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【嵌入式】Libmodbus源码分析(三)-modbus相关函数分析

程序员文章站 2022-07-02 08:45:01
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00. 目录

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 <[email protected]>
 *
 * 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 <[email protected]>
 *
 * 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 <[email protected]>
 *
 * 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 <[email protected]> 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

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05. 附录