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csocketspointersstructraw-sockets

Type casting struct pointer

I'm trying to get into Socket programming and came across an article at https://www.tenouk.com/Module43a.html I'm having difficulty understanding as how a char array is cast into struct pointer

char buffer[PCKT_LEN];
struct ipheader *ip = (struct ipheader *) buffer;
//some code here
ip->iph_ihl = 5;
ip->iph_ver = 4;
ip->iph_tos = 16;

As per my understanding, pointer ip will now hold the address of buffer and values for members of struct ipheader will now be stored in buffer. Please help understanding the same. If I'm right, then how would we be able to print values stored in buffer?

Solution

  • You understanding is correct. The pointer *ip will point to buffer. char buffer[PCKT_LEN] is an array of size sizeof(char) * PCKT_LEN. Since a char is usually 1 byte long it is just a chunk of memory of PCKT_LEN bytes. PCKT_LEN is defined to be 8192

    The amount of bytes needed to store a struct ipheader is much less than this. Try int a = sizeof(ipheader) and use a debugger to see the value assigned to a. For me it is 24 bytes, but it could be slightly different for you. This means that buffer can hold much more data than the struct ipheader needs. I haven't looked to deeply into the code, and I don't know much about socket programming. But one use for this could be to augment buffer with additional data outside of the struct. Since you know struct ipheader takes up sizeof(ipheader) bytes you will have sizeof(char)*8192 - sizeof(ipheader) left to augment the array.

    Edit: Upun further inspection, this is kinda what is happening:

    struct ipheader *ip = (struct ipheader *) buffer;
struct udpheader *udp = (struct udpheader *) (buffer + sizeof(struct ipheader));

    It tries to store the ip header at the beginning of the buffer, then it augments that same buffer with an udp header. By using buffer + sizeof(struct ipheader) it makes sure that it stores the udp header after ipheader by offsetting buffer by sizeof(struct ipheader) bytes. Basically struct ipheader *ip points to the beginning of the buffer and struct udpheader *udp points to buffer + sizeof(struct ipheader). I hope this makes sense. Obviously there is still a lot of space left over in buffer so you could potentially augment it even further.