is enclosed within a pair of forward slash and asterisk combinations (/**/). 

            {
        char *sub_topic = "/a18wP******/LightSwitch/user/get";
        res = aiot_mqtt_sub(mqtt_handle, sub_topic, NULL, 1, NULL);
        if (res < 0) {
            printf("aiot_mqtt_sub failed, res: -0x%04X\n", -res);
            return -1;
        }
    }

        Parameter

        Example

        Description

        sub_topic

        /a18wP******/LightSwitch/user/get

        The topic on which the device has the Subscribe permission. The device can receive messages from IoT Platform by using this topic.

        You can view custom topics in the IoT Platform console. On the Product Details page, click the Topic Categories tab. On the tab that appears, click Topic Category. For more information, see What is a topic.

        In this example, the default custom topic is used. Parameter description:

        • a1oGs****** indicates the ProductKey of the device.

        • LightSwitch indicates the DeviceName of the device.

      • Specify a topic that can be used by the device to send messages. Example: /a18wP******/LightSwitch/user/update. Then, specify content for a message that you want to send. Sample code:

        Note

        If you want to use the feature, uncomment the code that is enclosed within a pair of forward slash and asterisk combinations (/**/). 

             {
        char *pub_topic = "/a18wP******/LightSwitch/user/update";
        char *pub_payload = "{\"id\":\"1\",\"version\":\"1.0\",\"params\":{\"LightSwitch\":0}}";
        res = aiot_mqtt_pub(mqtt_handle, pub_topic, (uint8_t *)pub_payload, (uint32_t)strlen(pub_payload), 0);
        if (res < 0) {
            printf("aiot_mqtt_sub failed, res: -0x%04X\n", -res);
            return -1;
        }
    }

        Parameter

        Example

        Description

        pub_topic

        /a18wP******/LightSwitch/user/update

        The topic on which the device has the Publish permission. The device can use the topic to send messages to IoT Platform.

        You can view custom topics in the IoT Platform console. On the Product Details page, click the Topic Categories tab. On the tab that appears, click Topic Category. For more information, see What is a topic.

        In this example, the default custom topic is used. Parameter description:

        • a1oGs****** indicates the ProductKey of the device.

        • LightSwitch indicates the DeviceName of the device.

        pub_payload

        {\"id\":\"1\",\"version\":\"1.0\",\"params\":{\"LightSwitch\":0}}

        The message that is sent to IoT Platform.

        In this example, a custom topic is used. You can use custom message formats.

    3. In the SDK root directory, run the make command.

      The ./output/mqtt-basic-demo executable sample code file is generated.

    Execution result

    In the SDK root directory, run the following command to run the executable file: 

    ./output/mqtt-basic-demo

    • You can view the execution result on the device.

      • The following log data indicates that the LightSwitch device is connected to IoT Platform. 

        [1619768570.433][LK-0313] MQTT user calls aiot_mqtt_connect api, connect
[1619768570.433][LK-0317] LightSwitch&a18wP******
[1619768570.433][LK-0318] 933FE333F78F42E2AAD384F629C771277E06D459577942EA1F04ACFDFE******
core_sysdep_network_establish host a18wP******.iot-as-mqtt.cn-shanghai.aliyuncs.com port 8883, type 0
establish tcp connection with server(host='a18wP******.iot-as-mqtt.cn-shanghai.aliyuncs.com', port=[8883])
success to establish tcp, fd=3
local port: 49122
[1619768570.599][LK-1000] establish mbedtls connection with server(host='a18wP******.iot-as-mqtt.cn-shanghai.aliyuncs.com', port=[8883])
[1619768570.600][LK-1000] success to establish mbedtls connection， (cost 45134 bytes in total, max used 47870 bytes)
[1619768570.644][LK-0313] MQTT connect success in 212 ms
AIOT_MQTTEVT_CONNECT

      • The following log data indicates that the LightSwitch device subscribes to the /a18wP******/LightSwitch/user/get topic. 

        [1620373302.144][LK-0309] sub: /a18wP******/LightSwitch/user/get

        After the device successfully subscribes to the topic, you can go back to the IoT Platform console. On the Topics tab of the Device Details page, find the topic and click Publish Message in the Actions column. In the Publish Message dialog box, enter a message, for example, This is a test message from Alibaba Iot Platform., to simulate sending a message from IoT Platform to the device.

      • The following log data indicates that the LightSwitch device uses the /a18wP******/LightSwitch/user/update topic to submit a message to IoT Platform. 

        [1619768570.644][LK-0309] pub: /a18wP******/LightSwitch/user/update
[LK-030A] > 7B 22 69 64 22 3A 22 31  22 2C 22 76 65 72 73 69 | {"id":"1","versi
[LK-030A] > 6F 6E 22 3A 22 31 2E 30  22 2C 22 70 61 72 61 6D | on":"1.0","param
[LK-030A] > 73 22 3A 7B 22 4C 69 67  68 74 53 77 69 74 63 68 | s":{"LightSwitch
[LK-030A] > 22 3A 30 7D 7D

    • You can log on to the IoT Platform console to view the device status and operation logs.

      • In the left-side navigation pane, choose Devices > Devices. Then, find the device and view the device status. If the value in the State/Enabled column is Online, the device is connected to IoT Platform.

      • In the left-side navigation pane, choose Maintenance > Device Log. On the Device Log page, select the NightLightSwitch product. You can view logs that record when the devices of the product went online, subscribed to a topic, and submitted messages.

    If an error occurs during debugging, fix the error as prompted. For more information, see Error codes received by devices.

    What to do next

    • After the device is connected to IoT Platform, you can manage and monitor the device. For more information about the features of IoT Platform, see Features.

    • For more information about how to use Link SDK for C to configure advanced features, see Overview.