[musl] Re: realloci(): A realloc() variant that works in-place

[Paul Eggert]

On 10/31/25 10:22, Alejandro Colomar wrote:
> Paul, for context, this is a discussion for adding a function
> 
> 	int realloci(void *p, size_t n);
> 
> that changes the size of a memory block without moving it.  (And thus,
> fails rather often, for some implementations of allocators.)

Reading the threads leading into this, the motivation for this seems to 
be C++ and similar memory allocators that want a cheap way to grow an 
object - if the object doesn't move they can skip some reinitialization 
work, otherwise they have more work to do.

With that in mind, the proposed API is not the best way to go about the 
problem. What these users want is a function that acts just like 
R=realloc(P,N) EXCEPT that it lets you compare R==P, and if the two 
values are the same pointer you know the object did not move and you can 
skip some work. This is simpler than realloci because it means that you 
need only one call (not two) in the common case when realloci returns 
the null pointer.

In other words, these uses want the realloc function the way it was in 
7th Edition Unix, before sanitizers got in the way and insisted that 
it's an error to compare realloc's first argument with its result even 
if they happen to have the same value.

There's an easy way to change the C standard to support these uses: just 
change the spec for realloc to support this usage. There is no need to 
change the C library, or musl, or any of the commonly used production C 
libraries. The only change you'd need to make is to the C standard and 
to picky sanitizers.

This would be *much* better than adding a new, hard-to-explain realloci API.