cranium 2003 wrote:
You dont see any difference here since , ETH_ALEN is defined as 6, so 6+6 = 12 , that is a word boundary and then h_proto is alligned from that address , try adding a char in between and then see the difference with and withoutHello Mandeep, On Mon, 31 Jan 2005 16:29:52 +0530, Mandeep Sandhu <Mandeep_Sandhu@xxxxxxxxxxx> wrote:
On Mon, 2005-01-31 at 16:20 +0530, cranium 2003 wrote:
Hello, In linux kernel source etheernet header is defined as structure as struct ethhdr { unsigned char h_dest[ETH_ALEN]; /* destination eth addr */ unsigned char h_source[ETH_ALEN]; /* source ether addr */ unsigned short h_proto; /* packet type ID field */ } __attribute__((packed)); I want to know what __attribute__((packed)) meance? It is not looking as a structure object then what is thati?
Your ethernet header is 14 bytes long so the "packed" attribute tells the compiler to to pack the structure to 14 bytes and not try to byte align it on a word boundary (as it would do without specifying this keyword).
Why its necessary to tell compiler to consider it as 14 bytes? becasue sizeof(struct ethhdr) is 14 bytes so while putting ethhdr in packet 14 bytes are always put so why question arises to have word boundry alignment? regards, cranium
__attribute__((packed));
with __attribute__ tag you wud get -> 15; without __attribute__ tag u get -> 16
struct ethhdr
{
unsigned char h_dest[ETH_ALEN]; /* destination eth addr */
unsigned char h_source[ETH_ALEN]; /* source ether addr */
char c; /* added for testing */
unsigned short h_proto; /* packet type ID field */
} __attribute__((packed));on some archietectures unalligned memory access will not give you the right values for word's.
hth, Cheers, Amith
-- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
