Typecast struct pointer

[sergiuxxcracker]

Guys I have a small problem could anyone explain please what does the following line does please ?

char packet[65536];
   IPV4_HDR *v4hdr = NULL; //(IPV4_HDR is a structure)
   v4hdr = (IPV4_HDR *)packet; //<- what does this line do ?

[Moschops]

It makes the pointer v4hdr point to the array packet, as if the array was an object of type IPV4_HDR.

[sergiuxxcracker]

Can you please explain a little bit more please ? What do I need this for ? Couldn't I just add the packet[65536] into the structure ?

[Moschops]

Couldn't I just add the packet[65536] into the structure ?

No. The inside of a structure is defined once and once only. Here's an example:

struct breakfast
{
  int eggs;
  int bacon;
}

It's set. You cannot just add things into it mid-program. If you want to change the structure, you have to rewrite it.

What do I need this for ?

It looks like a simple way of setting aside 65536 bytes for use as a IPV4_HDR object. Seems an awfully roundabout way of doing it when you could just make an actual IPV4_HDR like this:

IPV4_HDR aNiceNewObject;

[Lowest0ne]

You have to cast the char* to an IPV4_HDR* in order for it to work syntactically.

I would say those lines are too far out of context to make sense. We can only assume that sizeof(IPV4_HDR) == 65536.

One way to try to put it into context:

class MemAlloc
{
private:
  char* memory;
public:
  MemAlloc(unsigned int size) { memory = new char[size]; }
  ~memAlloc(void) { delete [] memory; }

  char* getMemory(unsigned int amount)
  {
    // Find a free block of "memory" the size of amount
    return thatBlock;
  }
  // A bunch of other functions for handling the "memory" array's blocks
};

int main(void)
{
  MemAlloc myMemoryAllocator(1000000);  // A 1,000,000 size char array is dynamically allocated
  IPV4_HDR *v4hdr = (IPV4_HDR *) myMemoryAllocator.getMemory(sizeof(IPV4_HDR));
  // Do things 

That method was something I picked up in C class, not sure how real-life it is. This way your program can sort of handle it's own memory. Rather than always using new and delete you have your own little block to play with.