write data on device/card

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I started struggling with something I didn't think would be that complicated, so I would really appreciate some advice. I will tell you my situation.

I write data to a device/card. The writing procedure is easy and goes usually like this:

C++

unsigned char sendBuffer[32];

sendBuffer[0] = headerInfo;
sendBuffer[1] = headerInfo;
sendBuffer[2] = data[0];
sendBuffer[3] = data[1];
...
sendBuffer[17] = data[15];

The data I write is usually 16 bytes, or multiple of 16. So, storing is easy, I just call: SendData(sendBuffer, length), and done, data is stored.

1) My question is say I want to store now integer 517 on the card - what data shall I send to the device? How to embed it in the sendBuffer array?
2) Similarly. Say I want to send string "Hello world xyz" to the device. How to embed it in the sendBuffer?

Some highlights. I am the receiver end. I will be reading the data back. And also depending where I stored it I may in advance know whether it is an int or string.
I also don't have endianness issues.

Example
---------
Below I will just provide working example which manages to write and read 16 byte HEX strings and hope this will be helpful to you to answer my questions better.

Say I want to write this string: "00000000000000000000AABBEEAABBEE"

We convert it to byte array:

C#

for(int i=0,m=0; i < size; i+=2,m++)
{
        char ch1, ch2;
        ch1=(char)str[i]; // str is our hex string
        ch2=(char)str[i+1];
        int dig1, dig2;
        if(isdigit(ch1)) dig1 = ch1 - '0';
        else if(ch1>='A' && ch1<='F') dig1 = ch1 - 'A' + 10;
        else if(ch1>='a' && ch1<='f') dig1 = ch1 - 'a' + 10;
        if(isdigit(ch2)) dig2 = ch2 - '0';
        else if(ch2>='A' && ch2<='F') dig2 = ch2 - 'A' + 10;
        else if(ch2>='a' && ch2<='f') dig2 = ch2 - 'a' + 10;
       
 array1[m] = (char)(dig1*16 + dig2); // output byte array
}

Now, we copy this byte array array1 to the sendBuffer as above, e.g.,

C#

for(int i=0;i<16;i++)
     sendBuff[i]=array1[i];

and done we call send command as I mentioned before.

Reading is also easy, I call: Receive(receiveBuffer) and now receiveBuffer contains the byte array I wrote previously. Then I need to convert it back to hex like this:

C#

for(k=0;k<16;k++)
 {
      data[k*2]=hexval[((receiveBuffer[k]>>4)&0xF)];
      data[(k*2)+1]=hexval[receiveBuffer[k]&0x0F];
 }

And done, data now contains my initial hex string: "00000000000000000000AABBEEAABBEE".

Posted 19-Sep-13 22:27pm

_coder

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CPallini · Accepted Answer · 2013-09-19T23:29:00

Solution 1

Embedding a integer or a (short) string is simple.
You may use 4 (or 2) bytes for storing each integer. Suppose you choose 4 bytes an integer, then

int k = whatever; // assuming 32-bit integers
sendbuffer[2] = (unsigned char)(k >> 24);
sendbuffer[3] = (unsigned char)(k >> 16);
sendbuffer[4] = (unsigned char)(k >> 8);
sendbuffer[5] = (unsigned char) k;

While for a simple string

const char * s = "hello";
int n;
// TODO: check if it fits
for (n=0; n=strlen(s); ++n) 
{
  sendbuffer[2+n] = s[n];
}
s[n] = '\0';

Of course you have to specify somehow what sendbuffer contains (you could use the headerinfo bytes for the purpose, I suppose).

Posted 19-Sep-13 23:29pm

CPallini

Comments

_coder 20-Sep-13 5:36am

CPallini thank you. I think I may not need to specify what sendBuffer contains, as I write the data and I read back(I know where I will store what). About string example I was thinking memcpy could also work, to memcpy string (e.g., "hello world") to sendBuffer, right? The only problem I can think of now is that sendBuffer must contain 16 bytes or multiple of 16. If string I want to write is say 11 bytes, I would probably have to pad it right?

CPallini 20-Sep-13 6:26am

You don't need to pad it: just add the '\0' terminator, like I did (or prefix it with its length).

_coder 24-Sep-13 4:43am

CPallini, what if I want to embed say two integers into the buffer, how do I do this? Similarly to your code? (e.g.,start embedding second number from index 6 of buffer) Are there any gotchas?? Thank you.

CPallini 24-Sep-13 4:50am

Yes, you may do that. In the header you could specify how many valid ints are there.