Originally Posted by adamj2

Hi there,

What I am trying to do is implement my own sequence number program.

I have a sequence number i.e 0001 and then I try to convert this to a binary string in byte format of 4 bytes: 00 01 00 00

And then I am appending this in the middle of a byte payload that I have created, which is just a group of numbers. ie choose 12345678 which got converted to something like 32 33 34 35 etc for byte format..

I then want to extract the sequence number portion, which is bytes 4-8 of the 8 byte payload. This is where I am stuck and not sure how to progress.
ie my payload looks like 32 33 34 35 00 01 00 00
and I want to extract 00 01 00 00 from this and then convert this back to my original integer.

I am trying to use the substr command but am not just trying to extract data from a string, rather a string of bytes. I am just extracting the sequence number part of the last calculated sequence number for now to try and get it working.

Here is my code, any help would be great, Many thanks!

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#include <stdio.h>

#include <iostream>

#include <string>
using std::string;


#include "hex.h"

unsigned char temp[4];
unsigned int seq;
bool flag = true;
unsigned int currentseq;
unsigned int previousseq = 0;

int testing =2;

int main()

{

byte payload[8] = {"1234567"};

for (int i=0; i<10; i++)
    {

if (flag == true)
{
    seq = 0000;
    flag = false;
}

seq=seq++;

printf("Sender sequence number = %0004x", seq);
printf("\n");

// BYTE CONVERSION


    for( int i=0;i<4;i++)
    temp[i]= '\0';
    memcpy(temp,&seq,4);
    printf("TEMP = %02x %02x %02x %02x \n", temp[0], temp[1], temp[2], temp[3]);


    payload[4] = temp[0];
    payload[5] = temp[1];
    payload[6] = temp[2];
    payload[7] = temp[3];
    printf ("payload[4] =  %02x \n", temp[0]);
    printf("\n");


    }

    //this is where my problems are, am trying to extract by sequence number in byte format from the binary string, but not sure how
    string str = payload;
    const char *p = str.substr(2,4).c_str();
    std::cout << "\n" << p << "\n";
    

}

#include <stdio.h> #include <iostream> #include <string> using std::string; #include "hex.h" unsigned char temp[4]; unsigned int seq; bool flag = true; unsigned int currentseq; unsigned int previousseq = 0; int testing =2; int main() { byte payload[8] = {"1234567"}; for (int i=0; i<10; i++) { if (flag == true) { seq = 0000; flag = false; } seq=seq++; printf("Sender sequence number = %0004x", seq); printf("\n"); // BYTE CONVERSION for( int i=0;i<4;i++) temp[i]= '\0'; memcpy(temp,&seq,4); printf("TEMP = %02x %02x %02x %02x \n", temp[0], temp[1], temp[2], temp[3]); payload[4] = temp[0]; payload[5] = temp[1]; payload[6] = temp[2]; payload[7] = temp[3]; printf ("payload[4] = %02x \n", temp[0]); printf("\n"); } //this is where my problems are, am trying to extract by sequence number in byte format from the binary string, but not sure how string str = payload; const char *p = str.substr(2,4).c_str(); std::cout << "\n" << p << "\n"; }

Originally Posted by VernonDozier

C++ doesn't have a byte type. Do you mean unsigned char? And hex.h is something you wrote yourself, correct? So you are trying to extract the "0001" from the string using substr, but that doesn't work. But why doesn't it work? Is it that you need the 0, 0, 0, and 1 stored as integers and you are getting the ASCII values for 0, 0, 0, and 1 instead?

Originally Posted by adamj2

which gives me 01000000 for number 1 which is 4 bytes (they give 2 characters per byte for some reason)...etc...

Originally Posted by adamj2

And then I am appending this in the middle of a byte payload that I have created, which is just a group of numbers. ie choose 12345678 which got converted to something like 32 33 34 35 etc for byte format..

Originally Posted by adamj2

I have a sequence number i.e 0001 and then I try to convert this to a binary string in byte format of 4 bytes: 00 01 00 00

Originally Posted by VernonDozier

Who are they? Is this a function somewhere? The fact that you are given 2 characters per byte (what "gives" you this, by the way?) suggests to me that there is some function that you pass a number from 0 to 255, say 47, and you are returned the following two character string:

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2F

2F

which is the string representation of the hexadecimal representation of 47.

This earlier quote:


suggests that you 1 is being converted to "32", 2 is converted to "33", 3 is converted to "34", etc.

The ASCII value of '1' is 0x31, the ASCII value of '2' is 0x32, the ASCII value of '3' is 0x33, so it appears like you are dealing with ASCII values but are one off.




Why is this not 00 00 00 01?


I'm not sure we are using the same definition of "binary string". In your definition, does the number 1 get stored as one byte of 0x1 or as one byte of 0x31 (ASCII representation of '1') or as two bytes (one byte of 0x30, which is ASCII representation of '0', followed by one byte of 0x31, which is ASCII representation of '1')? Do you ever have bytes that are not in the range of 0x30 through 0x39 or 0x41 through 0x46 (Ascii representations of '0' through '9' and 'A' through 'F')?

Originally Posted by adamj2

I have been given some code for an new packet header processing. And all the data is appended to the payload as one long string. I am trying to add a seq num bit to it and just testing it out in a single prog. first.

These are some methods that are used for conversion to the format representing the data as strings of bytes (or whatever the correct definition is?)

i= this->_payload.length();
byte *p;
p= new byte [i];
memcpy (p, this->_payload.c_str(),i) ;

int ULE_SID=4444;
int uli_netval=htonl(ULE_SID); //convert host to network for integers
unsigned char temp[4];
for( int i=0;i<4;i++)
temp[i]= '\0';
memcpy(temp,&uli_netval,4);

I have tried adapting these methods to convert my seq number into a binary string of 4 bytes.

for some reason for say seq num 0001 I always get 01 00 00 00 from those methods.

I then want to append my 4 byte seq num to a payload (which is a string of data...ie header length first 4 bytes, seq num is next 4 bytes), I have just created a imaginery one of 12345678:
byte payload[8] = {"1234567"}; >> but maybe that is the wrong way to go about it?

I dont really know what is going on with the bytes and how they are represented etc so sorry am not much help with providing the necessary info. WIll just have to go back to my supervisor at Uni for further help and to find out exactly how the payload data is stored. I know its a string, and the seq num should be represented over 4 bytes, so guess it is over 4 bytes as unsigned chars.

Originally Posted by adamj2

Thanks for responces, will check out that link in a bit.

Sorry for not making my questions clear.

I now know it is a string of chars that I want to append my 4 byte sequence number to.

I then want to extract the sequence number from that string of chars.

At the moment am just stuck on how to extract from a string of chars rather than a normal string. Any example code will be great. Thanks.

Originally Posted by VernonDozier

Give this a try. Basically you have four integers from 0 to If it's not, this won't work. I think (not 100% positive) that some compilers set aside more than one byte for a char.