In malloc-ng, does realloc to a smaller size return a different pointer? (If you want to be pedantic I am asking about returning a pointer with a different representation, provenance is always different but I'm not asking about that .)
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The question I was really interested in was whether realloc to a smaller size can return NULL, and I thought that the answer to that was so clearly “obviously not” that I didn't dare ask it, but it seems it is a situation that can arise?
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Since even munmap can fail, realloc to a smaller size can fail for large allocations in any allocator. It's up to them if they report it, but I expect most report it.
Most modern allocators use slab allocation. If the size class changes, they need to allocate and copy to that.
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I wonder how many C developers believe that realloc to a smaller size will never fail. I was one of them this morning. (I do check the result, but it is in preparing the explanation “yeah it's boilerplate but you know…” to go with it that I ended up wondering about it.)
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It can fail even with dlmalloc/glibc if you go from an allocation they manage as a separate region to a smaller size they're willing to manage via free lists within regions. They still essentially have 2 types of size classes and glibc determines the approach dynamically.
The dlmalloc approach is that most sizes are all mixed together and power of two size class buckets are used to do approximate best-fit with low allocator overhead. It can shrink allocations in-place within those regions. A more modern allocator generally can't do that though.
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The jemalloc approach is to use a certain number of size classes per doubling of size. It puts a low bound on space wasted from size class rounding.
It also means you can only have 4 actual reallocations to double in size even if the calling code keeps incrementing by 1 element.
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