Connection issues
This topic describes potential issues and solutions when you connect devices using C Link software development kit (SDK) versions 3.x and 2.3.0.
Background information
If you have not yet connected to IoT Platform, use the latest version of the SDK. For more information, see Overview.
Why is the heartbeat interval passed by the SDK different from the connection parameter in the MQTT Connect package?
The SDK doubles the heartbeat interval that you provide and sends the result to IoT Platform as the MQTT Connect parameter.
To adjust the heartbeat interval, modify the corresponding parameter in the files for the following SDK versions.
| SDK version | File | Parameter |
| 3.x | ./src/mqtt/impl/iotx_mqtt_client.c | _mqtt_connect |
| 2.3.0 | ./src/protocol/mqtt/client/mqtt_client.c | iotx_mc_connect |
How to modify general configurations
When you compile the SDK library or an executable file, you can search for the default macro definition in the config file or the SDK source code and modify it.
The following are commonly used macro definitions:
| Macro definition | Description |
| CONFIG_MQTT_TX_MAXLEN | MQTT send buffer |
| CONFIG_MQTT_RX_MAXLEN | MQTT receive buffer |
| IOTX_MC_REQUEST_TIMEOUT_DEFAULT_MS | Heartbeat interval |
| IOTX_MC_KEEPALIVE_PROBE_MAX | Maximum number of lost heartbeats |
What do I do if the "mqtt read buffer is too short" error occurs during an MQTT connection?
The default MQTT buffer size in the SDK is 1024 bytes. This error occurs when the length of a single received MQTT message exceeds this value. To resolve this issue, you must increase the MQTT buffer size.
To resolve this issue, search for the CONFIG_MQTT_RX_MAXLEN macro in the SDK source code and set it to the required size.
Similarly, if this error occurs when you send a message, modify the value of CONFIG_MQTT_TX_MAXLEN to resolve the issue.
What do I do if the "The message length 2693 is too long" error occurs during a CoAP connection?
This error indicates that the reported message is too long. To resolve this issue, modify the value of COAP_MSG_MAX_PDU_LEN.