Parse and read data frame in C?

Question

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|
----------------------------------------------------

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

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")

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

• get the data in octet 1,2 in the Data field
• get the data in octet 3,4 in the Data field
• get the value in CRC field to check the consistency of the data

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;
}

Does anyone please have some ideas?

Answer 1

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

to read octet n use:

char octet_n = result[n];

So to do what you want you need:

// 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];

Edit: An explanation for this line:

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

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|
            --------------------

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];

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;

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