[SOLVED] The best way to perform bitwise operation on hexadecimal value ?

Originally Posted by kikilinux Dear Syndacate thanks to answer, it is not completely same but ur solution is work for me. I want the to set a specific bit of specific byte for example "first bit of 21st byte". i problem solved. Best

Just shift whatever bit you want to set into the offset of the byte, so to modify my code to set ONLY the 1st bit (and not clear the rest), just do:

char *ip_hdr_ptr = (char*)&ip_header;
*(ip_hdr_ptr + 20) |= 1;

If you want to set the 5th bit of the 21st byte, then just shift the 1:

char *ip_hdr_ptr = (char*)&ip_header;
*(ip_hdr_ptr + 20) |= 1 << 5;

If you want to clear the 5th bit of the 21st byte, then invert it and AND remaining bits (you can just take out the shift to clear 1st bit):

char *ip_hdr_ptr = (char*)&ip_header;
*(ip_hdr_ptr + 20) &= ~(1 << 5);

Hope that works better for ya. No need to worry about memory allocations and temporary values and the like.

I believe i can't correct my errors
Can anyone help me to have an OR operation on 22st byte of ip header (ip_header)?


struct iphdr *ip_header;
struct tcphdr *tcp_header;

// Get the ip header
ip_header = (struct iphdr *)skb_network_header(skb);
skb_set_transport_header(skb, ip_header->ihl * 4);
tcp_header = (struct tcphdr *)skb_transport_header(skb);

How can we point to a specific byte of ip_header?
Is it possible to perform OR bitwise operation on 22st or 21st bytes (option field) with no more variable declaration ?
At the end please suggest me a book for networking programming in kernel(if exists).

any help would be greatly appreciated.
Best Regards.

Originally Posted by kikilinux I believe i can't correct my errors Can anyone help me to have an OR operation on 22st byte of ip header (ip_header)? struct iphdr *ip_header; struct tcphdr *tcp_header; // Get the ip header ip_header = (struct iphdr *)skb_network_header(skb); skb_set_transport_header(skb, ip_header->ihl * 4); tcp_header = (struct tcphdr *)skb_transport_header(skb); How can we point to a specific byte of ip_header? Is it possible to perform OR bitwise operation on 22st or 21st bytes (option field) with no more variable declaration ? At the end please suggest me a book for networking programming in kernel(if exists). any help would be greatly appreciated. Best Regards.

I don't know any network programming books that are good, offhand.

As for the operation...w/o declaring any more variables you have to use a few parenthesis..some of them aren't necessary, but I like to be very clear w/ my intentions:
(Note: For the offsets I treated values as 0 based)

    struct iphdr *ip_header;
    struct tcphdr *tcp_header;

    // Get the ip header
    ip_header = (struct iphdr *)skb_network_header(skb);
    skb_set_transport_header(skb, ip_header->ihl * 4);
    tcp_header = (struct tcphdr *)skb_transport_header(skb);

    // Set 21st byte, bit 1 to 1
    *(((uint8_t*)ip_header) + 21) |= 1;

    // Set 22nd byte, bit 4 to 1
    *(((uint8_t*)ip_header) + 22) |= 1 << 4;

    // Clear 21st byte, bit 3
    *(((uint8_t*)ip_header) + 21) &= ~(1 << 3);

Hope that explains it a little better..it should work .

NOTE: "uint8_t" is guaranteed to be 8 bits, unlike a char.

Yes it worked
thanks to answer.