Payload modification - If or initiation problem

Hi,

I want to change N-th byte of payload. This code works but does not do what I want. I see that a-th byte of payload is set to 0xAA. What I am doing wrong here?

Code:

---

unsigned char data[4]={0x00,0x00,0x01,0xb3};
int a=55;
 int k = 0;
//int j;
int N = 51;
while(k < m->data_len) {

 
    if (m->payload[k] != 0x00) {
        k++;
                      }
    else {
    if (memcmp(&m->payload[k],data,sizeof(data))==0) {
int j;
if(j % N == 0) {
m->payload[a]=0xAA;   
m->payload[j]=0xFF; // ??????

}
    else {break;}
 j++;                        }
else {
k++;  }
    }
                  }

---

Quote:

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Originally Posted by MiniComa

I want to change N-th byte of payload. This code works but does not do what I want. I see that a-th byte of payload is set to 0xAA. What I am doing wrong here?

---

For starters, you need to indent your code properly, e.g.,

Code:

---

unsigned char data[4] = {0x00, 0x00, 0x01, 0xb3};
int a = 55;
int k = 0;
//int j;
int N = 51;
while (k < m->data_len) {
    if (m->payload[k] != 0x00) {
        k++;
    }
    else {
        if (memcmp(&m->payload[k], data, sizeof(data)) == 0) {
            int j;
            if (j % N == 0) {
                m->payload[a] = 0xAA;
                m->payload[j] = 0xFF; // ??????
            }
            else {
                break;
            }
            j++;
        }
        else {
            k++;
        }
    }
}

---

Anyway, if you want to change the Nth byte of payload instead of the jth byte, then just write:

Code:

---

m->payload[N] = 0xFF;

---

I want to change every N-th byte of payload. And if I set m->payload[N] = 0xFF; it will change only first one. Sorry if my question was obscure.

P.S. How do you made code to look so nice?

Quote:

---

Originally Posted by MiniComa

I want to change every N-th byte of payload. And if I set m->payload[N] = 0xFF; it will change only first one.

---

Right. In that case:

Code:

---

int n = N;

// ...

m->payload[n] = 0xFF;
n += N;

---

Quote:

---

Originally Posted by MiniComa

How do you made code to look so nice?

---

You're already using code bbcode tags, so that part is done :)
Notice that in each block of code, I indent by one level, fixed to 4 spaces.

In the code below , j has junk in it (not initialized ) and may or may not be evenly divisible by N, regardless of what k is.

Code:

---

int j;
if (j % N == 0) {

---

Did I understand you correctly and code should look like this? Because it gives me the same output.

Code:

---

unsigned char data[4]={0x00,0x00,0x01,0xb3};
 int k = 0;
int j;
int N=5;
int n=N;
while(k < m->data_len) {
    if (m->payload[k] != 0x00) {
                k++;
    }
    else {
        if (memcmp(&m->payload[k],data,sizeof(data))==0) {
            if(j % N == 0) {
            m->payload[a]=0xAA;   
            m->payload[n]=0xFF;
            n += N;
    }
          else{
          break;
    }
          j++;                }
    else {
          k++; 
    }
  }
}

---

Quote:

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Originally Posted by sparkomemphis

In the code below , j has junk in it (not initialized ) and may or may not be evenly divisible by N, regardless of what k is.

Code:

---

int j;
if (j % N == 0) {

---

---

What can I do to avoid that?

Taking into account sparkomemphis' observation and your use of j in your original code, I think that you were trying to use j as my example's n, except that you did it wrongly. I would expect something along these lines:

Code:

---

unsigned char data[4] = {0x00, 0x00, 0x01, 0xb3};
int k = 0;
int j = 0;
int N = 5;
while (k < m->data_len) {
    if (m->payload[k] == 0x00 && memcmp(&m->payload[k], data, sizeof(data)) == 0) {
        if (j % N == 0) {
            m->payload[a] = 0xAA;
            m->payload[j] = 0xFF;
        }
        else {
            break;
        }
        j++;
    }
    else {
        k++;
    }
}

---

I have taken the liberty of condensing your if statement logic. Notice that j is correctly initialised. Nonetheless, I don't expect this code to work: you need to get your variables right, and because I don't actually know what you are trying to do, I cannot correct them for you.

Could you say what do you mean by saying this "you need to get your variables right"? Because English is not my first language and I did not understand what you mean.
Talking about whole code, I can comment a little. I work with iptables and libipq in Ubuntu. I put some MPEG2 video packets in QUEUE, take them out, modify and put back to QUEUE. Unsigned char data is beginning of byte sequence of MPEG2 sequence header. So I try to identify these packets and modify payload. After all I try to evaluate how quality of video has changed after modification.

Things like: are you incrementing j and k at the correct places? What is the variable named a?

Quote:

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Originally Posted by laserlight

Things like: are you incrementing j and k at the correct places? What is the variable named a?

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Would you look into full code if I post a link? With "a" variable I try to divide stream into two parts where in one part I have modified packets with sequence header and in the other I have all other packets .

Link: [C] C code - Pastebin.com

I have figured it out by myself. Code looks like this:

Code:

---

unsigned char data[4] = {0x00, 0x00, 0x01, 0xb3};
int k = 0;
int j = 0;
int N = 5;
while (k < m->data_len) {
    if (m->payload[k] == 0x00 && memcmp(&m->payload[k], data, sizeof(data)) == 0) {
        if (j % N == 0) {
            m->payload[a] = 0xAA;
            m->payload[j] = 0xFF;
        }
        else {
            j++;
        }
        j++;
    }
    else {
        k++;
    }
}

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Have to change only one line. Thank you laserlight. You have helped me a lot.