使用C语言编写modbus转mqtt json

要使用C语言编写modbus转mqtt json，您需要使用一些库来处理modbus和mqtt协议，以及json数据的转换。以下是一些可能有用的库：

- libmodbus：用于处理modbus协议
- Paho MQTT：用于处理MQTT协议
- cJSON：用于处理JSON数据

以下是一个简单的示例代码，演示如何读取modbus寄存器并将其转换为JSON格式的数据，然后发布到MQTT主题：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <modbus/modbus.h>
#include <MQTTClient.h>
#include <cJSON.h>

#define MODBUS_SERVER "localhost"
#define MODBUS_PORT 502
#define MODBUS_SLAVE_ID 1
#define MODBUS_REGISTER_ADDRESS 0
#define MODBUS_REGISTER_COUNT 10

#define MQTT_SERVER "tcp://localhost:1883"
#define MQTT_CLIENT_ID "modbus2mqtt"
#define MQTT_TOPIC "modbus_data"

int main(int argc, char *argv[]) {
    modbus_t *ctx;
    uint16_t reg[MODBUS_REGISTER_COUNT];
    int rc;
    char *payload;
    cJSON *root, *data;
    MQTTClient client;
    MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;

    // Initialize modbus client
    ctx = modbus_new_tcp(MODBUS_SERVER, MODBUS_PORT);
    modbus_set_slave(ctx, MODBUS_SLAVE_ID);

    // Initialize MQTT client
    MQTTClient_create(&client, MQTT_SERVER, MQTT_CLIENT_ID, MQTTCLIENT_PERSISTENCE_NONE, NULL);
    conn_opts.keepAliveInterval = 20;
    conn_opts.cleansession = 1;

    // Connect to MQTT broker
    if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS) {
        printf("Failed to connect to MQTT broker, return code %d\n", rc);
        exit(EXIT_FAILURE);
    }

    // Read modbus registers
    rc = modbus_read_registers(ctx, MODBUS_REGISTER_ADDRESS, MODBUS_REGISTER_COUNT, reg);
    if (rc == -1) {
        printf("Failed to read modbus registers: %s\n", modbus_strerror(errno));
        exit(EXIT_FAILURE);
    }

    // Convert modbus data to JSON
    root = cJSON_CreateObject();
    cJSON_AddItemToObject(root, "type", cJSON_CreateString("modbus_data"));
    data = cJSON_AddArrayToObject(root, "data");
    for (int i = 0; i < MODBUS_REGISTER_COUNT; i++) {
        cJSON_AddNumberToObject(data, "", reg[i]);
    }
    payload = cJSON_Print(root);

    // Publish to MQTT topic
    MQTTClient_message message = MQTTClient_message_initializer;
    message.payload = payload;
    message.payloadlen = strlen(payload);
    message.qos = 1;
    MQTTClient_publishMessage(client, MQTT_TOPIC, &message, NULL);

    // Clean up
    modbus_close(ctx);
    modbus_free(ctx);
    MQTTClient_disconnect(client, 10000);
    MQTTClient_destroy(&client);
    cJSON_Delete(root);
    free(payload);

    return 0;
}

请注意，此示例代码仅用于演示目的。实际应用程序需要更全面的错误处理和配置选项。