NAME

NOTES

Data Types

Integer

every offset or length(len) value in this documentation will be an unsigned 32bit integer in big endian byte order(32bit unsigned big int or uint32(big) ).

String

All strings, mentioned in this documentation are ASCII encoded, and not nullterminated

Values

The actual values of the individual xpak entries are stored as Strings.

Vertical Bars

SYNOPSIS

tarball

     |<-xpak_offset->|
<tar>|< xpak >|<xpak_offset>"STOP"

xpak

"XPAKPACK"<index_len><data_len><index><data>"XPAKSTOP"

index

|<-------------index_len------------->|
|<index1><index2><index3><index4><...>|

indexN

          |<-name_len->|
<name_len>|< name >|<data_offset><data_len>

data

|<--------------data_len------------->|
|<-dataN_offset->|<-dataN_len->|
|< data >|< data_N >|<data>|

DETAILED

xpak

If you look at a gentoo binary package (binpkg) with a hex-editor you'll notice the behinf the data, which belongs to the tarball you find a binary blob - the xpak , an offset which holds the bytes from the start of the xpak to the end of the file - xpak_offset and finally the String STOP.

       |<xpak_offset>|
 <tbz2>|<---xpak---->|<xpak_offset>"STOP"|

Here you see the tbz2 archive, and the attached xpak blob, the xpak-offset and the string STOP at the end.

If we read the offset value and count offset bytes backwards from the start of xpak_offset , we have found the start of the xpak Block which starts with the String XPAKPACK. This xpak block consists of the string XPAKPACK , the length of the index block - index-len , the length of the data block - data-len , an index-len bytes long binary blob with the index , a data-len bytes long binary blob with the data and the string XPAKSTOP at the end:

                                |<index_len>|<data_len>|
 "XPAKPACK"<index_len><data_len>|<--index-->|<--data-->|"XPAKSTOP"

To actually get the index and the data , we cut out index_len bytes after the end of data_len
 for the index block and then cut out the next  data_len bytes for the data block. If we have done everything right up to this point, the following bytes would be the ASCII formatted string XPAKSTOP

The actual data is truncated into one big block - so if we want to read it we need the actual positions of each information in this big data block, this information can be obtained using the indices which are stored in the index block.

Index block

 |<-----------------------index_len---------------------->|
 |<index1><index2><index3><index4><index5><index6><index7>|

The index block holds all information we need to find the data we want in the data block. It consists of truncated index elements with a length index_len. Each of those index elements stands for one information in the data block and consists of the length of its name ( name_len> ), a name_len
 bytes long string (the Name of the data block), this index belongs to, the offset of the data block ( data_offset ) and the length of that data block ( data_len ):

           |<name_len>|
 <name_len>|<--name-->|<dataN_offset><dataN_len>

Data block

 |<------------------------data_len------------------------>|
 |<data1><data2><data3><data4><data5><data6><data7><data...>|

This binary block is data_len bytes long and consists of truncated data.

To select one data element, we need the data_offset and the data_len from the index , if we have those we can count data_offset bytes from the start of the data block, and then cut out the next data_len bytes. there we got our data block:

 |<-----dataN_offset----->|<--dataN_len->|
 |<data1data2data3data...>|<data-we-want>|  

AUTHORS

SEE ALSO

Index

NAME

NOTES

Data Types

Vertical Bars

SYNOPSIS

DETAILED

xpak

Index block

Data block

AUTHORS

SEE ALSO