Is there ever a situation (with modern mallocs) where realloc() does something that you can’t just do with malloc_size()? It really seems like an attractive nuisance at this point.
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realloc can grow an allocated block into free space available after it, whereas malloc_size will only tell you the size of the allocated block, right? (Or are you saying modern mallocs don't actually do this?)
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Modern mallocs bucket by size, so are rarely going to be able to expand except into the rest of the current bucket, and malloc_size tells you how much room you have there.
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Performance-oriented allocators like jemalloc and many others manage the memory for large allocations beyond their slab allocation size classes. They can still do real in-place growth. Being able to take advantage of mremap to move pages rather than using memcpy also helps a lot.
Using mremap is particularly helpful when combined with larger page sizes. Transparent huge pages combined with the allocator aligning to the huge page size makes mremap extremely advantageous. There's per-process global locking for mremap but at huge sizes it's a major win.
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The malloc_object_size API is non-standard and is seriously flawed. It's undefined behavior to use more of the allocation than you requested. It's a real problem with both Clang and GCC due to the alloc_size attributes on the malloc API. It's broken to use more of the allocation.
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