libmodbus 是支持Linux，Mac OS X，QNX和win32 的一个modbus库，支持modbus TCP和RTU 两种模式。

libmodbus 的官网：https://www.libmodbus.org/

当前版本为：v3.1.6

 编译

我在win10 下的wsl 下编译的，相信在ubuntu linux 下也大同小异。

   下载解压 libmodbus 的源码，F:lib modbus-3.1.6文件夹中。

    进入wsl 的console，输入下述命令完成编译

  cd /mnt/f/libmodbus-3.1.6

  ./configure

  make

  libmodbus 包中包含了tests 目录，其中有一些测试程序，已经同时被编译。

 运行测试程序

   ./bandwidth_server_one

另外开启一个wsl console 窗口

 输入 ./bandwidth_client

 可以看到输出信息

  bandwidth_client /bandwidth_server是用来测试modbus TCP 速率的测试程序。相对比较简单。下面列出了bandwidth_client 的程序源码

/*
 * Copyright © 2008-2014 Stéphane Raimbault <stephane.raimbault@gmail.com>
 *
 * SPDX-License-Identifier: BSD-3-Clause
 */

#include <stdio.h>
#ifndef _MSC_VER
#include <unistd.h>
#include <sys/time.h>
#endif
#include <string.h>
#include <stdlib.h>
#include <time.h>
#include <errno.h>

#include <modbus.h>

#define G_MSEC_PER_SEC 1000

static uint32_t gettime_ms(void)
{
    struct timeval tv;
#if !defined(_MSC_VER)
    gettimeofday(&tv, NULL);
    return (uint32_t) tv.tv_sec * 1000 + tv.tv_usec / 1000;
#else
    return GetTickCount();
#endif
}

enum {
    TCP,
    RTU
};

/* Tests based on PI-MBUS-300 documentation */
int main(int argc, char *argv[])
{
    uint8_t *tab_bit;
    uint16_t *tab_reg;
    modbus_t *ctx;
    int i;
    int nb_points;
    double elapsed;
    uint32_t start;
    uint32_t end;
    uint32_t bytes;
    uint32_t rate;
    int rc;
    int n_loop;
    int use_backend;

    if (argc > 1) {
        if (strcmp(argv[1], "tcp") == 0) {
            use_backend = TCP;
            n_loop = 100000;
        } else if (strcmp(argv[1], "rtu") == 0) {
            use_backend = RTU;
            n_loop = 100;
        } else {
            printf("Usage:\n  %s [tcp|rtu] - Modbus client to measure data bandwith\n\n", argv[0]);
            exit(1);
        }
    } else {
        /* By default */
        use_backend = TCP;
        n_loop = 100000;
    }

    if (use_backend == TCP) {
        ctx = modbus_new_tcp("127.0.0.1", 1502);
    } else {
        ctx = modbus_new_rtu("/dev/ttyUSB1", 115200, 'N', 8, 1);
        modbus_set_slave(ctx, 1);
    }
    if (modbus_connect(ctx) == -1) {
        fprintf(stderr, "Connection failed: %s\n",
                modbus_strerror(errno));
        modbus_free(ctx);
        return -1;
    }

    /* Allocate and initialize the memory to store the status */
    tab_bit = (uint8_t *) malloc(MODBUS_MAX_READ_BITS * sizeof(uint8_t));
    memset(tab_bit, 0, MODBUS_MAX_READ_BITS * sizeof(uint8_t));

    /* Allocate and initialize the memory to store the registers */
    tab_reg = (uint16_t *) malloc(MODBUS_MAX_READ_REGISTERS * sizeof(uint16_t));
    memset(tab_reg, 0, MODBUS_MAX_READ_REGISTERS * sizeof(uint16_t));

    printf("READ BITS\n\n");

    nb_points = MODBUS_MAX_READ_BITS;
    start = gettime_ms();
    for (i=0; i<n_loop; i++) {
        rc = modbus_read_bits(ctx, 0, nb_points, tab_bit);
        if (rc == -1) {
            fprintf(stderr, "%s\n", modbus_strerror(errno));
            return -1;
        }
    }
    end = gettime_ms();
    elapsed = end - start;

    rate = (n_loop * nb_points) * G_MSEC_PER_SEC / (end - start);
    printf("Transfert rate in points/seconds:\n");
    printf("* %d points/s\n", rate);
    printf("\n");

    bytes = n_loop * (nb_points / 8) + ((nb_points % 8) ? 1 : 0);
    rate = bytes / 1024 * G_MSEC_PER_SEC / (end - start);
    printf("Values:\n");
    printf("* %d x %d values\n", n_loop, nb_points);
    printf("* %.3f ms for %d bytes\n", elapsed, bytes);
    printf("* %d KiB/s\n", rate);
    printf("\n");

    /* TCP: Query and reponse header and values */
    bytes = 12 + 9 + (nb_points / 8) + ((nb_points % 8) ? 1 : 0);
    printf("Values and TCP Modbus overhead:\n");
    printf("* %d x %d bytes\n", n_loop, bytes);
    bytes = n_loop * bytes;
    rate = bytes / 1024 * G_MSEC_PER_SEC / (end - start);
    printf("* %.3f ms for %d bytes\n", elapsed, bytes);
    printf("* %d KiB/s\n", rate);
    printf("\n\n");

    printf("READ REGISTERS\n\n");

    nb_points = MODBUS_MAX_READ_REGISTERS;
    start = gettime_ms();
    for (i=0; i<n_loop; i++) {
        rc = modbus_read_registers(ctx, 0, nb_points, tab_reg);
        if (rc == -1) {
            fprintf(stderr, "%s\n", modbus_strerror(errno));
            return -1;
        }
    }
    end = gettime_ms();
    elapsed = end - start;

    rate = (n_loop * nb_points) * G_MSEC_PER_SEC / (end - start);
    printf("Transfert rate in points/seconds:\n");
    printf("* %d registers/s\n", rate);
    printf("\n");

    bytes = n_loop * nb_points * sizeof(uint16_t);
    rate = bytes / 1024 * G_MSEC_PER_SEC / (end - start);
    printf("Values:\n");
    printf("* %d x %d values\n", n_loop, nb_points);
    printf("* %.3f ms for %d bytes\n", elapsed, bytes);
    printf("* %d KiB/s\n", rate);
    printf("\n");

    /* TCP:Query and reponse header and values */
    bytes = 12 + 9 + (nb_points * sizeof(uint16_t));
    printf("Values and TCP Modbus overhead:\n");
    printf("* %d x %d bytes\n", n_loop, bytes);
    bytes = n_loop * bytes;
    rate = bytes / 1024 * G_MSEC_PER_SEC / (end - start);
    printf("* %.3f ms for %d bytes\n", elapsed, bytes);
    printf("* %d KiB/s\n", rate);
    printf("\n\n");

    printf("WRITE AND READ REGISTERS\n\n");

    nb_points = MODBUS_MAX_WR_WRITE_REGISTERS;
    start = gettime_ms();
    for (i=0; i<n_loop; i++) {
        rc = modbus_write_and_read_registers(ctx,
                                             0, nb_points, tab_reg,
                                             0, nb_points, tab_reg);
        if (rc == -1) {
            fprintf(stderr, "%s\n", modbus_strerror(errno));
            return -1;
        }
    }
    end = gettime_ms();
    elapsed = end - start;

    rate = (n_loop * nb_points) * G_MSEC_PER_SEC / (end - start);
    printf("Transfert rate in points/seconds:\n");
    printf("* %d registers/s\n", rate);
    printf("\n");

    bytes = n_loop * nb_points * sizeof(uint16_t);
    rate = bytes / 1024 * G_MSEC_PER_SEC / (end - start);
    printf("Values:\n");
    printf("* %d x %d values\n", n_loop, nb_points);
    printf("* %.3f ms for %d bytes\n", elapsed, bytes);
    printf("* %d KiB/s\n", rate);
    printf("\n");

    /* TCP:Query and reponse header and values */
    bytes = 12 + 9 + (nb_points * sizeof(uint16_t));
    printf("Values and TCP Modbus overhead:\n");
    printf("* %d x %d bytes\n", n_loop, bytes);
    bytes = n_loop * bytes;
    rate = bytes / 1024 * G_MSEC_PER_SEC / (end - start);
    printf("* %.3f ms for %d bytes\n", elapsed, bytes);
    printf("* %d KiB/s\n", rate);
    printf("\n");

    /* Free the memory */
    free(tab_bit);
    free(tab_reg);

    /* Close the connection */
    modbus_close(ctx);
    modbus_free(ctx);

    return 0;
}

上面的例子测试读10万个点，读10万个寄存器，读写十万次的时间。

主要用法

上下文类型context

前面已经提到，libmodbus 支持TCP和RTU 两种通信方式，前一种是支持TCP/IP 协议的，而RTU 是支持RS485 串口的协议。libmodbus 库的设计将TCP/RTU 模式的实现细节在用户端屏蔽掉了。modbus 的类型数据放置在一个结构类型中，当新建一个modbus 接口时，填入有关的参数，并将这个包含接口参数的结构指针返回给用户，这个指针称为上下文类型。

例如：

 在以后的各种操作中，用户只要带有contex 参数就可以了，不再关心是modbus TCP 还是modbus RTU 。这两种模式的各种函数都是一样的。

modbus_t *ctx;

 

ctx = modbus_new_tcp("127.0.0.1", 1502);

if (ctx == NULL) {

    fprintf(stderr, "Unable to allocate libmodbus context\n");

    return -1;

}

 

if (modbus_connect(ctx) == -1) {

    fprintf(stderr, "Connection failed: %s\n", modbus_strerror(errno));

    modbus_free(ctx);

    return -1;

}

显然，这是一种优雅的程序设计方式。

读比特位

rc = modbus_write_bit(ctx, addr, tab_rq_bits[0]);

 

写比特位

rc = modbus_read_bits(ctx, addr, 1, tab_rp_bits);

读寄存器

int modbus_read_registers(modbus_t *ctx, int addr, int nb, uint16_t *dest);

写寄存器

int modbus_write_register(modbus_t *ctx, int addr, const uint16_t value);

读写寄存器

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, const uint16_t *dest);