本文共 10842 字，大约阅读时间需要 36 分钟。

00. 目录

01. 常见类型声明

便于理解和跨平台使用。

stdint.h内容如下：

//// stdint.h////      Copyright (c) Microsoft Corporation. All rights reserved.//// The C Standard Library 
   
     header.//#pragma once#define _STDINT#include 
    
     #if _VCRT_COMPILER_PREPROCESSOR#pragma warning(push)#pragma warning(disable: _VCRUNTIME_DISABLED_WARNINGS)typedef signed char        int8_t;typedef short              int16_t;typedef int                int32_t;typedef long long          int64_t;typedef unsigned char      uint8_t;typedef unsigned short     uint16_t;typedef unsigned int       uint32_t;typedef unsigned long long uint64_t;typedef signed char        int_least8_t;typedef short              int_least16_t;typedef int                int_least32_t;typedef long long          int_least64_t;typedef unsigned char      uint_least8_t;typedef unsigned short     uint_least16_t;typedef unsigned int       uint_least32_t;typedef unsigned long long uint_least64_t;typedef signed char        int_fast8_t;typedef int                int_fast16_t;typedef int                int_fast32_t;typedef long long          int_fast64_t;typedef unsigned char      uint_fast8_t;typedef unsigned int       uint_fast16_t;typedef unsigned int       uint_fast32_t;typedef unsigned long long uint_fast64_t;typedef long long          intmax_t;typedef unsigned long long uintmax_t;// These macros must exactly match those in the Windows SDK's intsafe.h.#define INT8_MIN         (-127i8 - 1)#define INT16_MIN        (-32767i16 - 1)#define INT32_MIN        (-2147483647i32 - 1)#define INT64_MIN        (-9223372036854775807i64 - 1)#define INT8_MAX         127i8#define INT16_MAX        32767i16#define INT32_MAX        2147483647i32#define INT64_MAX        9223372036854775807i64#define UINT8_MAX        0xffui8#define UINT16_MAX       0xffffui16#define UINT32_MAX       0xffffffffui32#define UINT64_MAX       0xffffffffffffffffui64#define INT_LEAST8_MIN   INT8_MIN#define INT_LEAST16_MIN  INT16_MIN#define INT_LEAST32_MIN  INT32_MIN#define INT_LEAST64_MIN  INT64_MIN#define INT_LEAST8_MAX   INT8_MAX#define INT_LEAST16_MAX  INT16_MAX#define INT_LEAST32_MAX  INT32_MAX#define INT_LEAST64_MAX  INT64_MAX#define UINT_LEAST8_MAX  UINT8_MAX#define UINT_LEAST16_MAX UINT16_MAX#define UINT_LEAST32_MAX UINT32_MAX#define UINT_LEAST64_MAX UINT64_MAX#define INT_FAST8_MIN    INT8_MIN#define INT_FAST16_MIN   INT32_MIN#define INT_FAST32_MIN   INT32_MIN#define INT_FAST64_MIN   INT64_MIN#define INT_FAST8_MAX    INT8_MAX#define INT_FAST16_MAX   INT32_MAX#define INT_FAST32_MAX   INT32_MAX#define INT_FAST64_MAX   INT64_MAX#define UINT_FAST8_MAX   UINT8_MAX#define UINT_FAST16_MAX  UINT32_MAX#define UINT_FAST32_MAX  UINT32_MAX#define UINT_FAST64_MAX  UINT64_MAX#ifdef _WIN64    #define INTPTR_MIN   INT64_MIN    #define INTPTR_MAX   INT64_MAX    #define UINTPTR_MAX  UINT64_MAX#else    #define INTPTR_MIN   INT32_MIN    #define INTPTR_MAX   INT32_MAX    #define UINTPTR_MAX  UINT32_MAX#endif#define INTMAX_MIN       INT64_MIN#define INTMAX_MAX       INT64_MAX#define UINTMAX_MAX      UINT64_MAX#define PTRDIFF_MIN      INTPTR_MIN#define PTRDIFF_MAX      INTPTR_MAX#ifndef SIZE_MAX    // SIZE_MAX definition must match exactly with limits.h for modules support.    #ifdef _WIN64        #define SIZE_MAX 0xffffffffffffffffui64    #else        #define SIZE_MAX 0xffffffffui32    #endif#endif#define SIG_ATOMIC_MIN   INT32_MIN#define SIG_ATOMIC_MAX   INT32_MAX#define WCHAR_MIN        0x0000#define WCHAR_MAX        0xffff#define WINT_MIN         0x0000#define WINT_MAX         0xffff#define INT8_C(x)    (x)#define INT16_C(x)   (x)#define INT32_C(x)   (x)#define INT64_C(x)   (x ## LL)#define UINT8_C(x)   (x)#define UINT16_C(x)  (x)#define UINT32_C(x)  (x ## U)#define UINT64_C(x)  (x ## ULL)#define INTMAX_C(x)  INT64_C(x)#define UINTMAX_C(x) UINT64_C(x)#pragma warning(pop) // _VCRUNTIME_DISABLED_WARNINGS#endif // _VCRT_COMPILER_PREPROCESSOR
    
   

02. 常量声明

在modbus.h文件中，通过宏定义libmodbus库目前支持的所有Modbus功能码

/* Modbus function codes */#define MODBUS_FC_READ_COILS                0x01#define MODBUS_FC_READ_DISCRETE_INPUTS      0x02#define MODBUS_FC_READ_HOLDING_REGISTERS    0x03#define MODBUS_FC_READ_INPUT_REGISTERS      0x04#define MODBUS_FC_WRITE_SINGLE_COIL         0x05#define MODBUS_FC_WRITE_SINGLE_REGISTER     0x06#define MODBUS_FC_READ_EXCEPTION_STATUS     0x07#define MODBUS_FC_WRITE_MULTIPLE_COILS      0x0F#define MODBUS_FC_WRITE_MULTIPLE_REGISTERS  0x10#define MODBUS_FC_REPORT_SLAVE_ID           0x11#define MODBUS_FC_MASK_WRITE_REGISTER       0x16#define MODBUS_FC_WRITE_AND_READ_REGISTERS  0x17

在modbus.h文件中定义了最大可读/可写线圈数量，最大可读/可写寄存器数量。

//广播地址#define MODBUS_BROADCAST_ADDRESS    0/* Modbus_Application_Protocol_V1_1b.pdf (chapter 6 section 1 page 12) * Quantity of Coils to read (2 bytes): 1 to 2000 (0x7D0) * (chapter 6 section 11 page 29) * Quantity of Coils to write (2 bytes): 1 to 1968 (0x7B0) */#define MODBUS_MAX_READ_BITS              2000#define MODBUS_MAX_WRITE_BITS             1968/* Modbus_Application_Protocol_V1_1b.pdf (chapter 6 section 3 page 15) * Quantity of Registers to read (2 bytes): 1 to 125 (0x7D) * (chapter 6 section 12 page 31) * Quantity of Registers to write (2 bytes) 1 to 123 (0x7B) * (chapter 6 section 17 page 38) * Quantity of Registers to write in R/W registers (2 bytes) 1 to 121 (0x79) */#define MODBUS_MAX_READ_REGISTERS          125#define MODBUS_MAX_WRITE_REGISTERS         123#define MODBUS_MAX_WR_WRITE_REGISTERS      121#define MODBUS_MAX_WR_READ_REGISTERS       125/* The size of the MODBUS PDU is limited by the size constraint inherited from * the first MODBUS implementation on Serial Line network (max. RS485 ADU = 256 * bytes). Therefore, MODBUS PDU for serial line communication = 256 - Server * address (1 byte) - CRC (2 bytes) = 253 bytes. */#define MODBUS_MAX_PDU_LENGTH              253/* Consequently: * - RTU MODBUS ADU = 253 bytes + Server address (1 byte) + CRC (2 bytes) = 256 *   bytes. * - TCP MODBUS ADU = 253 bytes + MBAP (7 bytes) = 260 bytes. * so the maximum of both backend in 260 bytes. This size can used to allocate * an array of bytes to store responses and it will be compatible with the two * backends. */#define MODBUS_MAX_ADU_LENGTH              260/* Random number to avoid errno conflicts */#define MODBUS_ENOBASE 112345678

错误码常量

/* Random number to avoid errno conflicts */#define MODBUS_ENOBASE 112345678/* Protocol exceptions */enum {
       MODBUS_EXCEPTION_ILLEGAL_FUNCTION = 0x01,   //非法的功能码    MODBUS_EXCEPTION_ILLEGAL_DATA_ADDRESS,      //非法的数据地址    MODBUS_EXCEPTION_ILLEGAL_DATA_VALUE,        //非法的数据值    MODBUS_EXCEPTION_SLAVE_OR_SERVER_FAILURE,   //从站设备故障    MODBUS_EXCEPTION_ACKNOWLEDGE,               //ACK异常    MODBUS_EXCEPTION_SLAVE_OR_SERVER_BUSY,      //从站设备忙    MODBUS_EXCEPTION_NEGATIVE_ACKNOWLEDGE,      //否定应答    MODBUS_EXCEPTION_MEMORY_PARITY,             //内存奇偶校验错误    MODBUS_EXCEPTION_NOT_DEFINED,               //未定义    MODBUS_EXCEPTION_GATEWAY_PATH,              //网关路径不可用    MODBUS_EXCEPTION_GATEWAY_TARGET,            //目标设备未能回应    MODBUS_EXCEPTION_MAX};#define EMBXILFUN  (MODBUS_ENOBASE + MODBUS_EXCEPTION_ILLEGAL_FUNCTION)#define EMBXILADD  (MODBUS_ENOBASE + MODBUS_EXCEPTION_ILLEGAL_DATA_ADDRESS)#define EMBXILVAL  (MODBUS_ENOBASE + MODBUS_EXCEPTION_ILLEGAL_DATA_VALUE)#define EMBXSFAIL  (MODBUS_ENOBASE + MODBUS_EXCEPTION_SLAVE_OR_SERVER_FAILURE)#define EMBXACK    (MODBUS_ENOBASE + MODBUS_EXCEPTION_ACKNOWLEDGE)#define EMBXSBUSY  (MODBUS_ENOBASE + MODBUS_EXCEPTION_SLAVE_OR_SERVER_BUSY)#define EMBXNACK   (MODBUS_ENOBASE + MODBUS_EXCEPTION_NEGATIVE_ACKNOWLEDGE)#define EMBXMEMPAR (MODBUS_ENOBASE + MODBUS_EXCEPTION_MEMORY_PARITY)#define EMBXGPATH  (MODBUS_ENOBASE + MODBUS_EXCEPTION_GATEWAY_PATH)#define EMBXGTAR   (MODBUS_ENOBASE + MODBUS_EXCEPTION_GATEWAY_TARGET)/* Native libmodbus error codes */#define EMBBADCRC  (EMBXGTAR + 1)   //无效的CRC    #define EMBBADDATA (EMBXGTAR + 2)   //无效的数据#define EMBBADEXC  (EMBXGTAR + 3)   //无效的异常码#define EMBUNKEXC  (EMBXGTAR + 4)   //保留 未使用#define EMBMDATA   (EMBXGTAR + 5)   //数据过多#define EMBBADSLAVE (EMBXGTAR + 6)  //响应与查询地址不匹配

03. _modbus结构体声明

在文件modbus-private.h中定义了_modbus结构体，具体定义如下：

struct _modbus {
       /* Slave address */    int slave;      //从站设备地址    /* Socket or file descriptor */    int s;  //TCP模式下为套接字 RTU模式下为串口句柄    int debug;         //是否启用debug模式    int error_recovery; //错误恢复模式    struct timeval response_timeout;    //响应超时设置    struct timeval byte_timeout;        //字节超时设置    struct timeval indication_timeout;    //包含一系列通用函数指针    const modbus_backend_t *backend;    void *backend_data; //TCP模式下特殊配置数据 RTU模式下特殊配置数据};

04. modbus_backend_t结构体声明

modbus_backend_t作为一个重要的结构体，包含了各种处理函数。其定义如下：

typedef struct _modbus_backend {
       unsigned int backend_type;  //modbus_backend_type_t类型    unsigned int header_length; //HBMP长度    unsigned int checksum_length; //错误校验字段长度    unsigned int max_adu_length; //ADU最大长度    int (*set_slave) (modbus_t *ctx, int slave); //设置从站设备地址    //构造查询报文的基本通信帧    int (*build_request_basis) (modbus_t *ctx, int function, int addr,                                int nb, uint8_t *req);    //构造响应报文的基本通信帧    int (*build_response_basis) (sft_t *sft, uint8_t *rsp);    //构造响应报文TID参数    int (*prepare_response_tid) (const uint8_t *req, int *req_length);    //发送报文前的预处理    int (*send_msg_pre) (uint8_t *req, int req_length);    //发送报文    ssize_t (*send) (modbus_t *ctx, const uint8_t *req, int req_length);    //接收报文    int (*receive) (modbus_t *ctx, uint8_t *req);    //接收报文 该函数被receive函数调用    ssize_t (*recv) (modbus_t *ctx, uint8_t *rsp, int rsp_length);    //用于数据完整性检查    int (*check_integrity) (modbus_t *ctx, uint8_t *msg,                            const int msg_length);    //确认响应报文的帧头是否一致    int (*pre_check_confirmation) (modbus_t *ctx, const uint8_t *req,                                   const uint8_t *rsp, int rsp_length);    //建立连接    int (*connect) (modbus_t *ctx);    //关闭连接    void (*close) (modbus_t *ctx);    //清空缓冲区    int (*flush) (modbus_t *ctx);    //用于设置超时并读取通信事件，以检测是否存在待接收数据    int (*select) (modbus_t *ctx, fd_set *rset, struct timeval *tv, int msg_length);    //释放内存    void (*free) (modbus_t *ctx);} modbus_backend_t;

05. modbus_mapping_t结构体声明

modbus_mapping_t声明在modbus.h文件中，其声明如下：

typedef struct _modbus modbus_t;typedef struct _modbus_mapping_t {
       int nb_bits;                    //线圈寄存器的数量    int start_bits;                 //线圈寄存器的起始地址    int nb_input_bits;              //离散输入寄存器的数量           int start_input_bits;           //离散输入寄存器的起始地址    int nb_input_registers;         //输入寄存器的数量    int start_input_registers;      //输入寄存器的起始地址    int nb_registers;               //保持寄存器的数量    int start_registers;            //保持寄存器的起始地址    uint8_t *tab_bits;              //指向线圈寄存器的值    uint8_t *tab_input_bits;        //指向离散输入寄存器的值    uint16_t *tab_input_registers;  //指向输入寄存器的值    uint16_t *tab_registers;        //指向保持寄存器的值} modbus_mapping_t;

06. 附录

libmodbus源码下载：

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