How to change saved address after reallocation

Question

struct AD_SINT32Type* = NULL;
foo = (struct mystructArray*)malloc(sizeof(struct mystructArray));
foo[0].x = 45;
foo[0].y = 90;
coords[0] = &foo[0];


foo = (struct mystructArray*)realloc(foo, 2 * sizeof(struct mystructArray));
foo[1].x = 30;
foo[1].y = 15;
coords[1] = &foo[1];

After this code "coords[1]" points as intended, but "coords[0]" points to an the old address before the reallocation. Is there a way to automatic adapt the address "coords[0]" points to?

Welcome to Stack Overflow! [Please see this discussion on why not to cast the return value of `malloc()` and family in `C`.](http://stackoverflow.com/q/605845/2173917). — Sourav Ghosh, Dec 14 '15 at 14:18
Note that your code is unable to handle errors from `realloc`. — Kerrek SB, Dec 14 '15 at 14:19

John Zwinck · Accepted Answer · 2015-12-14T16:09:27.263

3

There is no fully "automatic" way to do this, but in cases like this where reallocation is needed, you often see the use of "offset pointers." So instead of this:

coords[0] = &foo[0];

You'd change the type of coords to something like ptrdiff_t[] and do this:

coords[0] = &foo[0] - foo;

This way, you save not the actual pointer, but the offset from the beginning of the allocation. And that value will never need to change.

edited Dec 14 '15 at 16:09

answered Dec 14 '15 at 14:21

John Zwinck

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The difference of two pointers yields a `ptrdiff_t`. Any other type might loose information. On 64 bit architectures (P64), an `uint32_t` **will** loose information. – too honest for this site Dec 14 '15 at 14:48
@Olaf: I have updated my answer to use `ptrdiff_t`. At least now nobody can complain that I've done something dirty. On 64-bit you will only lose information storing into a 32-bit int if you have a huge array. Just so we're clear. – John Zwinck Dec 14 '15 at 16:10
"something like" still is suspectible imo. `ptrdiff_t` is exactly designed for such. However, as the difference will always be `>= 0` and you actually calculate an index, `size_t` would also be acceptable. – too honest for this site Dec 14 '15 at 16:23

score 1 · Answer 2 · edited May 23 '17 at 11:44

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Is there a way to automatic adapt the address "coords[0]" points to?

No.

See How to update other pointers when realloc moves the memory block?

edited May 23 '17 at 11:44

Community

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answered Dec 14 '15 at 14:21

Andrew Henle

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score 0 · Answer 3 · answered Dec 14 '15 at 14:22

Is there a way to automatic adapt the address "coords[0]" points to?

No. C does not keep track where you store addresses. If you need this, it's your job to keep track.

Another idea is to think outside the box:

can you restructure your data in a way you don't need to store more than one pointer to the start of the dynamic array?
can you work with offsets instead?

score 0 · Answer 4 · answered Dec 14 '15 at 14:55

Just make coords a size_t [] (this is the correct type) and store indexes into foo instead of absolute addresses.

Alternatively, after a realloc of foo, you can update all coords entries in a loop.

But the first version migh not be much slower than using addresses, you benchmark before going complicated.

How to change saved address after reallocation

4 Answers4