解析并读取C中的数据帧? [英] Parse and read data frame in C?

问题描述

我正在编写一个程序，该程序从Linux上的串行端口读取数据. 数据是由另一台设备以以下帧格式发送的:

I am writing a program that reads the data from the serial port on Linux. The data are sent by another device with the following frame format:

|start | Command | Data               | CRC  | End |
|0x02  | 0x41    | (0-127 octets)     |      | 0x03|
----------------------------------------------------

数据字段包含127个八位位组，如图所示，八位位组1,2包含一种类型的数据；字节3,4包含另一个数据.我需要获取这些数据

The Data field contains 127 octets as shown and octet 1,2 contains one type of data; octet 3,4 contains another data. I need to get these data

我知道如何在Linux的串行端口中读写数据，但这只是在读写一个简单的字符串(如"ABD")

I know how to write and read data to and from a serial port in Linux, but it is just to write and read a simple string (like "ABD")

我的问题是我不知道如何解析如上所述格式的数据帧，以便可以:

My issue is that I do not know how to parse the data frame formatted as above so that I can:

• 在数据"字段中获取八位字节1,2中的数据
• 在数据"字段中获取3,4字节的数据
• 在CRC字段中获取值以检查数据的一致性

以下示例代码片段可在Linux中的串行端口之间读写简单的字符串:

Here the sample snip code that read and write a simple string from and to a serial port in Linux:

int writeport(int fd, char *chars) {
    int len = strlen(chars);
    chars[len] = 0x0d; // stick a <CR> after the command
    chars[len+1] = 0x00; // terminate the string properly
    int n = write(fd, chars, strlen(chars));
    if (n < 0) {
        fputs("write failed!\n", stderr);
        return 0;
    }
    return 1;                                                                                                           
}

int readport(int fd, char *result) {
    int iIn = read(fd, result, 254);
    result[iIn-1] = 0x00;
    if (iIn < 0) {
        if (errno == EAGAIN) {
            printf("SERIAL EAGAIN ERROR\n");
            return 0;
        } else {
            printf("SERIAL read error %d %s\n", errno, strerror(errno));
            return 0;
        }
    }                    
    return 1;
}

有人请问一些想法吗?

推荐答案

result是由char s组成的数组，其宽度为1个八位位组.

result is an array of chars, which are 1 octet wide.

要读取八位位组 n ，请使用:

to read octet n use:

char octet_n = result[n];

要做你想做的事

// skip the start and command fields
char *data_field = result + 2; 

int octet_1_2 = data_field[1] | (data_field[2] << 8);
int octet_3_4 = data_field[3] | (data_field[4] << 8);

// crc is at byte 128 + 2 = 130
int crc = result[130];

修改:此行的说明:

int octet_1_2 = data_field[1] | (data_field[2] << 8);

您想将两个连续的八位位组读取为一个16位字:

You want to read two consecutive octets into one 16-bit word:

            1
       bits 5        8 7       0
            --------------------
octet_1_2 = | octet 2 | octet 1|
            --------------------

因此，您想获取八位位组1的7:0位，并将它们放入octet_1_2的7:0位:

So you want to take bits 7:0 of octet 1 and put them in bits 7:0 of octet_1_2:

octet_1_2 = data_field[1];

然后，您要获取八位位组2的7:0位，并将它们放入octet_1_2的15:8位.您可以通过将八位位组2向左移动8位，并将结果OR'放入octet_1_2:

Then you want to take bits 7:0 of octet 2 and put them in bits 15:8 of octet_1_2. You do this by shifting octet 2 8 bits to the left, and OR'ing the result into octet_1_2:

octet_1_2 |= data_field[2] << 8;

这两行可以像我上面那样合并为一.

These two lines can be merged into one as I did above.

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