The C Preprocessor Constants in this section are defined in wbdefs.h which is derived from wbdefs.scm.
Each segment (file-based or not) maintains a freelist of disk blocks. In a newly created segment, nearly all of its block numbers are in the freelist.
flc_len must be larger than 2 times the maximum number of
blocks which would ever be needed for a freelist split. The minimum
flc_len is 10.