【嵌入式】Libmodbus源码分析(三)-modbus相关函数分析

港控/mmm° 2023-01-20 14:53 52阅读 0赞

## 00. 目录 ##

### 文章目录 ###

*   *  00. 目录
     *  01. modbus-private.h文件
     *  02. modbus.h文件
     *  03. modbus.c文件
     *  04. 预留
     *  05. 附录

## 01. modbus-private.h文件 ##

libmodbus内部使用的结构和函数的声明

/* * Copyright © 2010-2012 Stéphane Raimbault <stephane.raimbault@gmail.com> * * SPDX-License-Identifier: LGPL-2.1-or-later */
    
    #ifndef MODBUS_PRIVATE_H
    #define MODBUS_PRIVATE_H
    
    #ifndef _MSC_VER
    # include <stdint.h>
    # include <sys/time.h>
    #else
    # include "stdint.h"
    # include <time.h>
    typedef int ssize_t;
    #endif
    #include <sys/types.h>
    #include <config.h>
    
    #include "modbus.h"
    
    MODBUS_BEGIN_DECLS
    
    /* It's not really the minimal length (the real one is report slave ID * in RTU (4 bytes)) but it's a convenient size to use in RTU or TCP * communications to read many values or write a single one. * Maximum between : * - HEADER_LENGTH_TCP (7) + function (1) + address (2) + number (2) * - HEADER_LENGTH_RTU (1) + function (1) + address (2) + number (2) + CRC (2) */
    #define _MIN_REQ_LENGTH 12
    
    #define _REPORT_SLAVE_ID 180
    
    #define _MODBUS_EXCEPTION_RSP_LENGTH 5
    
    /* Timeouts in microsecond (0.5 s) */
    #define _RESPONSE_TIMEOUT 500000
    #define _BYTE_TIMEOUT 500000
    
    typedef enum { 
        _MODBUS_BACKEND_TYPE_RTU=0,
        _MODBUS_BACKEND_TYPE_TCP
    } modbus_backend_type_t;
    
    /* * ---------- Request Indication ---------- * | Client | ---------------------->| Server | * ---------- Confirmation Response ---------- */
    typedef enum { 
        /* Request message on the server side */
        MSG_INDICATION,
        /* Request message on the client side */
        MSG_CONFIRMATION
    } msg_type_t;
    
    /* This structure reduces the number of params in functions and so * optimizes the speed of execution (~ 37%). */
    typedef struct _sft { 
        int slave;
        int function;
        int t_id;
    } sft_t;
    
    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;
    
    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;  //请求超时设置
        //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
    };
    
    void _modbus_init_common(modbus_t *ctx);
    void _error_print(modbus_t *ctx, const char *context);
    int _modbus_receive_msg(modbus_t *ctx, uint8_t *msg, msg_type_t msg_type);
    
    #ifndef HAVE_STRLCPY
    size_t strlcpy(char *dest, const char *src, size_t dest_size);
    #endif
    
    MODBUS_END_DECLS
    
    #endif /* MODBUS_PRIVATE_H */

## 02. modbus.h文件 ##

libmodbus对外开放的API接口

/* * Copyright © 2001-2013 Stéphane Raimbault <stephane.raimbault@gmail.com> * * SPDX-License-Identifier: LGPL-2.1-or-later */
    
    #ifndef MODBUS_H
    #define MODBUS_H
    
    /* Add this for macros that defined unix flavor */
    #if (defined(__unix__) || defined(unix)) && !defined(USG)
    #include <sys/param.h>
    #endif
    
    #ifndef _MSC_VER
    #include <stdint.h>
    #else
    #include "stdint.h"
    #endif
    
    #include "modbus-version.h"
    
    #if defined(_MSC_VER)
    # if defined(DLLBUILD)
    /* define DLLBUILD when building the DLL */
    # define MODBUS_API __declspec(dllexport)
    # else
    # define MODBUS_API __declspec(dllimport)
    # endif
    #else
    # define MODBUS_API
    #endif
    
    #ifdef __cplusplus
    # define MODBUS_BEGIN_DECLS extern "C" { 
    # define MODBUS_END_DECLS }
    #else
    # define MODBUS_BEGIN_DECLS
    # define MODBUS_END_DECLS
    #endif
    
    MODBUS_BEGIN_DECLS
    
    #ifndef FALSE
    #define FALSE 0
    #endif
    
    #ifndef TRUE
    #define TRUE 1
    #endif
    
    #ifndef OFF
    #define OFF 0
    #endif
    
    #ifndef ON
    #define ON 1
    #endif
    
    /* Modbus function codes */ //功能码
    #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
    //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
    
    /* 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
    
    extern const unsigned int libmodbus_version_major;
    extern const unsigned int libmodbus_version_minor;
    extern const unsigned int libmodbus_version_micro;
    
    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;
    
    typedef enum
    { 
        MODBUS_ERROR_RECOVERY_NONE          = 0,        //不恢复
        MODBUS_ERROR_RECOVERY_LINK          = (1<<1),   //链接层恢复
        MODBUS_ERROR_RECOVERY_PROTOCOL      = (1<<2)    //协议层恢复
    } modbus_error_recovery_mode;
    
    //设置从站地址
    MODBUS_API int modbus_set_slave(modbus_t* ctx, int slave);
    //获取从站地址
    MODBUS_API int modbus_get_slave(modbus_t* ctx);
    //设置错误恢复模式
    MODBUS_API int modbus_set_error_recovery(modbus_t *ctx, modbus_error_recovery_mode error_recovery);
    //设置当前socket或者串口句柄
    MODBUS_API int modbus_set_socket(modbus_t *ctx, int s);
    //获取当前socket或者窗口句柄
    MODBUS_API int modbus_get_socket(modbus_t *ctx);
    
    //获取响应超时
    MODBUS_API int modbus_get_response_timeout(modbus_t *ctx, uint32_t *to_sec, uint32_t *to_usec);
    //设置响应超时
    MODBUS_API int modbus_set_response_timeout(modbus_t *ctx, uint32_t to_sec, uint32_t to_usec);
    //获取连续字节之间的超时时间
    MODBUS_API int modbus_get_byte_timeout(modbus_t *ctx, uint32_t *to_sec, uint32_t *to_usec);
    //设置连续字节之间的超时时间
    MODBUS_API int modbus_set_byte_timeout(modbus_t *ctx, uint32_t to_sec, uint32_t to_usec);
    
    //获取服务端等待客户端请求超时时间
    MODBUS_API int modbus_get_indication_timeout(modbus_t *ctx, uint32_t *to_sec, uint32_t *to_usec);
    //设置服务端等待客户端请求超时时间
    MODBUS_API int modbus_set_indication_timeout(modbus_t *ctx, uint32_t to_sec, uint32_t to_usec);
    //获取报文头长度
    MODBUS_API int modbus_get_header_length(modbus_t *ctx);
    //用于主站设备与从站设备建立连接
    MODBUS_API int modbus_connect(modbus_t *ctx);
    //关闭连接
    MODBUS_API void modbus_close(modbus_t *ctx);
    //释放内存
    MODBUS_API void modbus_free(modbus_t *ctx);
    //刷新缓冲区
    MODBUS_API int modbus_flush(modbus_t *ctx);
    //是否设置为debug模式
    MODBUS_API int modbus_set_debug(modbus_t *ctx, int flag);
    //获取当前错误信息
    MODBUS_API const char *modbus_strerror(int errnum);
    
    //-------------------------------------------------------------------------------
    //读取线圈或者离散量输出状态(功能码 0x1)
    MODBUS_API int modbus_read_bits(modbus_t *ctx, int addr, int nb, uint8_t *dest);
    //读取离散量输入值(功能码 0x2)
    MODBUS_API int modbus_read_input_bits(modbus_t *ctx, int addr, int nb, uint8_t *dest);
    //读取保持寄存器(功能码 0x3)
    MODBUS_API int modbus_read_registers(modbus_t *ctx, int addr, int nb, uint16_t *dest);
    //读取输入寄存器(功能码 0x4)
    MODBUS_API int modbus_read_input_registers(modbus_t *ctx, int addr, int nb, uint16_t *dest);
    //写单个线圈或者单个离散量(功能码 0x5)
    MODBUS_API int modbus_write_bit(modbus_t *ctx, int coil_addr, int status);
    //写单个保持寄存器(功能码 0x6)
    MODBUS_API int modbus_write_register(modbus_t *ctx, int reg_addr, const uint16_t value);
    //写多个线圈(功能码 0xF)
    MODBUS_API int modbus_write_bits(modbus_t *ctx, int addr, int nb, const uint8_t *data);
    //写多个保持寄存器(功能码 0x10)
    MODBUS_API int modbus_write_registers(modbus_t *ctx, int addr, int nb, const uint16_t *data);
    MODBUS_API int modbus_mask_write_register(modbus_t *ctx, int addr, uint16_t and_mask, uint16_t or_mask);
    MODBUS_API int modbus_write_and_read_registers(modbus_t *ctx, int write_addr, int write_nb,
                                                   const uint16_t *src, int read_addr, int read_nb,
                                                   uint16_t *dest);
    //报告从站ID(功能码 0x11)
    MODBUS_API int modbus_report_slave_id(modbus_t *ctx, int max_dest, uint8_t *dest);
    
    MODBUS_API modbus_mapping_t* modbus_mapping_new_start_address(
        unsigned int start_bits, unsigned int nb_bits,
        unsigned int start_input_bits, unsigned int nb_input_bits,
        unsigned int start_registers, unsigned int nb_registers,
        unsigned int start_input_registers, unsigned int nb_input_registers);
    
    MODBUS_API modbus_mapping_t* modbus_mapping_new(int nb_bits, int nb_input_bits,
                                                    int nb_registers, int nb_input_registers);
    MODBUS_API void modbus_mapping_free(modbus_mapping_t *mb_mapping);
    
    MODBUS_API int modbus_send_raw_request(modbus_t *ctx, const uint8_t *raw_req, int raw_req_length);
    
    MODBUS_API int modbus_receive(modbus_t *ctx, uint8_t *req);
    
    MODBUS_API int modbus_receive_confirmation(modbus_t *ctx, uint8_t *rsp);
    
    MODBUS_API int modbus_reply(modbus_t *ctx, const uint8_t *req,
                                int req_length, modbus_mapping_t *mb_mapping);
    MODBUS_API int modbus_reply_exception(modbus_t *ctx, const uint8_t *req,
                                          unsigned int exception_code);
    
    /** * UTILS FUNCTIONS **/
    //获取高字节
    #define MODBUS_GET_HIGH_BYTE(data) (((data) >> 8) & 0xFF)
    //获取低字节
    #define MODBUS_GET_LOW_BYTE(data) ((data) & 0xFF)
    #define MODBUS_GET_INT64_FROM_INT16(tab_int16, index) \ (((int64_t)tab_int16[(index) ] << 48) | \ ((int64_t)tab_int16[(index) + 1] << 32) | \ ((int64_t)tab_int16[(index) + 2] << 16) | \ (int64_t)tab_int16[(index) + 3])
    #define MODBUS_GET_INT32_FROM_INT16(tab_int16, index) \ (((int32_t)tab_int16[(index) ] << 16) | \ (int32_t)tab_int16[(index) + 1])
    #define MODBUS_GET_INT16_FROM_INT8(tab_int8, index) \ (((int16_t)tab_int8[(index) ] << 8) | \ (int16_t)tab_int8[(index) + 1])
    #define MODBUS_SET_INT16_TO_INT8(tab_int8, index, value) \ do { \ ((int8_t*)(tab_int8))[(index) ] = (int8_t)((value) >> 8); \ ((int8_t*)(tab_int8))[(index) + 1] = (int8_t)(value); \ } while (0)
    #define MODBUS_SET_INT32_TO_INT16(tab_int16, index, value) \ do { \ ((int16_t*)(tab_int16))[(index) ] = (int16_t)((value) >> 16); \ ((int16_t*)(tab_int16))[(index) + 1] = (int16_t)(value); \ } while (0)
    #define MODBUS_SET_INT64_TO_INT16(tab_int16, index, value) \ do { \ ((int16_t*)(tab_int16))[(index) ] = (int16_t)((value) >> 48); \ ((int16_t*)(tab_int16))[(index) + 1] = (int16_t)((value) >> 32); \ ((int16_t*)(tab_int16))[(index) + 2] = (int16_t)((value) >> 16); \ ((int16_t*)(tab_int16))[(index) + 3] = (int16_t)(value); \ } while (0)
    
    MODBUS_API void modbus_set_bits_from_byte(uint8_t *dest, int idx, const uint8_t value);
    MODBUS_API void modbus_set_bits_from_bytes(uint8_t *dest, int idx, unsigned int nb_bits,
                                           const uint8_t *tab_byte);
    MODBUS_API uint8_t modbus_get_byte_from_bits(const uint8_t *src, int idx, unsigned int nb_bits);
    MODBUS_API float modbus_get_float(const uint16_t *src);
    MODBUS_API float modbus_get_float_abcd(const uint16_t *src);
    MODBUS_API float modbus_get_float_dcba(const uint16_t *src);
    MODBUS_API float modbus_get_float_badc(const uint16_t *src);
    MODBUS_API float modbus_get_float_cdab(const uint16_t *src);
    
    MODBUS_API void modbus_set_float(float f, uint16_t *dest);
    MODBUS_API void modbus_set_float_abcd(float f, uint16_t *dest);
    MODBUS_API void modbus_set_float_dcba(float f, uint16_t *dest);
    MODBUS_API void modbus_set_float_badc(float f, uint16_t *dest);
    MODBUS_API void modbus_set_float_cdab(float f, uint16_t *dest);
    
    #include "modbus-tcp.h"
    #include "modbus-rtu.h"
    
    MODBUS_END_DECLS
    
    #endif /* MODBUS_H */

## 03. modbus.c文件 ##

modbus.c 核心文件，实现Modbus协议层，定义通用的Modbus消息发送和接收函数、各功能码对应的函数

/* * Copyright © 2001-2011 Stéphane Raimbault <stephane.raimbault@gmail.com> * * SPDX-License-Identifier: LGPL-2.1-or-later * * This library implements the Modbus protocol. * http://libmodbus.org/ */
    
    #include <stdio.h>
    #include <string.h>
    #include <stdlib.h>
    #include <stdarg.h>
    #include <errno.h>
    #include <limits.h>
    #include <time.h>
    #ifndef _MSC_VER
    #include <unistd.h>
    #endif
    
    #include <config.h>
    
    #include "modbus.h"
    #include "modbus-private.h"
    
    /* Internal use */
    #define MSG_LENGTH_UNDEFINED -1
    
    /* Exported version */
    const unsigned int libmodbus_version_major = LIBMODBUS_VERSION_MAJOR;
    const unsigned int libmodbus_version_minor = LIBMODBUS_VERSION_MINOR;
    const unsigned int libmodbus_version_micro = LIBMODBUS_VERSION_MICRO;
    
    /* Max between RTU and TCP max adu length (so TCP) */
    #define MAX_MESSAGE_LENGTH 260
    
    /* 3 steps are used to parse the query */
    typedef enum { 
        _STEP_FUNCTION,
        _STEP_META,
        _STEP_DATA
    } _step_t;
    
    //错误解析
    const char *modbus_strerror(int errnum) { 
        switch (errnum) { 
        case EMBXILFUN:
            return "Illegal function";
        case EMBXILADD:
            return "Illegal data address";
        case EMBXILVAL:
            return "Illegal data value";
        case EMBXSFAIL:
            return "Slave device or server failure";
        case EMBXACK:
            return "Acknowledge";
        case EMBXSBUSY:
            return "Slave device or server is busy";
        case EMBXNACK:
            return "Negative acknowledge";
        case EMBXMEMPAR:
            return "Memory parity error";
        case EMBXGPATH:
            return "Gateway path unavailable";
        case EMBXGTAR:
            return "Target device failed to respond";
        case EMBBADCRC:
            return "Invalid CRC";
        case EMBBADDATA:
            return "Invalid data";
        case EMBBADEXC:
            return "Invalid exception code";
        case EMBMDATA:
            return "Too many data";
        case EMBBADSLAVE:
            return "Response not from requested slave";
        default:
            return strerror(errnum);
        }
    }
    //错误输出
    void _error_print(modbus_t *ctx, const char *context)
    { 
        if (ctx->debug) { 
            fprintf(stderr, "ERROR %s", modbus_strerror(errno));
            if (context != NULL) { 
                fprintf(stderr, ": %s\n", context);
            } else { 
                fprintf(stderr, "\n");
            }
        }
    }
    
    //响应超时
    static void _sleep_response_timeout(modbus_t *ctx)
    { 
        /* Response timeout is always positive */
    #ifdef _WIN32
        /* usleep doesn't exist on Windows */
        Sleep((ctx->response_timeout.tv_sec * 1000) +
              (ctx->response_timeout.tv_usec / 1000));
    #else
        /* usleep source code */
        struct timespec request, remaining;
        request.tv_sec = ctx->response_timeout.tv_sec;
        request.tv_nsec = ((long int)ctx->response_timeout.tv_usec) * 1000;
        while (nanosleep(&request, &remaining) == -1 && errno == EINTR) { 
            request = remaining;
        }
    #endif
    }
    
    //刷新缓冲区
    int modbus_flush(modbus_t *ctx)
    { 
        int rc;
    
        if (ctx == NULL) { 
            errno = EINVAL;
            return -1;
        }
    
        rc = ctx->backend->flush(ctx);
        if (rc != -1 && ctx->debug) { 
            /* Not all backends are able to return the number of bytes flushed */
            printf("Bytes flushed (%d)\n", rc);
        }
        return rc;
    }
    
    /* Computes the length of the expected response */
    static unsigned int compute_response_length_from_request(modbus_t *ctx, uint8_t *req)
    { 
        int length;
        const int offset = ctx->backend->header_length;
    
        switch (req[offset]) { 
        case MODBUS_FC_READ_COILS:
        case MODBUS_FC_READ_DISCRETE_INPUTS: { 
            /* Header + nb values (code from write_bits) */
            int nb = (req[offset + 3] << 8) | req[offset + 4];
            length = 2 + (nb / 8) + ((nb % 8) ? 1 : 0);
        }
            break;
        case MODBUS_FC_WRITE_AND_READ_REGISTERS:
        case MODBUS_FC_READ_HOLDING_REGISTERS:
        case MODBUS_FC_READ_INPUT_REGISTERS:
            /* Header + 2 * nb values */
            length = 2 + 2 * (req[offset + 3] << 8 | req[offset + 4]);
            break;
        case MODBUS_FC_READ_EXCEPTION_STATUS:
            length = 3;
            break;
        case MODBUS_FC_REPORT_SLAVE_ID:
            /* The response is device specific (the header provides the length) */
            return MSG_LENGTH_UNDEFINED;
        case MODBUS_FC_MASK_WRITE_REGISTER:
            length = 7;
            break;
        default:
            length = 5;
        }
    
        return offset + length + ctx->backend->checksum_length;
    }
    
    /* Sends a request/response */
    static int send_msg(modbus_t *ctx, uint8_t *msg, int msg_length)
    { 
        int rc;
        int i;
        //进行消息预处理
        msg_length = ctx->backend->send_msg_pre(msg, msg_length);
    
        if (ctx->debug) { 
            for (i = 0; i < msg_length; i++)
                printf("[%.2X]", msg[i]);
            printf("\n");
        }
    
        /* In recovery mode, the write command will be issued until to be successful! Disabled by default. */
        do { 
            rc = ctx->backend->send(ctx, msg, msg_length);
            if (rc == -1) { 
                _error_print(ctx, NULL);
                if (ctx->error_recovery & MODBUS_ERROR_RECOVERY_LINK) { 
                    int saved_errno = errno;
    
                    if ((errno == EBADF || errno == ECONNRESET || errno == EPIPE)) { 
                        modbus_close(ctx);
                        _sleep_response_timeout(ctx);
                        modbus_connect(ctx);
                    } else { 
                        _sleep_response_timeout(ctx);
                        modbus_flush(ctx);
                    }
                    errno = saved_errno;
                }
            }
        } while ((ctx->error_recovery & MODBUS_ERROR_RECOVERY_LINK) &&
                 rc == -1);
    
        if (rc > 0 && rc != msg_length) { 
            errno = EMBBADDATA;
            return -1;
        }
    
        return rc;
    }
    
    int modbus_send_raw_request(modbus_t *ctx, const uint8_t *raw_req, int raw_req_length)
    { 
        sft_t sft;
        uint8_t req[MAX_MESSAGE_LENGTH];
        int req_length;
    
        if (ctx == NULL) { 
            errno = EINVAL;
            return -1;
        }
    
        if (raw_req_length < 2 || raw_req_length > (MODBUS_MAX_PDU_LENGTH + 1)) { 
            /* The raw request must contain function and slave at least and must not be longer than the maximum pdu length plus the slave address. */
            errno = EINVAL;
            return -1;
        }
    
        sft.slave = raw_req[0];
        sft.function = raw_req[1];
        /* The t_id is left to zero */
        sft.t_id = 0;
        /* This response function only set the header so it's convenient here */
        req_length = ctx->backend->build_response_basis(&sft, req);
    
        if (raw_req_length > 2) { 
            /* Copy data after function code */
            memcpy(req + req_length, raw_req + 2, raw_req_length - 2);
            req_length += raw_req_length - 2;
        }
    
        return send_msg(ctx, req, req_length);
    }
    
    /* * ---------- Request Indication ---------- * | Client | ---------------------->| Server | * ---------- Confirmation Response ---------- */
    
    /* Computes the length to read after the function received */
    static uint8_t compute_meta_length_after_function(int function,
                                                      msg_type_t msg_type)
    { 
        int length;
    
        if (msg_type == MSG_INDICATION) { 
            if (function <= MODBUS_FC_WRITE_SINGLE_REGISTER) { 
                length = 4;
            } else if (function == MODBUS_FC_WRITE_MULTIPLE_COILS ||
                       function == MODBUS_FC_WRITE_MULTIPLE_REGISTERS) { 
                length = 5;
            } else if (function == MODBUS_FC_MASK_WRITE_REGISTER) { 
                length = 6;
            } else if (function == MODBUS_FC_WRITE_AND_READ_REGISTERS) { 
                length = 9;
            } else { 
                /* MODBUS_FC_READ_EXCEPTION_STATUS, MODBUS_FC_REPORT_SLAVE_ID */
                length = 0;
            }
        } else { 
            /* MSG_CONFIRMATION */
            switch (function) { 
            case MODBUS_FC_WRITE_SINGLE_COIL:
            case MODBUS_FC_WRITE_SINGLE_REGISTER:
            case MODBUS_FC_WRITE_MULTIPLE_COILS:
            case MODBUS_FC_WRITE_MULTIPLE_REGISTERS:
                length = 4;
                break;
            case MODBUS_FC_MASK_WRITE_REGISTER:
                length = 6;
                break;
            default:
                length = 1;
            }
        }
    
        return length;
    }
    
    /* Computes the length to read after the meta information (address, count, etc) */
    static int compute_data_length_after_meta(modbus_t *ctx, uint8_t *msg,
                                              msg_type_t msg_type)
    { 
        int function = msg[ctx->backend->header_length];
        int length;
    
        if (msg_type == MSG_INDICATION) { 
            switch (function) { 
            case MODBUS_FC_WRITE_MULTIPLE_COILS:
            case MODBUS_FC_WRITE_MULTIPLE_REGISTERS:
                length = msg[ctx->backend->header_length + 5];
                break;
            case MODBUS_FC_WRITE_AND_READ_REGISTERS:
                length = msg[ctx->backend->header_length + 9];
                break;
            default:
                length = 0;
            }
        } else { 
            /* MSG_CONFIRMATION */
            if (function <= MODBUS_FC_READ_INPUT_REGISTERS ||
                function == MODBUS_FC_REPORT_SLAVE_ID ||
                function == MODBUS_FC_WRITE_AND_READ_REGISTERS) { 
                length = msg[ctx->backend->header_length + 1];
            } else { 
                length = 0;
            }
        }
    
        length += ctx->backend->checksum_length;
    
        return length;
    }
    
    
    /* Waits a response from a modbus server or a request from a modbus client. This function blocks if there is no replies (3 timeouts). The function shall return the number of received characters and the received message in an array of uint8_t if successful. Otherwise it shall return -1 and errno is set to one of the values defined below: - ECONNRESET - EMBBADDATA - EMBUNKEXC - ETIMEDOUT - read() or recv() error codes */
    
    int _modbus_receive_msg(modbus_t *ctx, uint8_t *msg, msg_type_t msg_type)
    { 
        int rc;
        fd_set rset;
        struct timeval tv;
        struct timeval *p_tv;
        int length_to_read;
        int msg_length = 0;
        _step_t step;
        if (ctx->debug) { 
            if (msg_type == MSG_INDICATION) { 
                //表示正在等待查询报文
                printf("Waiting for an indication...\n");
            } else { 
                //表示发送查询报文后等待接收响应
                printf("Waiting for a confirmation...\n");
            }
        }
    
        /* Add a file descriptor to the set */
        FD_ZERO(&rset);
        FD_SET(ctx->s, &rset);
    
        /* We need to analyse the message step by step. At the first step, we want * to reach the function code because all packets contain this * information. */
        step = _STEP_FUNCTION;
        length_to_read = ctx->backend->header_length + 1;
    
        if (msg_type == MSG_INDICATION) { 
            /* Wait for a message, we don't know when the message will be * received */
            if (ctx->indication_timeout.tv_sec == 0 && ctx->indication_timeout.tv_usec == 0) { 
                /* By default, the indication timeout isn't set */
                p_tv = NULL;
            } else { 
                /* Wait for an indication (name of a received request by a server, see schema) */
                tv.tv_sec = ctx->indication_timeout.tv_sec;
                tv.tv_usec = ctx->indication_timeout.tv_usec;
                p_tv = &tv;
            }
        } else { 
            tv.tv_sec = ctx->response_timeout.tv_sec;
            tv.tv_usec = ctx->response_timeout.tv_usec;
            p_tv = &tv;
        }
    
        while (length_to_read != 0) { 
            rc = ctx->backend->select(ctx, &rset, p_tv, length_to_read);
            if (rc == -1) { 
                _error_print(ctx, "select");
                if (ctx->error_recovery & MODBUS_ERROR_RECOVERY_LINK) { 
                    int saved_errno = errno;
    
                    if (errno == ETIMEDOUT) { 
                        _sleep_response_timeout(ctx);
                        modbus_flush(ctx);
                    } else if (errno == EBADF) { 
                        modbus_close(ctx);
                        modbus_connect(ctx);
                    }
                    errno = saved_errno;
                }
                return -1;
            }
    
            rc = ctx->backend->recv(ctx, msg + msg_length, length_to_read);
            if (rc == 0) { 
                errno = ECONNRESET;
                rc = -1;
            }
    
            if (rc == -1) { 
                _error_print(ctx, "read");
                if ((ctx->error_recovery & MODBUS_ERROR_RECOVERY_LINK) &&
                    (errno == ECONNRESET || errno == ECONNREFUSED ||
                     errno == EBADF)) { 
                    int saved_errno = errno;
                    modbus_close(ctx);
                    modbus_connect(ctx);
                    /* Could be removed by previous calls */
                    errno = saved_errno;
                }
                return -1;
            }
    
            /* Display the hex code of each character received */
            if (ctx->debug) { 
                int i;
                for (i=0; i < rc; i++)
                    printf("<%.2X>", msg[msg_length + i]);
            }
    
            /* Sums bytes received */
            msg_length += rc;
            /* Computes remaining bytes */
            length_to_read -= rc;
    
            if (length_to_read == 0) { 
                switch (step) { 
                case _STEP_FUNCTION:
                    /* Function code position */
                    length_to_read = compute_meta_length_after_function(
                        msg[ctx->backend->header_length],
                        msg_type);
                    if (length_to_read != 0) { 
                        step = _STEP_META;
                        break;
                    } /* else switches straight to the next step */
                case _STEP_META:
                    length_to_read = compute_data_length_after_meta(
                        ctx, msg, msg_type);
                    if ((msg_length + length_to_read) > (int)ctx->backend->max_adu_length) { 
                        errno = EMBBADDATA;
                        _error_print(ctx, "too many data");
                        return -1;
                    }
                    step = _STEP_DATA;
                    break;
                default:
                    break;
                }
            }
    
            if (length_to_read > 0 &&
                (ctx->byte_timeout.tv_sec > 0 || ctx->byte_timeout.tv_usec > 0)) { 
                /* If there is no character in the buffer, the allowed timeout interval between two consecutive bytes is defined by byte_timeout */
                tv.tv_sec = ctx->byte_timeout.tv_sec;
                tv.tv_usec = ctx->byte_timeout.tv_usec;
                p_tv = &tv;
            }
            /* else timeout isn't set again, the full response must be read before expiration of response timeout (for CONFIRMATION only) */
        }
    
        if (ctx->debug)
            printf("\n");
    
        return ctx->backend->check_integrity(ctx, msg, msg_length);
    }
    
    /* Receive the request from a modbus master */
    int modbus_receive(modbus_t *ctx, uint8_t *req)
    { 
        if (ctx == NULL) { 
            errno = EINVAL;
            return -1;
        }
    
        return ctx->backend->receive(ctx, req);
    }
    
    /* Receives the confirmation. The function shall store the read response in rsp and return the number of values (bits or words). Otherwise, its shall return -1 and errno is set. The function doesn't check the confirmation is the expected response to the initial request. */
    int modbus_receive_confirmation(modbus_t *ctx, uint8_t *rsp)
    { 
        if (ctx == NULL) { 
            errno = EINVAL;
            return -1;
        }
    
        return _modbus_receive_msg(ctx, rsp, MSG_CONFIRMATION);
    }
    
    static int check_confirmation(modbus_t *ctx, uint8_t *req,
                                  uint8_t *rsp, int rsp_length)
    { 
        int rc;
        int rsp_length_computed;
        const int offset = ctx->backend->header_length;
        const int function = rsp[offset];
    
        if (ctx->backend->pre_check_confirmation) { 
            rc = ctx->backend->pre_check_confirmation(ctx, req, rsp, rsp_length);
            if (rc == -1) { 
                if (ctx->error_recovery & MODBUS_ERROR_RECOVERY_PROTOCOL) { 
                    _sleep_response_timeout(ctx);
                    modbus_flush(ctx);
                }
                return -1;
            }
        }
    
        rsp_length_computed = compute_response_length_from_request(ctx, req);
    
        /* Exception code */
        if (function >= 0x80) { 
            if (rsp_length == (offset + 2 + (int)ctx->backend->checksum_length) &&
                req[offset] == (rsp[offset] - 0x80)) { 
                /* Valid exception code received */
    
                int exception_code = rsp[offset + 1];
                if (exception_code < MODBUS_EXCEPTION_MAX) { 
                    errno = MODBUS_ENOBASE + exception_code;
                } else { 
                    errno = EMBBADEXC;
                }
                _error_print(ctx, NULL);
                return -1;
            } else { 
                errno = EMBBADEXC;
                _error_print(ctx, NULL);
                return -1;
            }
        }
    
        /* Check length */
        if ((rsp_length == rsp_length_computed ||
             rsp_length_computed == MSG_LENGTH_UNDEFINED) &&
            function < 0x80) { 
            int req_nb_value;
            int rsp_nb_value;
    
            /* Check function code */
            if (function != req[offset]) { 
                if (ctx->debug) { 
                    fprintf(stderr,
                            "Received function not corresponding to the request (0x%X != 0x%X)\n",
                            function, req[offset]);
                }
                if (ctx->error_recovery & MODBUS_ERROR_RECOVERY_PROTOCOL) { 
                    _sleep_response_timeout(ctx);
                    modbus_flush(ctx);
                }
                errno = EMBBADDATA;
                return -1;
            }
    
            /* Check the number of values is corresponding to the request */
            switch (function) { 
            case MODBUS_FC_READ_COILS:
            case MODBUS_FC_READ_DISCRETE_INPUTS:
                /* Read functions, 8 values in a byte (nb * of values in the request and byte count in * the response. */
                req_nb_value = (req[offset + 3] << 8) + req[offset + 4];
                req_nb_value = (req_nb_value / 8) + ((req_nb_value % 8) ? 1 : 0);
                rsp_nb_value = rsp[offset + 1];
                break;
            case MODBUS_FC_WRITE_AND_READ_REGISTERS:
            case MODBUS_FC_READ_HOLDING_REGISTERS:
            case MODBUS_FC_READ_INPUT_REGISTERS:
                /* Read functions 1 value = 2 bytes */
                req_nb_value = (req[offset + 3] << 8) + req[offset + 4];
                rsp_nb_value = (rsp[offset + 1] / 2);
                break;
            case MODBUS_FC_WRITE_MULTIPLE_COILS:
            case MODBUS_FC_WRITE_MULTIPLE_REGISTERS:
                /* N Write functions */
                req_nb_value = (req[offset + 3] << 8) + req[offset + 4];
                rsp_nb_value = (rsp[offset + 3] << 8) | rsp[offset + 4];
                break;
            case MODBUS_FC_REPORT_SLAVE_ID:
                /* Report slave ID (bytes received) */
                req_nb_value = rsp_nb_value = rsp[offset + 1];
                break;
            default:
                /* 1 Write functions & others */
                req_nb_value = rsp_nb_value = 1;
            }
    
            if (req_nb_value == rsp_nb_value) { 
                rc = rsp_nb_value;
            } else { 
                if (ctx->debug) { 
                    fprintf(stderr,
                            "Quantity not corresponding to the request (%d != %d)\n",
                            rsp_nb_value, req_nb_value);
                }
    
                if (ctx->error_recovery & MODBUS_ERROR_RECOVERY_PROTOCOL) { 
                    _sleep_response_timeout(ctx);
                    modbus_flush(ctx);
                }
    
                errno = EMBBADDATA;
                rc = -1;
            }
        } else { 
            if (ctx->debug) { 
                fprintf(stderr,
                        "Message length not corresponding to the computed length (%d != %d)\n",
                        rsp_length, rsp_length_computed);
            }
            if (ctx->error_recovery & MODBUS_ERROR_RECOVERY_PROTOCOL) { 
                _sleep_response_timeout(ctx);
                modbus_flush(ctx);
            }
            errno = EMBBADDATA;
            rc = -1;
        }
    
        return rc;
    }
    
    static int response_io_status(uint8_t *tab_io_status,
                                  int address, int nb,
                                  uint8_t *rsp, int offset)
    { 
        int shift = 0;
        /* Instead of byte (not allowed in Win32) */
        int one_byte = 0;
        int i;
    
        for (i = address; i < address + nb; i++) { 
            one_byte |= tab_io_status[i] << shift;
            if (shift == 7) { 
                /* Byte is full */
                rsp[offset++] = one_byte;
                one_byte = shift = 0;
            } else { 
                shift++;
            }
        }
    
        if (shift != 0)
            rsp[offset++] = one_byte;
    
        return offset;
    }
    
    /* Build the exception response */
    static int response_exception(modbus_t *ctx, sft_t *sft,
                                  int exception_code, uint8_t *rsp,
                                  unsigned int to_flush,
                                  const char* template, ...)
    { 
        int rsp_length;
    
        /* Print debug message */
        if (ctx->debug) { 
            va_list ap;
    
            va_start(ap, template);
            vfprintf(stderr, template, ap);
            va_end(ap);
        }
    
        /* Flush if required */
        if (to_flush) { 
            _sleep_response_timeout(ctx);
            modbus_flush(ctx);
        }
    
        /* Build exception response */
        sft->function = sft->function + 0x80;
        rsp_length = ctx->backend->build_response_basis(sft, rsp);
        rsp[rsp_length++] = exception_code;
    
        return rsp_length;
    }
    
    /* Send a response to the received request. Analyses the request and constructs a response. If an error occurs, this function construct the response accordingly. */
    int modbus_reply(modbus_t *ctx, const uint8_t *req,
                     int req_length, modbus_mapping_t *mb_mapping)
    { 
        int offset;
        int slave;
        int function;
        uint16_t address;
        uint8_t rsp[MAX_MESSAGE_LENGTH];
        int rsp_length = 0;
        sft_t sft;
    
        if (ctx == NULL) { 
            errno = EINVAL;
            return -1;
        }
    
        offset = ctx->backend->header_length;
        slave = req[offset - 1];
        function = req[offset];
        address = (req[offset + 1] << 8) + req[offset + 2];
    
        sft.slave = slave;
        sft.function = function;
        sft.t_id = ctx->backend->prepare_response_tid(req, &req_length);
    
        /* Data are flushed on illegal number of values errors. */
        switch (function) { 
        case MODBUS_FC_READ_COILS:
        case MODBUS_FC_READ_DISCRETE_INPUTS: { 
            unsigned int is_input = (function == MODBUS_FC_READ_DISCRETE_INPUTS);
            int start_bits = is_input ? mb_mapping->start_input_bits : mb_mapping->start_bits;
            int nb_bits = is_input ? mb_mapping->nb_input_bits : mb_mapping->nb_bits;
            uint8_t *tab_bits = is_input ? mb_mapping->tab_input_bits : mb_mapping->tab_bits;
            const char * const name = is_input ? "read_input_bits" : "read_bits";
            int nb = (req[offset + 3] << 8) + req[offset + 4];
            /* The mapping can be shifted to reduce memory consumption and it doesn't always start at address zero. */
            int mapping_address = address - start_bits;
    
            if (nb < 1 || MODBUS_MAX_READ_BITS < nb) { 
                rsp_length = response_exception(
                    ctx, &sft, MODBUS_EXCEPTION_ILLEGAL_DATA_VALUE, rsp, TRUE,
                    "Illegal nb of values %d in %s (max %d)\n",
                    nb, name, MODBUS_MAX_READ_BITS);
            } else if (mapping_address < 0 || (mapping_address + nb) > nb_bits) { 
                rsp_length = response_exception(
                    ctx, &sft,
                    MODBUS_EXCEPTION_ILLEGAL_DATA_ADDRESS, rsp, FALSE,
                    "Illegal data address 0x%0X in %s\n",
                    mapping_address < 0 ? address : address + nb, name);
            } else { 
                rsp_length = ctx->backend->build_response_basis(&sft, rsp);
                rsp[rsp_length++] = (nb / 8) + ((nb % 8) ? 1 : 0);
                rsp_length = response_io_status(tab_bits, mapping_address, nb,
                                                rsp, rsp_length);
            }
        }
            break;
        case MODBUS_FC_READ_HOLDING_REGISTERS:
        case MODBUS_FC_READ_INPUT_REGISTERS: { 
            unsigned int is_input = (function == MODBUS_FC_READ_INPUT_REGISTERS);
            int start_registers = is_input ? mb_mapping->start_input_registers : mb_mapping->start_registers;
            int nb_registers = is_input ? mb_mapping->nb_input_registers : mb_mapping->nb_registers;
            uint16_t *tab_registers = is_input ? mb_mapping->tab_input_registers : mb_mapping->tab_registers;
            const char * const name = is_input ? "read_input_registers" : "read_registers";
            int nb = (req[offset + 3] << 8) + req[offset + 4];
            /* The mapping can be shifted to reduce memory consumption and it doesn't always start at address zero. */
            int mapping_address = address - start_registers;
    
            if (nb < 1 || MODBUS_MAX_READ_REGISTERS < nb) { 
                rsp_length = response_exception(
                    ctx, &sft, MODBUS_EXCEPTION_ILLEGAL_DATA_VALUE, rsp, TRUE,
                    "Illegal nb of values %d in %s (max %d)\n",
                    nb, name, MODBUS_MAX_READ_REGISTERS);
            } else if (mapping_address < 0 || (mapping_address + nb) > nb_registers) { 
                rsp_length = response_exception(
                    ctx, &sft, MODBUS_EXCEPTION_ILLEGAL_DATA_ADDRESS, rsp, FALSE,
                    "Illegal data address 0x%0X in %s\n",
                    mapping_address < 0 ? address : address + nb, name);
            } else { 
                int i;
    
                rsp_length = ctx->backend->build_response_basis(&sft, rsp);
                rsp[rsp_length++] = nb << 1;
                for (i = mapping_address; i < mapping_address + nb; i++) { 
                    rsp[rsp_length++] = tab_registers[i] >> 8;
                    rsp[rsp_length++] = tab_registers[i] & 0xFF;
                }
            }
        }
            break;
        case MODBUS_FC_WRITE_SINGLE_COIL: { 
            int mapping_address = address - mb_mapping->start_bits;
    
            if (mapping_address < 0 || mapping_address >= mb_mapping->nb_bits) { 
                rsp_length = response_exception(
                    ctx, &sft, MODBUS_EXCEPTION_ILLEGAL_DATA_ADDRESS, rsp, FALSE,
                    "Illegal data address 0x%0X in write_bit\n",
                    address);
            } else { 
                int data = (req[offset + 3] << 8) + req[offset + 4];
    
                if (data == 0xFF00 || data == 0x0) { 
                    mb_mapping->tab_bits[mapping_address] = data ? ON : OFF;
                    memcpy(rsp, req, req_length);
                    rsp_length = req_length;
                } else { 
                    rsp_length = response_exception(
                        ctx, &sft,
                        MODBUS_EXCEPTION_ILLEGAL_DATA_VALUE, rsp, FALSE,
                        "Illegal data value 0x%0X in write_bit request at address %0X\n",
                        data, address);
                }
            }
        }
            break;
        case MODBUS_FC_WRITE_SINGLE_REGISTER: { 
            int mapping_address = address - mb_mapping->start_registers;
    
            if (mapping_address < 0 || mapping_address >= mb_mapping->nb_registers) { 
                rsp_length = response_exception(
                    ctx, &sft,
                    MODBUS_EXCEPTION_ILLEGAL_DATA_ADDRESS, rsp, FALSE,
                    "Illegal data address 0x%0X in write_register\n",
                    address);
            } else { 
                int data = (req[offset + 3] << 8) + req[offset + 4];
    
                mb_mapping->tab_registers[mapping_address] = data;
                memcpy(rsp, req, req_length);
                rsp_length = req_length;
            }
        }
            break;
        case MODBUS_FC_WRITE_MULTIPLE_COILS: { 
            int nb = (req[offset + 3] << 8) + req[offset + 4];
            int nb_bits = req[offset + 5];
            int mapping_address = address - mb_mapping->start_bits;
    
            if (nb < 1 || MODBUS_MAX_WRITE_BITS < nb || nb_bits * 8 < nb) { 
                /* May be the indication has been truncated on reading because of * invalid address (eg. nb is 0 but the request contains values to * write) so it's necessary to flush. */
                rsp_length = response_exception(
                    ctx, &sft, MODBUS_EXCEPTION_ILLEGAL_DATA_VALUE, rsp, TRUE,
                    "Illegal number of values %d in write_bits (max %d)\n",
                    nb, MODBUS_MAX_WRITE_BITS);
            } else if (mapping_address < 0 ||
                       (mapping_address + nb) > mb_mapping->nb_bits) { 
                rsp_length = response_exception(
                    ctx, &sft,
                    MODBUS_EXCEPTION_ILLEGAL_DATA_ADDRESS, rsp, FALSE,
                    "Illegal data address 0x%0X in write_bits\n",
                    mapping_address < 0 ? address : address + nb);
            } else { 
                /* 6 = byte count */
                modbus_set_bits_from_bytes(mb_mapping->tab_bits, mapping_address, nb,
                                           &req[offset + 6]);
    
                rsp_length = ctx->backend->build_response_basis(&sft, rsp);
                /* 4 to copy the bit address (2) and the quantity of bits */
                memcpy(rsp + rsp_length, req + rsp_length, 4);
                rsp_length += 4;
            }
        }
            break;
        case MODBUS_FC_WRITE_MULTIPLE_REGISTERS: { 
            int nb = (req[offset + 3] << 8) + req[offset + 4];
            int nb_bytes = req[offset + 5];
            int mapping_address = address - mb_mapping->start_registers;
    
            if (nb < 1 || MODBUS_MAX_WRITE_REGISTERS < nb || nb_bytes != nb * 2) { 
                rsp_length = response_exception(
                    ctx, &sft, MODBUS_EXCEPTION_ILLEGAL_DATA_VALUE, rsp, TRUE,
                    "Illegal number of values %d in write_registers (max %d)\n",
                    nb, MODBUS_MAX_WRITE_REGISTERS);
            } else if (mapping_address < 0 ||
                       (mapping_address + nb) > mb_mapping->nb_registers) { 
                rsp_length = response_exception(
                    ctx, &sft, MODBUS_EXCEPTION_ILLEGAL_DATA_ADDRESS, rsp, FALSE,
                    "Illegal data address 0x%0X in write_registers\n",
                    mapping_address < 0 ? address : address + nb);
            } else { 
                int i, j;
                for (i = mapping_address, j = 6; i < mapping_address + nb; i++, j += 2) { 
                    /* 6 and 7 = first value */
                    mb_mapping->tab_registers[i] =
                        (req[offset + j] << 8) + req[offset + j + 1];
                }
    
                rsp_length = ctx->backend->build_response_basis(&sft, rsp);
                /* 4 to copy the address (2) and the no. of registers */
                memcpy(rsp + rsp_length, req + rsp_length, 4);
                rsp_length += 4;
            }
        }
            break;
        case MODBUS_FC_REPORT_SLAVE_ID: { 
            int str_len;
            int byte_count_pos;
    
            rsp_length = ctx->backend->build_response_basis(&sft, rsp);
            /* Skip byte count for now */
            byte_count_pos = rsp_length++;
            rsp[rsp_length++] = _REPORT_SLAVE_ID;
            /* Run indicator status to ON */
            rsp[rsp_length++] = 0xFF;
            /* LMB + length of LIBMODBUS_VERSION_STRING */
            str_len = 3 + strlen(LIBMODBUS_VERSION_STRING);
            memcpy(rsp + rsp_length, "LMB" LIBMODBUS_VERSION_STRING, str_len);
            rsp_length += str_len;
            rsp[byte_count_pos] = rsp_length - byte_count_pos - 1;
        }
            break;
        case MODBUS_FC_READ_EXCEPTION_STATUS:
            if (ctx->debug) { 
                fprintf(stderr, "FIXME Not implemented\n");
            }
            errno = ENOPROTOOPT;
            return -1;
            break;
        case MODBUS_FC_MASK_WRITE_REGISTER: { 
            int mapping_address = address - mb_mapping->start_registers;
    
            if (mapping_address < 0 || mapping_address >= mb_mapping->nb_registers) { 
                rsp_length = response_exception(
                    ctx, &sft, MODBUS_EXCEPTION_ILLEGAL_DATA_ADDRESS, rsp, FALSE,
                    "Illegal data address 0x%0X in write_register\n",
                    address);
            } else { 
                uint16_t data = mb_mapping->tab_registers[mapping_address];
                uint16_t and = (req[offset + 3] << 8) + req[offset + 4];
                uint16_t or = (req[offset + 5] << 8) + req[offset + 6];
    
                data = (data & and) | (or & (~and));
                mb_mapping->tab_registers[mapping_address] = data;
                memcpy(rsp, req, req_length);
                rsp_length = req_length;
            }
        }
            break;
        case MODBUS_FC_WRITE_AND_READ_REGISTERS: { 
            int nb = (req[offset + 3] << 8) + req[offset + 4];
            uint16_t address_write = (req[offset + 5] << 8) + req[offset + 6];
            int nb_write = (req[offset + 7] << 8) + req[offset + 8];
            int nb_write_bytes = req[offset + 9];
            int mapping_address = address - mb_mapping->start_registers;
            int mapping_address_write = address_write - mb_mapping->start_registers;
    
            if (nb_write < 1 || MODBUS_MAX_WR_WRITE_REGISTERS < nb_write ||
                nb < 1 || MODBUS_MAX_WR_READ_REGISTERS < nb ||
                nb_write_bytes != nb_write * 2) { 
                rsp_length = response_exception(
                    ctx, &sft, MODBUS_EXCEPTION_ILLEGAL_DATA_VALUE, rsp, TRUE,
                    "Illegal nb of values (W%d, R%d) in write_and_read_registers (max W%d, R%d)\n",
                    nb_write, nb, MODBUS_MAX_WR_WRITE_REGISTERS, MODBUS_MAX_WR_READ_REGISTERS);
            } else if (mapping_address < 0 ||
                       (mapping_address + nb) > mb_mapping->nb_registers ||
                       mapping_address < 0 ||
                       (mapping_address_write + nb_write) > mb_mapping->nb_registers) { 
                rsp_length = response_exception(
                    ctx, &sft, MODBUS_EXCEPTION_ILLEGAL_DATA_ADDRESS, rsp, FALSE,
                    "Illegal data read address 0x%0X or write address 0x%0X write_and_read_registers\n",
                    mapping_address < 0 ? address : address + nb,
                    mapping_address_write < 0 ? address_write : address_write + nb_write);
            } else { 
                int i, j;
                rsp_length = ctx->backend->build_response_basis(&sft, rsp);
                rsp[rsp_length++] = nb << 1;
    
                /* Write first. 10 and 11 are the offset of the first values to write */
                for (i = mapping_address_write, j = 10;
                     i < mapping_address_write + nb_write; i++, j += 2) { 
                    mb_mapping->tab_registers[i] =
                        (req[offset + j] << 8) + req[offset + j + 1];
                }
    
                /* and read the data for the response */
                for (i = mapping_address; i < mapping_address + nb; i++) { 
                    rsp[rsp_length++] = mb_mapping->tab_registers[i] >> 8;
                    rsp[rsp_length++] = mb_mapping->tab_registers[i] & 0xFF;
                }
            }
        }
            break;
    
        default:
            rsp_length = response_exception(
                ctx, &sft, MODBUS_EXCEPTION_ILLEGAL_FUNCTION, rsp, TRUE,
                "Unknown Modbus function code: 0x%0X\n", function);
            break;
        }
    
        /* Suppress any responses when the request was a broadcast */
        return (ctx->backend->backend_type == _MODBUS_BACKEND_TYPE_RTU &&
                slave == MODBUS_BROADCAST_ADDRESS) ? 0 : send_msg(ctx, rsp, rsp_length);
    }
    
    int modbus_reply_exception(modbus_t *ctx, const uint8_t *req,
                               unsigned int exception_code)
    { 
        int offset;
        int slave;
        int function;
        uint8_t rsp[MAX_MESSAGE_LENGTH];
        int rsp_length;
        int dummy_length = 99;
        sft_t sft;
    
        if (ctx == NULL) { 
            errno = EINVAL;
            return -1;
        }
    
        offset = ctx->backend->header_length;
        slave = req[offset - 1];
        function = req[offset];
    
        sft.slave = slave;
        sft.function = function + 0x80;
        sft.t_id = ctx->backend->prepare_response_tid(req, &dummy_length);
        rsp_length = ctx->backend->build_response_basis(&sft, rsp);
    
        /* Positive exception code */
        if (exception_code < MODBUS_EXCEPTION_MAX) { 
            rsp[rsp_length++] = exception_code;
            return send_msg(ctx, rsp, rsp_length);
        } else { 
            errno = EINVAL;
            return -1;
        }
    }
    
    /* Reads IO status */
    static int read_io_status(modbus_t *ctx, int function,
                              int addr, int nb, uint8_t *dest)
    { 
        int rc;
        int req_length;
    
        uint8_t req[_MIN_REQ_LENGTH];
        uint8_t rsp[MAX_MESSAGE_LENGTH];
        //构造查询帧的基础部分根据RTU模式或者TCP模式分别调用不同的构造函数
        req_length = ctx->backend->build_request_basis(ctx, function, addr, nb, req);
    
        rc = send_msg(ctx, req, req_length);
        if (rc > 0) { 
            int i, temp, bit;
            int pos = 0;
            int offset;
            int offset_end;
    
            rc = _modbus_receive_msg(ctx, rsp, MSG_CONFIRMATION);
            if (rc == -1)
                return -1;
    
            rc = check_confirmation(ctx, req, rsp, rc);
            if (rc == -1)
                return -1;
            //响应码 MBAP+功能码(1字节)+数据长度(1字节)
            offset = ctx->backend->header_length + 2;
            offset_end = offset + rc;
            for (i = offset; i < offset_end; i++) { 
                /* Shift reg hi_byte to temp */
                temp = rsp[i];
    
                for (bit = 0x01; (bit & 0xff) && (pos < nb);) { 
                    dest[pos++] = (temp & bit) ? TRUE : FALSE;
                    bit = bit << 1;
                }
    
            }
        }
    
        return rc;
    }
    
    /* Reads the boolean status of bits and sets the array elements in the destination to TRUE or FALSE (single bits). */
    int modbus_read_bits(modbus_t *ctx, int addr, int nb, uint8_t *dest)
    { 
        int rc;
    
        if (ctx == NULL) { 
            errno = EINVAL;
            return -1;
        }
    
        if (nb > MODBUS_MAX_READ_BITS) { 
            if (ctx->debug) { 
                fprintf(stderr,
                        "ERROR Too many bits requested (%d > %d)\n",
                        nb, MODBUS_MAX_READ_BITS);
            }
            errno = EMBMDATA;
            return -1;
        }
    
        rc = read_io_status(ctx, MODBUS_FC_READ_COILS, addr, nb, dest);
    
        if (rc == -1)
            return -1;
        else
            return nb;
    }
    
    
    /* Same as modbus_read_bits but reads the remote device input table */
    int modbus_read_input_bits(modbus_t *ctx, int addr, int nb, uint8_t *dest)
    { 
        int rc;
    
        if (ctx == NULL) { 
            errno = EINVAL;
            return -1;
        }
    
        if (nb > MODBUS_MAX_READ_BITS) { 
            if (ctx->debug) { 
                fprintf(stderr,
                        "ERROR Too many discrete inputs requested (%d > %d)\n",
                        nb, MODBUS_MAX_READ_BITS);
            }
            errno = EMBMDATA;
            return -1;
        }
    
        rc = read_io_status(ctx, MODBUS_FC_READ_DISCRETE_INPUTS, addr, nb, dest);
    
        if (rc == -1)
            return -1;
        else
            return nb;
    }
    
    /* Reads the data from a remove device and put that data into an array */
    static int read_registers(modbus_t *ctx, int function, int addr, int nb,
                              uint16_t *dest)
    { 
        int rc;
        int req_length;
        uint8_t req[_MIN_REQ_LENGTH];
        uint8_t rsp[MAX_MESSAGE_LENGTH];
    
        //判断读取寄存器的数量是否在允许的范围之内
        if (nb > MODBUS_MAX_READ_REGISTERS) { 
            if (ctx->debug) { 
                fprintf(stderr,
                        "ERROR Too many registers requested (%d > %d)\n",
                        nb, MODBUS_MAX_READ_REGISTERS);
            }
            errno = EMBMDATA;
            return -1;
        }
        //构造查询帧的基础部分
        req_length = ctx->backend->build_request_basis(ctx, function, addr, nb, req);
        //发送消息帧
        rc = send_msg(ctx, req, req_length);
        if (rc > 0) { 
            int offset;
            int i;
            //接收响应报文
            rc = _modbus_receive_msg(ctx, rsp, MSG_CONFIRMATION);
            if (rc == -1)
                return -1;
    
            rc = check_confirmation(ctx, req, rsp, rc);
            if (rc == -1)
                return -1;
    
            offset = ctx->backend->header_length;
    
            for (i = 0; i < rc; i++) { 
                /* shift reg hi_byte to temp OR with lo_byte */
                dest[i] = (rsp[offset + 2 + (i << 1)] << 8) |
                    rsp[offset + 3 + (i << 1)];
            }
        }
    
        return rc;
    }
    
    /* Reads the holding registers of remote device and put the data into an array */
    int modbus_read_registers(modbus_t *ctx, int addr, int nb, uint16_t *dest)
    { 
        int status;
    
        if (ctx == NULL) { 
            errno = EINVAL;
            return -1;
        }
    
        if (nb > MODBUS_MAX_READ_REGISTERS) { 
            if (ctx->debug) { 
                fprintf(stderr,
                        "ERROR Too many registers requested (%d > %d)\n",
                        nb, MODBUS_MAX_READ_REGISTERS);
            }
            errno = EMBMDATA;
            return -1;
        }
    
        status = read_registers(ctx, MODBUS_FC_READ_HOLDING_REGISTERS,
                                addr, nb, dest);
        return status;
    }
    
    /* Reads the input registers of remote device and put the data into an array */
    int modbus_read_input_registers(modbus_t *ctx, int addr, int nb,
                                    uint16_t *dest)
    { 
        int status;
    
        if (ctx == NULL) { 
            errno = EINVAL;
            return -1;
        }
    
        if (nb > MODBUS_MAX_READ_REGISTERS) { 
            fprintf(stderr,
                    "ERROR Too many input registers requested (%d > %d)\n",
                    nb, MODBUS_MAX_READ_REGISTERS);
            errno = EMBMDATA;
            return -1;
        }
    
        status = read_registers(ctx, MODBUS_FC_READ_INPUT_REGISTERS,
                                addr, nb, dest);
    
        return status;
    }
    
    /* Write a value to the specified register of the remote device. Used by write_bit and write_register */
    static int write_single(modbus_t *ctx, int function, int addr, const uint16_t value)
    { 
        int rc;
        int req_length;
        uint8_t req[_MIN_REQ_LENGTH];
    
        if (ctx == NULL) { 
            errno = EINVAL;
            return -1;
        }
    
        req_length = ctx->backend->build_request_basis(ctx, function, addr, (int) value, req);
    
        rc = send_msg(ctx, req, req_length);
        if (rc > 0) { 
            /* Used by write_bit and write_register */
            uint8_t rsp[MAX_MESSAGE_LENGTH];
    
            rc = _modbus_receive_msg(ctx, rsp, MSG_CONFIRMATION);
            if (rc == -1)
                return -1;
    
            rc = check_confirmation(ctx, req, rsp, rc);
        }
    
        return rc;
    }
    
    /* Turns ON or OFF a single bit of the remote device */
    int modbus_write_bit(modbus_t *ctx, int addr, int status)
    { 
        if (ctx == NULL) { 
            errno = EINVAL;
            return -1;
        }
    
        return write_single(ctx, MODBUS_FC_WRITE_SINGLE_COIL, addr,
                            status ? 0xFF00 : 0);
    }
    
    /* Writes a value in one register of the remote device */
    int modbus_write_register(modbus_t *ctx, int addr, const uint16_t value)
    { 
        if (ctx == NULL) { 
            errno = EINVAL;
            return -1;
        }
    
        return write_single(ctx, MODBUS_FC_WRITE_SINGLE_REGISTER, addr, value);
    }
    
    /* Write the bits of the array in the remote device */
    int modbus_write_bits(modbus_t *ctx, int addr, int nb, const uint8_t *src)
    { 
        int rc;
        int i;
        int byte_count;
        int req_length;
        int bit_check = 0;
        int pos = 0;
        uint8_t req[MAX_MESSAGE_LENGTH];
    
        if (ctx == NULL) { 
            errno = EINVAL;
            return -1;
        }
    
        if (nb > MODBUS_MAX_WRITE_BITS) { 
            if (ctx->debug) { 
                fprintf(stderr, "ERROR Writing too many bits (%d > %d)\n",
                        nb, MODBUS_MAX_WRITE_BITS);
            }
            errno = EMBMDATA;
            return -1;
        }
    
        req_length = ctx->backend->build_request_basis(ctx,
                                                       MODBUS_FC_WRITE_MULTIPLE_COILS,
                                                       addr, nb, req);
        byte_count = (nb / 8) + ((nb % 8) ? 1 : 0);
        req[req_length++] = byte_count;
    
        for (i = 0; i < byte_count; i++) { 
            int bit;
    
            bit = 0x01;
            req[req_length] = 0;
    
            while ((bit & 0xFF) && (bit_check++ < nb)) { 
                if (src[pos++])
                    req[req_length] |= bit;
                else
                    req[req_length] &=~ bit;
    
                bit = bit << 1;
            }
            req_length++;
        }
    
        rc = send_msg(ctx, req, req_length);
        if (rc > 0) { 
            uint8_t rsp[MAX_MESSAGE_LENGTH];
    
            rc = _modbus_receive_msg(ctx, rsp, MSG_CONFIRMATION);
            if (rc == -1)
                return -1;
    
            rc = check_confirmation(ctx, req, rsp, rc);
        }
    
    
        return rc;
    }
    
    /* Write the values from the array to the registers of the remote device */
    int modbus_write_registers(modbus_t *ctx, int addr, int nb, const uint16_t *src)
    { 
        int rc;
        int i;
        int req_length;
        int byte_count;
        uint8_t req[MAX_MESSAGE_LENGTH];
    
        if (ctx == NULL) { 
            errno = EINVAL;
            return -1;
        }
    
        if (nb > MODBUS_MAX_WRITE_REGISTERS) { 
            if (ctx->debug) { 
                fprintf(stderr,
                        "ERROR Trying to write to too many registers (%d > %d)\n",
                        nb, MODBUS_MAX_WRITE_REGISTERS);
            }
            errno = EMBMDATA;
            return -1;
        }
    
        req_length = ctx->backend->build_request_basis(ctx,
                                                       MODBUS_FC_WRITE_MULTIPLE_REGISTERS,
                                                       addr, nb, req);
        byte_count = nb * 2;
        req[req_length++] = byte_count;
    
        for (i = 0; i < nb; i++) { 
            req[req_length++] = src[i] >> 8;
            req[req_length++] = src[i] & 0x00FF;
        }
    
        rc = send_msg(ctx, req, req_length);
        if (rc > 0) { 
            uint8_t rsp[MAX_MESSAGE_LENGTH];
    
            rc = _modbus_receive_msg(ctx, rsp, MSG_CONFIRMATION);
            if (rc == -1)
                return -1;
    
            rc = check_confirmation(ctx, req, rsp, rc);
        }
    
        return rc;
    }
    
    int modbus_mask_write_register(modbus_t *ctx, int addr, uint16_t and_mask, uint16_t or_mask)
    { 
        int rc;
        int req_length;
        /* The request length can not exceed _MIN_REQ_LENGTH - 2 and 4 bytes to * store the masks. The ugly subtraction is there to remove the 'nb' value * (2 bytes) which is not used. */
        uint8_t req[_MIN_REQ_LENGTH + 2];
    
        req_length = ctx->backend->build_request_basis(ctx,
                                                       MODBUS_FC_MASK_WRITE_REGISTER,
                                                       addr, 0, req);
    
        /* HACKISH, count is not used */
        req_length -= 2;
    
        req[req_length++] = and_mask >> 8;
        req[req_length++] = and_mask & 0x00ff;
        req[req_length++] = or_mask >> 8;
        req[req_length++] = or_mask & 0x00ff;
    
        rc = send_msg(ctx, req, req_length);
        if (rc > 0) { 
            /* Used by write_bit and write_register */
            uint8_t rsp[MAX_MESSAGE_LENGTH];
    
            rc = _modbus_receive_msg(ctx, rsp, MSG_CONFIRMATION);
            if (rc == -1)
                return -1;
    
            rc = check_confirmation(ctx, req, rsp, rc);
        }
    
        return rc;
    }
    
    /* Write multiple registers from src array to remote device and read multiple registers from remote device to dest array. */
    int modbus_write_and_read_registers(modbus_t *ctx,
                                        int write_addr, int write_nb,
                                        const uint16_t *src,
                                        int read_addr, int read_nb,
                                        uint16_t *dest)
    
    { 
        int rc;
        int req_length;
        int i;
        int byte_count;
        uint8_t req[MAX_MESSAGE_LENGTH];
        uint8_t rsp[MAX_MESSAGE_LENGTH];
    
        if (ctx == NULL) { 
            errno = EINVAL;
            return -1;
        }
    
        if (write_nb > MODBUS_MAX_WR_WRITE_REGISTERS) { 
            if (ctx->debug) { 
                fprintf(stderr,
                        "ERROR Too many registers to write (%d > %d)\n",
                        write_nb, MODBUS_MAX_WR_WRITE_REGISTERS);
            }
            errno = EMBMDATA;
            return -1;
        }
    
        if (read_nb > MODBUS_MAX_WR_READ_REGISTERS) { 
            if (ctx->debug) { 
                fprintf(stderr,
                        "ERROR Too many registers requested (%d > %d)\n",
                        read_nb, MODBUS_MAX_WR_READ_REGISTERS);
            }
            errno = EMBMDATA;
            return -1;
        }
        req_length = ctx->backend->build_request_basis(ctx,
                                                       MODBUS_FC_WRITE_AND_READ_REGISTERS,
                                                       read_addr, read_nb, req);
    
        req[req_length++] = write_addr >> 8;
        req[req_length++] = write_addr & 0x00ff;
        req[req_length++] = write_nb >> 8;
        req[req_length++] = write_nb & 0x00ff;
        byte_count = write_nb * 2;
        req[req_length++] = byte_count;
    
        for (i = 0; i < write_nb; i++) { 
            req[req_length++] = src[i] >> 8;
            req[req_length++] = src[i] & 0x00FF;
        }
    
        rc = send_msg(ctx, req, req_length);
        if (rc > 0) { 
            int offset;
    
            rc = _modbus_receive_msg(ctx, rsp, MSG_CONFIRMATION);
            if (rc == -1)
                return -1;
    
            rc = check_confirmation(ctx, req, rsp, rc);
            if (rc == -1)
                return -1;
    
            offset = ctx->backend->header_length;
            for (i = 0; i < rc; i++) { 
                /* shift reg hi_byte to temp OR with lo_byte */
                dest[i] = (rsp[offset + 2 + (i << 1)] << 8) |
                    rsp[offset + 3 + (i << 1)];
            }
        }
    
        return rc;
    }
    
    /* Send a request to get the slave ID of the device (only available in serial communication). */
    int modbus_report_slave_id(modbus_t *ctx, int max_dest, uint8_t *dest)
    { 
        int rc;
        int req_length;
        uint8_t req[_MIN_REQ_LENGTH];
    
        if (ctx == NULL || max_dest <= 0) { 
            errno = EINVAL;
            return -1;
        }
    
        req_length = ctx->backend->build_request_basis(ctx, MODBUS_FC_REPORT_SLAVE_ID,
                                                       0, 0, req);
    
        /* HACKISH, addr and count are not used */
        req_length -= 4;
    
        rc = send_msg(ctx, req, req_length);
        if (rc > 0) { 
            int i;
            int offset;
            uint8_t rsp[MAX_MESSAGE_LENGTH];
    
            rc = _modbus_receive_msg(ctx, rsp, MSG_CONFIRMATION);
            if (rc == -1)
                return -1;
    
            rc = check_confirmation(ctx, req, rsp, rc);
            if (rc == -1)
                return -1;
    
            offset = ctx->backend->header_length + 2;
    
            /* Byte count, slave id, run indicator status and additional data. Truncate copy to max_dest. */
            for (i=0; i < rc && i < max_dest; i++) { 
                dest[i] = rsp[offset + i];
            }
        }
    
        return rc;
    }
    
    void _modbus_init_common(modbus_t *ctx)
    { 
        /* Slave and socket are initialized to -1 */
        ctx->slave = -1;
        ctx->s = -1;
    
        ctx->debug = FALSE;
        ctx->error_recovery = MODBUS_ERROR_RECOVERY_NONE;
    
        ctx->response_timeout.tv_sec = 0;
        ctx->response_timeout.tv_usec = _RESPONSE_TIMEOUT;
    
        ctx->byte_timeout.tv_sec = 0;
        ctx->byte_timeout.tv_usec = _BYTE_TIMEOUT;
    
        ctx->indication_timeout.tv_sec = 0;
        ctx->indication_timeout.tv_usec = 0;
    }
    
    /* Define the slave number */
    int modbus_set_slave(modbus_t *ctx, int slave)
    { 
        if (ctx == NULL) { 
            errno = EINVAL;
            return -1;
        }
    
        return ctx->backend->set_slave(ctx, slave);
    }
    
    int modbus_get_slave(modbus_t *ctx)
    { 
        if (ctx == NULL) { 
            errno = EINVAL;
            return -1;
        }
    
        return ctx->slave;
    }
    
    int modbus_set_error_recovery(modbus_t *ctx,
                                  modbus_error_recovery_mode error_recovery)
    { 
        if (ctx == NULL) { 
            errno = EINVAL;
            return -1;
        }
    
        /* The type of modbus_error_recovery_mode is unsigned enum */
        ctx->error_recovery = (uint8_t) error_recovery;
        return 0;
    }
    
    int modbus_set_socket(modbus_t *ctx, int s)
    { 
        if (ctx == NULL) { 
            errno = EINVAL;
            return -1;
        }
    
        ctx->s = s;
        return 0;
    }
    
    int modbus_get_socket(modbus_t *ctx)
    { 
        if (ctx == NULL) { 
            errno = EINVAL;
            return -1;
        }
    
        return ctx->s;
    }
    
    /* Get the timeout interval used to wait for a response */
    int modbus_get_response_timeout(modbus_t *ctx, uint32_t *to_sec, uint32_t *to_usec)
    { 
        if (ctx == NULL) { 
            errno = EINVAL;
            return -1;
        }
    
        *to_sec = ctx->response_timeout.tv_sec;
        *to_usec = ctx->response_timeout.tv_usec;
        return 0;
    }
    
    int modbus_set_response_timeout(modbus_t *ctx, uint32_t to_sec, uint32_t to_usec)
    { 
        if (ctx == NULL ||
            (to_sec == 0 && to_usec == 0) || to_usec > 999999) { 
            errno = EINVAL;
            return -1;
        }
    
        ctx->response_timeout.tv_sec = to_sec;
        ctx->response_timeout.tv_usec = to_usec;
        return 0;
    }
    
    /* Get the timeout interval between two consecutive bytes of a message */
    int modbus_get_byte_timeout(modbus_t *ctx, uint32_t *to_sec, uint32_t *to_usec)
    { 
        if (ctx == NULL) { 
            errno = EINVAL;
            return -1;
        }
    
        *to_sec = ctx->byte_timeout.tv_sec;
        *to_usec = ctx->byte_timeout.tv_usec;
        return 0;
    }
    
    int modbus_set_byte_timeout(modbus_t *ctx, uint32_t to_sec, uint32_t to_usec)
    { 
        /* Byte timeout can be disabled when both values are zero */
        if (ctx == NULL || to_usec > 999999) { 
            errno = EINVAL;
            return -1;
        }
    
        ctx->byte_timeout.tv_sec = to_sec;
        ctx->byte_timeout.tv_usec = to_usec;
        return 0;
    }
    
    /* Get the timeout interval used by the server to wait for an indication from a client */
    int modbus_get_indication_timeout(modbus_t *ctx, uint32_t *to_sec, uint32_t *to_usec)
    { 
        if (ctx == NULL) { 
            errno = EINVAL;
            return -1;
        }
    
        *to_sec = ctx->indication_timeout.tv_sec;
        *to_usec = ctx->indication_timeout.tv_usec;
        return 0;
    }
    
    int modbus_set_indication_timeout(modbus_t *ctx, uint32_t to_sec, uint32_t to_usec)
    { 
        /* Indication timeout can be disabled when both values are zero */
        if (ctx == NULL || to_usec > 999999) { 
            errno = EINVAL;
            return -1;
        }
    
        ctx->indication_timeout.tv_sec = to_sec;
        ctx->indication_timeout.tv_usec = to_usec;
        return 0;
    }
    
    int modbus_get_header_length(modbus_t *ctx)
    { 
        if (ctx == NULL) { 
            errno = EINVAL;
            return -1;
        }
    
        return ctx->backend->header_length;
    }
    
    int modbus_connect(modbus_t *ctx)
    { 
        if (ctx == NULL) { 
            errno = EINVAL;
            return -1;
        }
    
        return ctx->backend->connect(ctx);
    }
    
    void modbus_close(modbus_t *ctx)
    { 
        if (ctx == NULL)
            return;
    
        ctx->backend->close(ctx);
    }
    
    void modbus_free(modbus_t *ctx)
    { 
        if (ctx == NULL)
            return;
    
        ctx->backend->free(ctx);
    }
    
    int modbus_set_debug(modbus_t *ctx, int flag)
    { 
        if (ctx == NULL) { 
            errno = EINVAL;
            return -1;
        }
    
        ctx->debug = flag;
        return 0;
    }
    
    /* Allocates 4 arrays to store bits, input bits, registers and inputs registers. The pointers are stored in modbus_mapping structure. The modbus_mapping_new_start_address() function shall return the new allocated structure if successful. Otherwise it shall return NULL and set errno to ENOMEM. */
    modbus_mapping_t* modbus_mapping_new_start_address(
        unsigned int start_bits, unsigned int nb_bits,
        unsigned int start_input_bits, unsigned int nb_input_bits,
        unsigned int start_registers, unsigned int nb_registers,
        unsigned int start_input_registers, unsigned int nb_input_registers)
    { 
        modbus_mapping_t *mb_mapping;
    
        mb_mapping = (modbus_mapping_t *)malloc(sizeof(modbus_mapping_t));
        if (mb_mapping == NULL) { 
            return NULL;
        }
    
        /* 0X */
        mb_mapping->nb_bits = nb_bits;
        mb_mapping->start_bits = start_bits;
        if (nb_bits == 0) { 
            mb_mapping->tab_bits = NULL;
        } else { 
            /* Negative number raises a POSIX error */
            mb_mapping->tab_bits =
                (uint8_t *) malloc(nb_bits * sizeof(uint8_t));
            if (mb_mapping->tab_bits == NULL) { 
                free(mb_mapping);
                return NULL;
            }
            memset(mb_mapping->tab_bits, 0, nb_bits * sizeof(uint8_t));
        }
    
        /* 1X */
        mb_mapping->nb_input_bits = nb_input_bits;
        mb_mapping->start_input_bits = start_input_bits;
        if (nb_input_bits == 0) { 
            mb_mapping->tab_input_bits = NULL;
        } else { 
            mb_mapping->tab_input_bits =
                (uint8_t *) malloc(nb_input_bits * sizeof(uint8_t));
            if (mb_mapping->tab_input_bits == NULL) { 
                free(mb_mapping->tab_bits);
                free(mb_mapping);
                return NULL;
            }
            memset(mb_mapping->tab_input_bits, 0, nb_input_bits * sizeof(uint8_t));
        }
    
        /* 4X */
        mb_mapping->nb_registers = nb_registers;
        mb_mapping->start_registers = start_registers;
        if (nb_registers == 0) { 
            mb_mapping->tab_registers = NULL;
        } else { 
            mb_mapping->tab_registers =
                (uint16_t *) malloc(nb_registers * sizeof(uint16_t));
            if (mb_mapping->tab_registers == NULL) { 
                free(mb_mapping->tab_input_bits);
                free(mb_mapping->tab_bits);
                free(mb_mapping);
                return NULL;
            }
            memset(mb_mapping->tab_registers, 0, nb_registers * sizeof(uint16_t));
        }
    
        /* 3X */
        mb_mapping->nb_input_registers = nb_input_registers;
        mb_mapping->start_input_registers = start_input_registers;
        if (nb_input_registers == 0) { 
            mb_mapping->tab_input_registers = NULL;
        } else { 
            mb_mapping->tab_input_registers =
                (uint16_t *) malloc(nb_input_registers * sizeof(uint16_t));
            if (mb_mapping->tab_input_registers == NULL) { 
                free(mb_mapping->tab_registers);
                free(mb_mapping->tab_input_bits);
                free(mb_mapping->tab_bits);
                free(mb_mapping);
                return NULL;
            }
            memset(mb_mapping->tab_input_registers, 0,
                   nb_input_registers * sizeof(uint16_t));
        }
    
        return mb_mapping;
    }
    
    modbus_mapping_t* modbus_mapping_new(int nb_bits, int nb_input_bits,
                                         int nb_registers, int nb_input_registers)
    { 
        return modbus_mapping_new_start_address(
            0, nb_bits, 0, nb_input_bits, 0, nb_registers, 0, nb_input_registers);
    }
    
    /* Frees the 4 arrays */
    void modbus_mapping_free(modbus_mapping_t *mb_mapping)
    { 
        if (mb_mapping == NULL) { 
            return;
        }
    
        free(mb_mapping->tab_input_registers);
        free(mb_mapping->tab_registers);
        free(mb_mapping->tab_input_bits);
        free(mb_mapping->tab_bits);
        free(mb_mapping);
    }
    
    #ifndef HAVE_STRLCPY
    /* * Function strlcpy was originally developed by * Todd C. Miller <Todd.Miller@courtesan.com> to simplify writing secure code. * See ftp://ftp.openbsd.org/pub/OpenBSD/src/lib/libc/string/strlcpy.3 * for more information. * * Thank you Ulrich Drepper... not! * * Copy src to string dest of size dest_size. At most dest_size-1 characters * will be copied. Always NUL terminates (unless dest_size == 0). Returns * strlen(src); if retval >= dest_size, truncation occurred. */
    size_t strlcpy(char *dest, const char *src, size_t dest_size)
    { 
        register char *d = dest;
        register const char *s = src;
        register size_t n = dest_size;
    
        /* Copy as many bytes as will fit */
        if (n != 0 && --n != 0) { 
            do { 
                if ((*d++ = *s++) == 0)
                    break;
            } while (--n != 0);
        }
    
        /* Not enough room in dest, add NUL and traverse rest of src */
        if (n == 0) { 
            if (dest_size != 0)
                *d = '\0'; /* NUL-terminate dest */
            while (*s++)
                ;
        }
    
        return (s - src - 1); /* count does not include NUL */
    }
    #endif

## 04. 预留 ##

## 05. 附录 ##