nx_tar_header.nx

tar_header.nx -- parse POSIX ustar tar headers (IEEE 1003.1). Each tar entry is a 512-byte header followed by its file data padded to a 512-byte boundary. The archive ends with at least two zero-filled 512-byte blocks. Header layout: offset size field 0 100 name (NUL-terminated if shorter) 100 8 mode (octal ASCII) 108 8 uid (octal ASCII) 116 8 gid (octal ASCII) 124 12 size (octal ASCII, bytes of file data) 136 12 mtime (octal ASCII seconds since epoch) 148 8 header checksum (octal ASCII) 156 1 typeflag ('0'=regular, '5'=dir, '2'=symlink, '1'=hardlink, 'x'=pax extended, 'g'=pax global) 157 100 linkname 257 6 magic "ustar\0" (POSIX) or "ustar " (GNU) 263 2 version "00" (POSIX) or " \0" (GNU) 265 32 uname (owner name) 297 32 gname (group name) 329 8 devmajor (octal) 337 8 devminor (octal) 345 155 prefix (prepended to name for long paths) Invariants: T1 Magic must be "ustar" -- the first 5 bytes at offset 257. T2 Size fields are parsed as big-endian octal ASCII. GNU tar optionally encodes large sizes as binary with MSB=0x80; we reject those (caller can detect and extend). T3 Checksum verification not performed here (future work); we parse the field value but don't validate.