Pointer arithmetic question

**moneil** · 03-25-2004

I need a little help with manipulating pointers. Here is the problem. I have two pointers declared as follows:

Header *h1, *h2;

Header is a struct in the program I'm working on. At this point in my program h1 already points to a location in memory and h2 is currently null. Now I want to make h2 point to a particular amount of bytes past h1 in memory. However, this displacement past h1 is not an integer multiple of the amount of bytes of a Header struct. For example, I need h2 to point to exactly 32 bytes past h1, but Headers are 12 bytes, so I cannot use regular pointer arithmetic (e.g. h2 = h1 + 3 would make h2 point to 36 bytes past h1, but that's the closest I can get to 32).

Does anyone have any ideas? Thanks for any possible input or advice.

**linuxdude** · 03-25-2004

will typecasting work? Why do you need this? If it is so the structs will be next to eachother remember that a structure has padding

**Dave_Sinkula** · 03-25-2004

>Does anyone have any ideas?

With caution flags...

Code:

#include <stdio.h>

struct sType
{
   char element[12];
};

int main(void)
{
   struct sType object = {0};
   struct sType *h1 = &object;
   struct sType *h2 = (char*)h1 + 32;
   printf("&object = %p, h1 = %p, h2 = %p\n",
          (void*)&object, (void*)h1, (void*)h2);
   return 0;
}

/* my output
&object = 0012FF80, h1 = 0012FF80, h2 = 0012FFA0
*/

**moneil** · 03-25-2004

The program is supposed to be a memory management program.

h1 points to a header. Directly following that header is a free chunk of memory that a user requested to use which I allocated for him.

The entire thing (header + memory requested) is 2^k bytes where k is some integer. I need to split that whole thing into halves. So h1 will still point to the same place, but h2 will now point to the middle of that 2^k block, and the whole block has essentially become two smallers ones which I can now use.

So say the header + free memory for the user is 2^5 = 32 bytes. What I want to do is have h2 point to 2^(5 - 1) = 16 bytes past where h1 points to. Then h1 and h2 would each point to 16 byte blocks of memory, the beginnign 12 of each which would be a header struct. The remaining 4 bytes each would be for the user who requested the memory.

I hope I explained that well enough.

Once I point h2 to that right location I will type-cast it as a Header pointer but how do I get h2 to point to that location?

I'm thinking something along the lines of:

h2 = (Header *)(&h1 + #bytes);

or something to that effect, but I'm not quite sure. Thanks.

**moneil** · 03-25-2004

Thanks Dave...i'm gonna give that a whirl.

When adding 32 does that add 32 bytes or 32 bytes * sizeOf(char)?

**Dave_Sinkula** · 03-25-2004

>When adding 32 does that add 32 bytes or 32 bytes * sizeOf(char)?

Yes. (Since sizeof(char) is 1 by definition, both would be the same.)

**moneil** · 03-25-2004

Yes I just realized that! Thanks for the help!

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