nx_tar.nx

nx_tar.nx -- USTAR archive: a COMPLETE format, reader and writer. The tree had nx_tar_header.nx -- a header STRUCT and nothing that could read or write an archive. This finishes it: build a multi-file archive, walk it back, and recover every name, size and byte. Symmetric, so it is provable by exact round-trip rather than against hand-picked constants. tar is unchanged since 1988 and is still the packaging substrate for everything Unix -- and, with the existing gzip, it is the .tar.gz path. THE CHECKSUM'S SELF-REFERENCE. The header checksum is the sum of all 512 header bytes WITH THE CHECKSUM FIELD ITSELF TREATED AS EIGHT SPACES. Both writer and reader must substitute spaces; a reader that sums the stored digits instead rejects every valid archive, and a writer that sums zeros produces archives GNU tar refuses. OCTAL, NOT BINARY. Every numeric field is ASCII octal, right-aligned and zero-padded, terminated by NUL. Writing them as binary integers produces a header that looks fine in a hex dump and is unreadable by every tar on earth. THE TWO-BLOCK TERMINATOR. An archive ends with 1024 zero bytes. One block is not enough -- readers scan for two consecutive zero blocks, and a single-block ending is treated as a truncated archive. genealogy_id: posix_ustar_1988 lineage_id: nx_tar_v1 license_tier: ORIGINAL