realloc without moving?

[Wolfram Gloger]

chris wrote:
> 
> > Is there a function in glibc like realloc, except in the case where it
> > is not possible to extend the existing memory then instead of allocating
> > a new block of memory and copying the old block into this area the
> > function simply returns?

Uli wrote:

> There is not code like this.  You can query in glibc's implementation
> the actual maximum block size with malloc_usable_size() although I don't
> know whether this works in all situations.  It simply never has been
> used much.

I can confirm that malloc_usable_size() is supposed to work in all
situations -- if it wouldn't work I'd consider it a bug.

However, I think malloc_usable_size will not help that much for the
realloc() decision described above.  malloc_usable_size is usually not
much larger than the chunk size originally requested (typically just a
few bytes), and it definitely does not account for a following,
available chunk.  So, despite malloc_usable_size telling you "not
enough memory in this chunk available", a realloc to some larger size
might actually succeed anyway, without changing the chunk address.

To Chris: What exactly are your requirements?  You are aware that
glibc on Linux also has efficient realloc for very large chunks thanks
to the mremap() system call?

Regards,
Wolfram.