1、*_t结尾的变量可以理解为type/typedef的缩写，表示它是通过typedef定义的，而不是其它数据类型。
2、关于下划线开头的变量与函数，比如_error_print、struct _modbus{}.
知乎上的回答：
——“某优雅语言我不太懂，但至少在C/C++里，标识符若以下划线开头，一般是在告诉你，这个东西，你不应该直接使用。”
——“标准规定单下划线加大写字母和双下划线开头的标识符都是预留给实现/扩展的，可移植的用户代码不该直接使用它们，也不该自行定义这种标识符。 反过来说，这也相当于鼓励编译器尽量用这些标识符做实现细节，以避免和用户代码的标识符冲突。”
https://blog.csdn.net/u010168781/article/details/73924974
https://blog.csdn.net/lhq_215/article/details/78730131
/**********************************************
*简介：Linux下modbusRTU测试程序
*作者：郭纬
*日期：2017-5-16
*版本：V1.0
**********************************************/
#include<stdio.h>
#include<stdlib.h>
#include"modbus.h"
#include <memory.h>
int main(void)
{
modbus_t *mb;
uint16_t tab_reg[64]={0};
//1-打开端口
//在下面函数中会把const modbus_backend_t _modbus_rtu_backend赋值给ctx->backend
mb = modbus_new_rtu("/dev/ttymxc1",19200,'E',8,1);
//2-设置从地址
modbus_set_slave(mb,1);
//3-建立连接
modbus_connect(mb);
//4-设置应答延时
struct timeval t;
t.tv_sec=0;
t.tv_usec=1000000;//1000ms
modbus_set_response_timeout(mb,&t);
//5-循环读
int num = 0;
while(1)
{
memset(tab_reg,0,64*2);
//6-读寄存器设置：寄存器地址、数量、数据缓冲
int regs=modbus_read_registers(mb, 0, 20, tab_reg);
printf("-------------------------------------------\n");
printf("[%4d][read num = %d]",num,regs);
num++;
int i;
for(i=0; i<20; i++)
{
printf("<%#x>",tab_reg[i]);
}
printf("\n");
printf("-------------------------------------------\n");
sleep(1);
}
//7-关闭modbus端口
modbus_close(mb);
//8-释放modbus资源
modbus_free(mb);
return 0;
}
---------------------
作者：郭老二
来源：CSDN
原文：https://blog.csdn.net/u010168781/article/details/73924974
版权声明：本文为博主原创文章，转载请附上博文链接！/*
0、两个最重要的数据结构
*/
//modbus-private.h文件中定义的_modbus
struct _modbus {
/* Slave address */
int slave;
/* Socket or file descriptor */
int s;
int debug;
int error_recovery;
struct timeval response_timeout;
struct timeval byte_timeout;
struct timeval indication_timeout;
const modbus_backend_t *backend;
void *backend_data;
};
//_modbus在modbus.h中被typedef成modbus_t
typedef struct _modbus modbus_t;
//modbus_backend_t的定义
typedef struct _modbus_backend {
unsigned int backend_type;
unsigned int header_length;
unsigned int checksum_length;
unsigned int max_adu_length;
int (*set_slave) (modbus_t *ctx, int slave);
int (*build_request_basis) (modbus_t *ctx, int function, int addr,
int nb, uint8_t *req);
int (*build_response_basis) (sft_t *sft, uint8_t *rsp);
int (*prepare_response_tid) (const uint8_t *req, int *req_length);
int (*send_msg_pre) (uint8_t *req, int req_length);
ssize_t (*send) (modbus_t *ctx, const uint8_t *req, int req_length);
int (*receive) (modbus_t *ctx, uint8_t *req);
ssize_t (*recv) (modbus_t *ctx, uint8_t *rsp, int rsp_length);
int (*check_integrity) (modbus_t *ctx, uint8_t *msg,
const int msg_length);
int (*pre_check_confirmation) (modbus_t *ctx, const uint8_t *req,
const uint8_t *rsp, int rsp_length);
int (*connect) (modbus_t *ctx);
void (*close) (modbus_t *ctx);
int (*flush) (modbus_t *ctx);
int (*select) (modbus_t *ctx, fd_set *rset, struct timeval *tv, int msg_length);
void (*free) (modbus_t *ctx);
} modbus_backend_t;
/*
1、初始化设备，下面是初始化RTU模式的代码
mb = modbus_new_rtu("/dev/ttymxc1",19200,'E',8,1);
在上面的函数中RTU模式的初始值被赋值成下面的常量。
*/
//RTU模式ctx->backend被赋值的的初始值(在文件modbus-rtu.c中)
const modbus_backend_t _modbus_rtu_backend = {
_MODBUS_BACKEND_TYPE_RTU,
_MODBUS_RTU_HEADER_LENGTH,
_MODBUS_RTU_CHECKSUM_LENGTH,
MODBUS_RTU_MAX_ADU_LENGTH,
_modbus_set_slave,
_modbus_rtu_build_request_basis,
_modbus_rtu_build_response_basis,
_modbus_rtu_prepare_response_tid,
_modbus_rtu_send_msg_pre,
_modbus_rtu_send,
_modbus_rtu_receive,
_modbus_rtu_recv,//如果是Linux系统，这里会使用read()函数
_modbus_rtu_check_integrity,
_modbus_rtu_pre_check_confirmation,
_modbus_rtu_connect,
_modbus_rtu_close,
_modbus_rtu_flush,
_modbus_rtu_select,
_modbus_rtu_free
};
//如果使用TCP模式的modbus_t* modbus_new_tcp(const char *ip, int port)函数，那么ctx->backend就会被赋值成下面的常量
//TCP模式ctx->backend被赋值的初始值（在文件modbus-tcp.c中）
const modbus_backend_t _modbus_tcp_backend = {
_MODBUS_BACKEND_TYPE_TCP,
_MODBUS_TCP_HEADER_LENGTH,
_MODBUS_TCP_CHECKSUM_LENGTH,
MODBUS_TCP_MAX_ADU_LENGTH,
_modbus_set_slave,
_modbus_tcp_build_request_basis,
_modbus_tcp_build_response_basis,
_modbus_tcp_prepare_response_tid,
_modbus_tcp_send_msg_pre,
_modbus_tcp_send,
_modbus_tcp_receive,
_modbus_tcp_recv,
_modbus_tcp_check_integrity,
_modbus_tcp_pre_check_confirmation,
_modbus_tcp_connect,
_modbus_tcp_close,
_modbus_tcp_flush,
_modbus_tcp_select,
_modbus_tcp_free
};
/*
2、modbus_set_slave(mb,1);
*/
int modbus_set_slave(modbus_t *ctx, int slave)
{
if (ctx == NULL) {
errno = EINVAL;
return -1;
}
return ctx->backend->set_slave(ctx, slave);
}
/*3、modbus_connect(mb);
实际上真正执行的还是_modbus_rtu_backend或者_modbus_tcp_backend的connect
*/
int modbus_connect(modbus_t *ctx)
{
if (ctx == NULL) {
errno = EINVAL;
return -1;
}
return ctx->backend->connect(ctx);
}
/*
4、 设置超时
t.tv_sec=0;
t.tv_usec=1000000;//1000ms
modbus_set_response_timeout(mb,&t);
*/
/*
5、读取一次寄存器的流程
*/
modbus_read_registers()
{
//1.发送请求出去
send_msg(ctx, req, req_length);
{
do {
rc = ctx->backend->send(ctx, msg, msg_length);
} while ((ctx->error_recovery & MODBUS_ERROR_RECOVERY_LINK) && rc == -1);
}
//2、接收应答
_modbus_receive_msg(ctx, rsp, MSG_CONFIRMATION);
{
while (length_to_read != 0)
{
//select与recv函数是调用的系统函数，在初始化的时候就已经指定好了。
ctx->backend->select(ctx, &rset, p_tv, length_to_read);
ctx->backend->recv(ctx, msg + msg_length, length_to_read);//如果是Linux系统，这里会使用read()函数
}
}
//校验应答
check_confirmation(ctx, req, rsp, rc);
}
libmodbus is a free software library to send/receive data with a device which respects the Modbus protocol. This library can use a serial port or an Ethernet connection.
The functions included in the library have been derived from the Modicon Modbus Protocol Reference Guide which can be obtained from Schneider at www.schneiderautomation.com.
The license of libmodbus is LGPL v2.1 or later.
The documentation is available as manual pages (man libmodbus to read general
description and list of available functions) or Web pages
www.libmodbus.org/documentation/. The
documentation is licensed under the Creative Commons Attribution-ShareAlike
License 3.0 (Unported) (http://creativecommons.org/licenses/by-sa/3.0/).
The official website is www.libmodbus.org.
The library is written in C and designed to run on Linux, Mac OS X, FreeBSD and QNX and Windows.
You will only need to install automake, autoconf, libtool and a C compiler (gcc or clang) to compile the library and asciidoc and xmlto to generate the documentation (optional).
To install, just run the usual dance, ./configure && make install. Run
./autogen.sh first to generate the configure script if required.
You can change installation directory with prefix option, eg. ./configure --prefix=/usr/local/. You have to check that the installation library path is
properly set up on your system (/etc/ld.so.conf.d) and library cache is up to
date (run ldconfig as root if required).
The library provides a libmodbus.pc file to use with pkg-config to ease your
program compilation and linking.
If you want to compile with Microsoft Visual Studio, you need to install https://github.com/chemeris/msinttypes to fill the absence of stdint.h.
To compile under Windows, install MinGW and MSYS then select the common packages (gcc, automake, libtool, etc). The directory ./src/win32/ contains a Visual C project.
To compile under OS X with homebrew, you
will need to install the following dependencies first: brew install autoconf automake libtool.
The documentation is available online or as manual pages after installation.
The documentation is based on
AsciiDoc. Only man pages are built
by default with make command, you can run make htmldoc in doc directory
to generate HTML files.
Some tests are provided in tests directory, you can freely edit the source code to fit your needs (it's Free Software :).
See tests/README for a description of each program.
For a quick test of libmodbus, you can run the following programs in two shells:
- ./unit-test-server
- ./unit-test-client
By default, all TCP unit tests will be executed (see --help for options).
It's also possible to run the unit tests with make check.
See CONTRIBUTING document.