This is a modern refactoring of the simple "storage allocator" found in the classic K&R TCPL book.

It retains the simplicity and efficiency of the original, adding a few improvements :

• Localised, encapsulated state :

  The K&R allocator uses a global freelist, so there is one global allocator. Instead, we load/store the freelist and other state in struct allocators, so there may be any number of independent, scope-bound allocator objects.

• Custom memory pools :

  Where the original obtains memory from the OS through a syscall, here the user adds memory region(s) to an allocator object via allocator_add().

• Safe, correct & portable :

  This allocator is thread-safe using lightweight syncronisation techniques - spinlocks composed of C11 atomics.

  It also checks for integer overflow and treats this as an allocation error and handles alignment in a machine-independant way. In fact, the library is virtually freestanding.

• Queryable :

  Like most modern allocators, this one too can query the real size of an allocation, with allocator_allocsz(). It also allows for querying each block's size in the freelist using allocator_for_blocks(). The K&R allocator is, in contrast, entirely opaque.