Adding a pointer to size_t in C89

Question

First, I know about this thread:

Now Assume I have this array:

size_t    count = some_value();
struct info    *pinfos = malloc(count * sizeof(struct info));

and I wanted to "navigate" it using the variable

struct info     *p;

Can I use this code to check the limit?

p   <   (char*)pinfos  +  count*sizeof(struct info)  -  1

Is this legal in C89?

Is there a more "appropriate" way to do so?

You do not need to fumble with a cast to `char*`. Pointer arithmetic works the same way as array indexing. The next element is `1` further along, regardless of the size of each element. — Weather Vane, Aug 05 '17 at 16:40
`info` is not defined, as in `sizeof(info)`. You defined `struct info`. — Weather Vane, Aug 05 '17 at 16:41
This is one reason why we like to see the [Minimal, Complete, and Verifiable example](http://stackoverflow.com/help/mcve) that shows the problem. — Weather Vane, Aug 05 '17 at 16:43
Then it would not be "Minimal". Typos and "something like my code" are not good in questions. They make them hard to answer. — Weather Vane, Aug 05 '17 at 16:45

score 3 · Accepted Answer · answered Aug 05 '17 at 16:37

3

You can do that indeed but you are overcomplicating it, just do

p < pinfos + count - 1

and it will work because pointer arithmetic is performed in terms of the size of the pointer type.

answered Aug 05 '17 at 16:37

Iharob Al Asimi

Why the -1 ?? You're checking too short. – Michaël Roy Aug 05 '17 at 16:38
@MichaëlRoy because p will be incremented later on – Bite Bytes Aug 05 '17 at 16:39
Does this mean I can add (unsigned) integers (size_t in this case) to pointers freely? – Bite Bytes Aug 05 '17 at 16:47
@BiteBytes Indeed you can, but be careful with UB. – Iharob Al Asimi Aug 05 '17 at 16:59

1 Answers1