unusual behaviour of my function

[ak47]

Hello everyone,
I have a function that creates a custom protocol for transmission of data to a serial port.
The function is below:

void calc()
{
/*cmd array and num integer are global*/
    unsigned char checksum = 0;
    cmd[0] = 0xAA;   //starting byte
    cmd[1] = 0x10;
    cmd[2] = 4;
    cmd[3] = 'G';
    cmd[4] = 'P';
    cmd[5] = ' ';
    cmd[6] = num & 0xFF;        //lsb
    cmd[7] = (num >> 8) & 0xFF; //msb

    //printf("lsb: %d",cmd[6]);
    //printf("\nmsb: %d",cmd[7]);

    checksum = 0;
    checksum ^= cmd[1];
    checksum ^= cmd[2];
    checksum ^= cmd[3];
    checksum ^= cmd[4];
    checksum ^= cmd[5];
    checksum ^= cmd[6];
    checksum ^= cmd[7];

    cmd[8] = checksum;
   //printf("\nchecksum:%d",cmd[8]);
    cmd[9] = 0xCC;     //closing byte
    cmd[10] = 0;
    cmd[11] = 0;
    //cmd[12] = 0;
}

Now in my main.c i am sending the cmd array:

calc(num);
        if(!WriteFile(hSerial,cmd,strlen(cmd),&dwBytesRead, NULL))
        {
            //if any error occurs in writing to serial port.
            printf("\n\nError occured while writting file. \n");
            exit(1);
        }
        else
        {
            printf("\nData sent.");
        }

Just to confirm that all the data was indeed sent to the serial port ,i looped the port back to my computer to re-read the data.

if(!ReadFile(hSerial, rzBuff, 10, &dwBytesRead, NULL))
        {
            system("cls");
            printf("Error occured while reading string data.\n");
            system("pause");
            exit(1);
        }
        else
        {
            printf("\n\nRead data: \n");
            printf("0:%x\n1:%x\n2:%d\n3:%c\n4:%c\n5:%c\n6:%d\n7:%d\n8:%d\n9:%x\n10:%d",rzBuff[0],rzBuff[1],rzBuff[2],rzBuff[3],rzBuff[4],rzBuff[5],rzBuff[6],rzBuff[7],rzBuff[8],rzBuff[9],rzBuff[10]);
        }
memset(rzBuff,0,strlen(rzBuff));
memset(cmd,0,strlen(cmd));

All is working fine except for one strange thing.
if num is less than 256 and greater than 799 the closing bit i.e cmd[9] always shows 0!!!
I am not able to understand what i am doing wrong,no matter what , cmd[9] should always be 0xcc.

I know this is more of a windows programming related discussion,but i am having trouble in the basic functionality and asignment of variables in a function.

Please help.

[Salem]

You're using strlen() on a buffer which contains binary data (eg checksum results).

> if(!ReadFile(hSerial, rzBuff, 10, &dwBytesRead, NULL))
Also check dwBytesRead is the number you expect.

Is it really 10 bytes?
If so, what are these here for?
cmd[10] = 0;
cmd[11] = 0;
//cmd[12] = 0;

[Hodor]

Can you paste the declaration of cmd?

I'm guessing that cmd is an array of unsigned char (or char). Looking at your printf() format string you have n8:%d. That expects an integer which is very unlikely to be the same size as char or unsigned char. So, it's probably printing cmd[8] and cmd[9] (at least, probably more) "combined".

[ak47]

You're using strlen() on a buffer which contains binary data (eg checksum results).

> if(!ReadFile(hSerial, rzBuff, 10, &dwBytesRead, NULL))
Also check dwBytesRead is the number you expect.

Is it really 10 bytes?
If so, what are these here for?
cmd[10] = 0;
cmd[11] = 0;
//cmd[12] = 0;

Actually my array is 10 bytes.

unsigned char cmd[10] = "\0";
unsigned char rzBuff[10] = "\0";

Originally cmd[] was 12 bytes long,i needed to confirm that data after cmd[9] was being looped back correctly or not. I have changed it to 10 bytes now and removed cmd[10] and cmd[11]

Secondly,about the strlen() in Readfile(),im confused i have used strlen() in Writefile().Yoy mean i should remove strlen() and write the size of the array? 10 in this case?

Thirdly,
I checked with dwBytesRead variable
Here are the results i got
0 to 255 : 7 bytes
256 above: 10 bytes

Edit: I can post the full program if you wish

[ak47]

Can you paste the declaration of cmd?

I'm guessing that cmd is an array of unsigned char (or char). Looking at your printf() format string you have n8:%d. That expects an integer which is very unlikely to be the same size as char or unsigned char. So, it's probably printing cmd[8] and cmd[9] (at least, probably more) "combined".

unsigned char cmd[10] = "\0"; 
unsigned char rzBuff[10] = "\0";

i thought that ,that %d would effect the next byte too,but it did not.I confirmed it by changing it to %c(1 byte) and checking,still cmd[9] value mysteriously disappears

[ak47]

Hey guys,i solved the issue,it was in my Writefile statement.

if(!WriteFile(hSerial,cmd,strlen(cmd),&dwBytesRead, NULL))

I removed the strlen() rightly pointed out by Salem and replaced it with the actual size,i.e 10 and wola..it worked

Thank you for all your help
Arun