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README.md: update example and add explanation 

Add an explanation of the readings that happen on the tar archive stream

Fixes #3
This commit is contained in:
Vincent Batts 2015-03-10 11:19:50 -04:00
parent 5c79f14ea5
commit 402c6217ac
1 changed files with 31 additions and 32 deletions
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README.md
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@ -72,40 +72,39 @@ Now run the example over the archive:
```
$ ./main tar-split.tar
2015/02/20 15:00:58 writing "tar-split.tar" to "tar-split.tar.out"
pax_global_header pre: 512 read: 52 post: 0
LICENSE pre: 972 read: 1075 post: 0
README.md pre: 973 read: 1004 post: 0
archive/ pre: 532 read: 0 post: 0
archive/tar/ pre: 512 read: 0 post: 0
archive/tar/common.go pre: 512 read: 7790 post: 0
archive/tar/example_test.go pre: 914 read: 1659 post: 0
archive/tar/reader.go pre: 901 read: 25303 post: 0
archive/tar/reader_test.go pre: 809 read: 17513 post: 0
archive/tar/stat_atim.go pre: 919 read: 414 post: 0
archive/tar/stat_atimespec.go pre: 610 read: 414 post: 0
archive/tar/stat_unix.go pre: 610 read: 716 post: 0
archive/tar/tar_test.go pre: 820 read: 6673 post: 0
archive/tar/testdata/ pre: 1007 read: 0 post: 0
archive/tar/testdata/gnu.tar pre: 512 read: 3072 post: 0
archive/tar/testdata/nil-uid.tar pre: 512 read: 1024 post: 0
archive/tar/testdata/pax.tar pre: 512 read: 10240 post: 0
archive/tar/testdata/small.txt pre: 512 read: 5 post: 0
archive/tar/testdata/small2.txt pre: 1019 read: 11 post: 0
archive/tar/testdata/sparse-formats.tar pre: 1013 read: 17920 post: 0
archive/tar/testdata/star.tar pre: 512 read: 3072 post: 0
archive/tar/testdata/ustar.tar pre: 512 read: 2048 post: 0
archive/tar/testdata/v7.tar pre: 512 read: 3584 post: 0
archive/tar/testdata/writer-big-long.tar pre: 512 read: 4096 post: 0
archive/tar/testdata/writer-big.tar pre: 512 read: 4096 post: 0
archive/tar/testdata/writer.tar pre: 512 read: 3584 post: 0
archive/tar/testdata/xattrs.tar pre: 512 read: 5120 post: 0
archive/tar/writer.go pre: 512 read: 11867 post: 0
archive/tar/writer_test.go pre: 933 read: 12436 post: 0
main.go pre: 876 read: 1568 post: 0
old.go pre: 992 read: 4918 post: 0
Size: 174080; Sum: 174080
pax_global_header pre: 512 read: 52
.travis.yml pre: 972 read: 374
DESIGN.md pre: 650 read: 1131
LICENSE pre: 917 read: 1075
README.md pre: 973 read: 4289
archive/ pre: 831 read: 0
archive/tar/ pre: 512 read: 0
archive/tar/common.go pre: 512 read: 7790
[...]
tar/storage/entry_test.go pre: 667 read: 1137
tar/storage/getter.go pre: 911 read: 2741
tar/storage/getter_test.go pre: 843 read: 1491
tar/storage/packer.go pre: 557 read: 3141
tar/storage/packer_test.go pre: 955 read: 3096
EOF padding: 1512
Remainder: 512
Size: 215040; Sum: 215040
```
*What are we seeing here?*
* `pre` is the header of a file entry, and potentially the padding from the
end of the prior file's payload. Also with particular tar extensions and pax
attributes, the header can exceed 512 bytes.
* `read` is the size of the file payload from the entry
* `EOF padding` is the expected 1024 null bytes on the end of a tar archive,
plus potential padding from the end of the prior file entry's payload
* `Remainder` is the remaining bytes of an archive. This is typically deadspace
as most tar implmentations will return after having reached the end of the
1024 null bytes. Though various implementations will include some amount of
bytes here, which will affect the checksum of the resulting tar archive,
therefore this must be accounted for as well.
Ideally the input tar and output `*.out`, will match:
```