.TAR File Format – End of archive

Padding file data

A file’s data starts in the block after its header and fills as many blocks as it needs. The last block is padded with zeros, so a file of n bytes takes n rounded up to the next multiple of 512. The next header always starts on a block boundary, and a reader finds it by rounding up the size in the header.

The two zero blocks

After the last entry come two blocks of 512 zero bytes. A reader stops at the first block that is all zeros, since a real header never is; its checksum would fail. GNU tar warns about “a lone zero block” when the first zero block is not followed by a second.

For hello.txt, the 13-byte file used on the header block page, the archive written by bsdtar is four blocks long:

Records and the blocking factor

Tape drives work best with large writes, so tar writes its blocks in groups called records. The number of blocks in a record is the blocking factor, and the default is 20, giving records of 10,240 bytes. The final record is filled with zeros after the two end blocks. This is why a tar file made by GNU tar is always a multiple of 10,240 bytes, so the one-file archive above would be 10,240 bytes long rather than 2,048. Writers that do not pad to a record, such as bsdtar writing to a disk file here, stop after the two zero blocks.

Appending and joining

To add entries to an archive, tar overwrites the two zero blocks with the new headers and writes new zero blocks after them. Joining two archives with a plain file copy leaves the first archive’s zero blocks in the middle, so most readers stop there. GNU tar’s --ignore-zeros option reads past them to the entries beyond.

A compressed tar file such as .tar.gz compresses the whole stream, padding and end blocks included. Runs of zeros compress to almost nothing, so the padding costs very little.