11.6 Allocation from Frob Blocks

The uninitialized memory required by a Lisp_Object of a particular type is allocated using ALLOCATE_FIXED_TYPE(). This only occurs inside of the lowest-level object-creating functions in alloc.c: Fcons(), make_float(), Fmake_byte_code(), Fmake_symbol(), allocate_extent(), allocate_event(), Fmake_marker(), and make_uninit_string(). The idea is that, for each type, there are a number of frob blocks (each 2K in size); each frob block is divided up into object-sized chunks. Each frob block will have some of these chunks that are currently assigned to objects, and perhaps some that are free. (If a frob block has nothing but free chunks, it is freed at the end of the garbage collection cycle.) The free chunks are stored in a free list, which is chained by storing a pointer in the first four bytes of the chunk. (Except for the free chunks at the end of the last frob block, which are handled using an index which points past the end of the last-allocated chunk in the last frob block.) ALLOCATE_FIXED_TYPE() first tries to retrieve a chunk from the free list; if that fails, it calls ALLOCATE_FIXED_TYPE_FROM_BLOCK(), which looks at the end of the last frob block for space, and creates a new frob block if there is none. (There are actually two versions of these macros, one of which is more defensive but less efficient and is used for error-checking.)