Thread: Detecting byte pattern

Hello,

Could you tell me what is wrong with this code:

Code:

unsigned char data[4]={0x00,0x00,0x01,0xb3}; //byte pattern
 int i = 0;
while(m->data_len > 100) {
 
    if (m->payload[i] != 0x00) {   
        i++;
                                 } //skipping through bytes until I find 0x00
    else {
    if (memcmp(&m->payload[i],data,sizeof(data)) == 0) {
                       printf("I frame");

break;                                                                      }

   
         }
                }

Full code: [C] Code - Pastebin.com

Hard to tell what the code is supposed to do.
One mistake could be that you do not advance i any more when 0x00 is found.
Kurt

Originally Posted by ZuK

Hard to tell what the code is supposed to do.
One mistake could be that you do not advance i any more when 0x00 is found.
Kurt

The main idea is to detect wanted bytes pattern. I must be doing smt wrong here. I skip through bytes until I find 0x00 and then with memcmp() function try to check if wanted pattern starts here. If not it should search further. Maybe my code search only for first 0x00 and then jumps to another packet?

This code works well If I know exact place of bytes pattern.

MC

Then you should also increment i when a 0x00 was found but memcmp() fails.
Kurt

Originally Posted by ZuK

Then you should also increment i when a 0x00 was found but memcmp() fails.
Kurt

Thank you. Cant believe I was so dumb to miss it.

Originally Posted by AndiPersti

What did you expect? What error messages you get?

Code:

unsigned char data[4]={0x00,0x00,0x01,0xb3}; //byte pattern
 int i = 0;
while(m->data_len > 100)
 {
    if (m->payload[i] != 0x00) 
    {   
        i++;
    } //skipping through bytes until I find 0x00
    else
    {
        if (memcmp(&m->payload[i],data,sizeof(data)) == 0)
        {
            printf("I frame");
            break;                                                                      
        }
    }
}

You never change m->data_len inside your while-loop thus you loop infinitely if m->data_len is greater than 100 and never find the pattern you want.
And you should learn to indent your code properly.

Bye, Andreas

Thank you Andreas. I am newbie in programming so all info are very useful to me.