Casting and Pointers

**kpreston** · 08-27-2008

One of the more difficult concepts to grasp, for me, are pointers and type casting. For instance, I don't know WHY it's possible to cast struct sockaddr_in to struct sockaddr when working with winsock, other than because I'm told I have to.

If possible, can someone break down the next few lines of code to me in lamemen's (sp?) terms for me? I'll attempt to, but I'll no doubt be way off...

hostent* localHost; //LINE 1
char* localIP; //LINE 2
localHost = gethostbyname(""); //LINE 3
localIP = inet_ntoa (*(struct in_addr *)*localHost->h_addr_list); //LINE 4

LINE 1: declares a null pointer of type hostent
LINE 2: delcares a null pointer of type char
LINE 3: localHost now points to an address in memory... somewhere.
LINE 4: Totally lost. Dereferencing localHost.h_addr_list[0], casting it to struct in_addr *, then dereferencing that again? Why not (struct in_addr)*localHost->h_addr_list other than because it won't work?

**noops** · 08-27-2008

What is h_addr_list?

The only time I've done something like this in my short history with C is when I used void pointers like:

Code:

int n = 8;
void* data = &n;
void** ptr = &data;
printf("&#37;d", *(int *)*ptr);

So maybe h_addr_list dereferences to a void pointer. Just my 2 cents, let's see what someone more experienced says.

edit: the reason for the cast is because you can't (or shouldn't?) dereference a void pointer (unless it dereferences to a pointer?).

**cpjust** · 08-27-2008

Wrong. None of those point to a NULL pointer because they aren't set to NULL.

1. Declares a pointer to a hostent type. Since it's no initialized, it points to some random garbage value.
2. Declares a pointer to a char string. Again, pointing to garbage.
3. Returns a pointer to a hostent type which points to a static variable (or maybe global) that is inside the gethostbyname() function.
4. Breaking it down step by step:
*localHost->h_addr_list
De-references the h_addr_list member.

*(struct in_addr *)
Casts the first element of the h_addr_list into a in_addr pointer, then de-references it.

localIP = inet_ntoa( ... )
Calls the inet_ntoa() function and assigns the result to localIP.

Why not (struct in_addr)*localHost->h_addr_list other than because it won't work?

Because h_addr_list is a char** (i.e. a pointer to a pointer), not a char*, so you need to de-reference it twice.

**kpreston** · 08-27-2008

Originally Posted by cpjust

Wrong. None of those point to a NULL pointer because they aren't set to NULL.

1. Declares a pointer to a hostent type. Since it's no initialized, it points to some random garbage value.
2. Declares a pointer to a char string. Again, pointing to garbage.

Thank you for correcting my assumtion.

*(struct in_addr *)
Because h_addr_list is a char** (i.e. a pointer to a pointer), not a char*, so you need to de-reference it twice.

That was exactly the answer I was looking for. Thank you for your quick response. Would (struct in_addr)*localHost->h_addr_list be correct if it was a char *?

**cpjust** · 08-27-2008

Originally Posted by kpreston

Thank you for correcting my assumtion.

That was exactly the answer I was looking for. Thank you for your quick response. Would (struct in_addr)*localHost->h_addr_list be correct if it was a char *?

I think in that case you'd need to use this:

Code:

*(struct in_addr*)localHost->h_addr_list

But maybe it would work the other way too, I'm not 100% sure? If it was something other than char* then you'd definitely need to cast it to a in_addr* first, and then de-reference it.

**kpreston** · 08-27-2008

Originally Posted by cpjust

If it was something other than char* then you'd definitely need to cast it to a in_addr* first, and then de-reference it.

Why?

**Elysia** · 08-28-2008

Originally Posted by cpjust

But maybe it would work the other way too, I'm not 100% sure?

I don't think it would work. Granted, I don't know C, but in C++, it would simply complain that it can't convert char to struct in_addr.
When we dereference it, we get a single char and all other information is lost anyway, so even if it works, it would be a bad idea™.

Originally Posted by kpreston

Why?

OK the thing here is that in C (and C++), everything stored in memory is stored in 0s and 1s. The type of the variable you use determine how that data is interpreted and used.
That's why such a thing is possible. Also in C, because it doesn't really support things as templates, usually functions that might expect several types only takes ONE type and from certain conditions and data is determines the actual type of the data passed into it. I believe this is the case with winsock.
So you just have to cast it to the correct type so the function can use the information correctly.

**QuantumPete** · 08-28-2008

Originally Posted by kpreston

lamemen's (sp?) terms

That's "layman's terms"

QuantumPete

**cpjust** · 08-28-2008

Originally Posted by QuantumPete

That's "layman's terms"

QuantumPete

LOL!!

**cpjust** · 08-28-2008

Originally Posted by Elysia

I don't think it would work. Granted, I don't know C, but in C++, it would simply complain that it can't convert char to struct in_addr.
When we dereference it, we get a single char and all other information is lost anyway, so even if it works, it would be a bad idea™.

I was thinking it needed to be cast to a pointer first because of alignment issues, but since char is 1 byte, maybe it wouldn't have the same issue as int or anything else? But either way, it's probably better to always cast to the right pointer type first, then de-reference it.

**kpreston** · 08-28-2008

Thank you all (even you, QuantumPete).

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