On Tue, Sep 20, 2022 at 9:58 AM Siddhesh Poyarekar wrote: > Adding support for something that's already declared as bad > programming practice seems like a step backwards. Instead, I hope we > find a way to discourage active use of malloc_usable_size more > strongly. BTW, if folks aren't aware, there is already work on the C++ side to expose an API which lets you request a heap allocation of _at least_ the given size, which rounds the actual size up in whatever way the allocator likes, and returns the pointer and actual size allocated. With this API, you declare an explicit intent that all of the memory -- up to the returned size -- is valid to use without needing to go back to the allocator to ask for more. The proposal is still making its way through the standardization process, but hopefully it'll make it into the next version of C++ after C++23. (Of course, that's not a sure thing until it happens.) Here's the doc, with more rationale/etc: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p0901r9.html Also, as noted in the doc, jemalloc experimentally implemented this functionality in its non-standard API, via a function it called "smallocx" -- though jemalloc hides the API so it can't be used by default. The API is effectively: typedef struct { void *ptr; size_t size; } smallocx_return_t; smallocx_return_t smallocx(size_t size, int flags); https://github.com/jemalloc/jemalloc/blob/a0734fd6ee326cd2059edbe4bca7092988a63684/src/jemalloc.c#L3414 (That's consistent with jemalloc's other non-standard APIs, which stick alignment/etc into a "flags" argument, but probably not suitable for a more-standardized cross-implementation API) tcmalloc implements similar functionality, as well, with family of functions named "tcmalloc_size_returning_operator_new": https://github.com/google/tcmalloc/blob/267aa2ec2817ab9d09b3fbb65ecb90193dd4348e/tcmalloc/malloc_extension.h#L549 which of course also isn't a suitable API to support cross-implementation. If someone wants to push forward this area, IMO, it would be really great to have an API exposing this functionality designed to be implemented in a common way across libc malloc implementations -- and eventually added to POSIX or C.