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The header files of the device SDK for C provided for you to call such as API function declarations, constants, and variable parameter definitions are in the include directory. This document describes the variable parameters and API functions in the directory.

Variable Parameter Configuration

You can configure corresponding parameters in the SDK for C based on the needs in specific scenarios to ensure the smooth operations of your businesses. Variable connection parameters include:

1. Timeout period of blocking MQTT calls (including connection, subscribing, and publishing) in milliseconds. 5000 ms is recommended.
2. Size of the buffer for message sending and receiving over the MQTT protocol, which is 2,048 bytes by default and can be up to 16 KB.
3. Size of the buffer for message sending and receiving over the CoAP protocol, which is 512 bytes by default and can be up to 1 KB.
4. MQTT heartbeat message sending interval in milliseconds, which can be up to 690s.
5. Maximum waiting time for reconnection in milliseconds. When a device is reconnected after disconnection, the waiting time will double if reconnection fails, and reconnection will stop when the maximum waiting time is exceeded.

You can modify the configuration of the corresponding connection parameters by modifying the following macro definitions in the include/qcloud_iot_export_variables.h file.
You need to recompile the SDK after modification. Below is the sample code:

/* default MQTT/CoAP timeout value when connect/pub/sub (unit: ms) */
#define QCLOUD_IOT_MQTT_COMMAND_TIMEOUT                             (5 * 1000)

/* default MQTT keep alive interval (unit: ms) */
#define QCLOUD_IOT_MQTT_KEEP_ALIVE_INTERNAL                         (240 * 1000)

/* default MQTT Tx buffer size, MAX: 16*1024 */
#define QCLOUD_IOT_MQTT_TX_BUF_LEN                                  (2048)

/* default MQTT Rx buffer size, MAX: 16*1024 */
#define QCLOUD_IOT_MQTT_RX_BUF_LEN                                  (2048)

/* default COAP Tx buffer size, MAX: 1*1024 */
#define COAP_SENDMSG_MAX_BUFLEN                                     (512)

/* default COAP Rx buffer size, MAX: 1*1024 */
#define COAP_RECVMSG_MAX_BUFLEN                                     (512)

/* MAX MQTT reconnect interval (unit: ms) */
#define MAX_RECONNECT_WAIT_INTERVAL                                 (60 * 1000)

API Function Description

The following describes the main features and corresponding APIs provided by the SDK for C v3.1.0 for you to compile business logic. For more information on API parameters and returned values, please see the comments in the header files of the SDK code such as include/exports/qcloud_iot_export_*.h.

MQTT APIs

Notes on use in multithreaded environment

To use MQTT APIs in a multithreaded environment, you need to pay attention to the following:

• You cannot use multiple threads to call IOT_MQTT_Yield, IOT_MQTT_Construct, or IOT_MQTT_Destroy.
• You can use multiple threads to call IOT_MQTT_Publish, IOT_MQTT_Subscribe, and IOT_MQTT_Unsubscribe.
• As the function to read MQTT messages from socket and process them, IOT_MQTT_Yield should have a certain execution time to prevent it from being suspended or preempted for a long time.

Device shadow APIs

For more information on the device shadow feature, please see Device Shadow Details.

CoAP APIs

OTA APIs

For more information on the OTA firmware download feature, please see Device Firmware Update.

Log APIs

For more information on the device log reporting feature, please see the log reporting section of the IoT Hub documentation in the SDK docs directory.

System time APIs

Gateway feature APIs

Fore more information on the gateway feature, please see the gateway product section of the IoT Hub documentation in the SDK docs directory.