在对recvfrom（）的调用中使用了哪种字节序来存储信息

Question

Here is a simple code in socket API:

sock_raw = socket(AF_PACKET,SOCK_RAW,htons(ETH_P_ALL));

//...

data_size = recvfrom(sock_raw , buffer , bufsize , 0 , (struct sockaddr*)&saddr , (socklen_t*)&saddr_size);

//...

void print_ethernet_header(unsigned char* buffer) { struct ethhdr *eth = (struct ethhdr *)buffer; fprintf(logfile , " |-Protocol : %x \\n",ntohs(eth->h_proto)); }

In this question I was explained that h_proto member of struct ethhdr is stored in big-endian format. Is it valid to say that the other members of this structure( h_source and h_dest are as well stored in big-endian format? And in such case is it valid to say that recvfrom() function stores data to buffer in big-endian format (I failed to find any info about this in man page to this function)? I think that this should be true because of rfc1042 , but I am not sure.

Answer 1

recvfrom reads data from a socket and stores it in a buffer, but it doesn't process the data: it stores it as it's received. The order of values in the data depends on the network protocol. Most network protocols use big-endian (“network order”), so the data provided by recvfrom is typically big-endian — but that's not a result of recvfrom .

In most cases you should use ntohs and ntohl to translate values as appropriate, and process arrays in the documented order. For h_source and h_dest , the documented order is indeed big-endian. If you're writing kernel code, there are usually macros or helper functions available to do this for you; even printk has %pM format specifiers to handle MAC addresses.