In the code here, there is a line:
struct iphdr * iph = (struct iphdr *)buffer;
in ProcessPacket function where buffer is of type char*. buffer has been given value by recvfrom in the main function. How is the simple string (buffer) converted to a structure and how is the data safely extracted?
iphdr:
struct iphdr {
#if defined(__LITTLE_ENDIAN_BITFIELD)
__u8 ihl:4,
version:4;
#elif defined (__BIG_ENDIAN_BITFIELD)
__u8 version:4,
ihl:4;
#else
#error "Please fix <asm/byteorder.h>"
#endif
__u8 tos;
__u16 tot_len;
__u16 id;
__u16 frag_off;
__u8 ttl;
__u8 protocol;
__u16 check;
__u32 saddr;
__u32 daddr;
/*The options start here. */
};
The first thing to understand is that the bits on memory stay exactly the same irrespective of the cast (struct iphdr *). Just that you are now saying that buffer is now to be treated as a pointer to struct iphdr instead of what it was before. You are just telling the compiler to look at the bits with a different pair of glasses and hence interpret accordingly. The compiler suddenly sees that buffer has become a struct iphdr *. And says "OK" that's all. What's important is you know exactly what buffer is and cast it to the proper type.
If you wanted, you could have type-casted buffer to int * (or any other pointer type) and the compiler would have said nothing. Although you would have problems later on.
Collected from the Internet
Please contact [email protected] to delete if infringement.
Comments