malloc_usable_size - obtain size of block of memory allocated from heap

       For an explanation of the terms used in this section, see attributes(7).
       ┌─────────────────────────────────────────────────────────────────────────────┬───────────────┬─────────┐
       │ Interface                                                                   │ Attribute     │ Value   │
       ├─────────────────────────────────────────────────────────────────────────────┼───────────────┼─────────┤
       │ malloc_usable_size()                                                        │ Thread safety │ MT-Safe │
       └─────────────────────────────────────────────────────────────────────────────┴───────────────┴─────────┘

       The value returned by malloc_usable_size() may be greater than  the  requested  size  of  the  allocation
       because  of  various  internal implementation details, none of which the programmer should rely on.  This
       function is intended to only be used for diagnostics and statistics; writing to the excess memory without
       first calling realloc(3) to resize the allocation is not supported.  The returned value is only valid  at
       the time of the call.

       This  function  can  be  used for diagnostics or statistics about allocations from malloc(3) or a related
       function.

       Standard C library (libc, -lc)

       malloc_usable_size - obtain size of block of memory allocated from heap

malloc_usable_size() returns a value no less than the size of the block of allocated memory pointed to by
       ptr.  If ptr is NULL, 0 is returned.

malloc(3)

Linux man-pages 6.9.1                              2024-05-02                              malloc_usable_size(3)

       GNU.

#include<malloc.h>size_tmalloc_usable_size(void*_Nullableptr);