1 .TH XPAK 5 "Oct 2011" "Portage VERSION" "Portage"
3 xpak \- The XPAK Data Format used with Portage binary packages
5 Every Gentoo binary package has a xpak attached to it which contains build
6 time information like the USE flags it was built with, the ebuild it was
7 built from, the environmental variables, CFLAGS, CXXFLAGS, etc...
10 The following conventions cover all occurrences in this documentation
12 All offsets/lengths are big endian unsigned 32bit integers
14 All strings are ASCII encoded, and not NUL terminated (quotes are for illustration only)
16 The actual values of the individual xpak entries are stored as Strings
19 The vertical bars '|' are not part of the file format; they are merely used to
20 illustrate how the offset values apply to the data.
24 <tar>|< xpak >|<xpak_offset>"STOP"
26 "XPAKPACK"<index_len><data_len><index><data>"XPAKSTOP"
28 |<-------------index_len------------->|
29 |<index1><index2><index3><index4><...>|
32 <name_len>|< name >|<data_offset><data_len>
34 |<--------------data_len------------->|
35 |<-dataN_offset->|<-dataN_len->|
36 |< data >|< data_N >|<data>|
40 If you look at a Gentoo binary package (binpkg) with a hex-editor you'll
41 notice that after the tarball of files, you find a binary blob - the
42 \fIxpak\fR, an offset which holds the bytes from the start of the
43 \fIxpak\fR to the end of the file - \fIxpak_offset\fR and finally the
47 <tar>|<---xpak---->|<xpak_offset>"STOP"
49 Here you see the \fItar\fR archive, the attached \fIxpak\fR blob, the
50 \fIxpak_offset\fR and the string \fI"STOP"\fR at the end. This metadata
51 is not considered "part" of the \fIxpak\fR, but rather part of the binpkg.
53 If we read the offset value and count \fIoffset\fR bytes backwards from
54 the start of \fIxpak_offset\fR, we have found the start of the \fIxpak\fR
55 block which starts with the String \fI"XPAKPACK"\fR.
57 This xpak block consists of the string \fI"XPAKPACK"\fR, the length of the
58 \fIindex\fR block (\fIindex_len\fR), the length of the \fIdata\fR block
59 (\fIdata_len\fR), an \fIindex_len\fR bytes long binary blob with the
60 \fIindex\fR, a \fIdata_len\fR bytes long binary blob with the \fIdata\fR,
61 and the string \fI"XPAKSTOP"\fR at the end:
63 |<index_len>|<data_len>|
64 "XPAKPACK"<index_len><data_len>|<--index-->|<--data-->|"XPAKSTOP"
66 To actually get the \fIindex\fR and the \fIdata\fR, we cut out \fIindex_len\fR
67 bytes after the end of \fIdata_len\fR for the \fIindex\fR block, and then cut
68 out the next \fIdata_len\fR bytes for the \fIdata\fR block. If we have done
69 everything right up to this point, the following bytes would be the ASCII
70 formatted string \fI"XPAKSTOP"\fR.
72 The actual \fIdata\fR is merged into one big block; so if we want to read it,
73 we need the actual positions of each information in this big data block. This
74 information can be obtained using the indices which are stored in the
78 The \fIindex\fR block consists of several indices:
80 |<-----------------------index_len---------------------->|
81 |<index1><index2><index3><index4><index5><index6><index7>|
83 The \fIindex\fR block holds all the information we need to find the data we
84 want in the \fIdata\fR block. It consists of multiple index elements, all of
85 which add up to the total length \fIindex_len\fR. It is not zero delimited
88 Each of those elements corresponds to one chunk of data in the \fIdata\fR
89 block: the length of that block's name (\fIname_len\fR), a \fIname_len\fR
90 bytes long string, the offset of that block (\fIdataN_offset\fR) and the
91 length of that block (\fIdataN_len\fR):
94 <name_len>|<--name-->|<dataN_offset><dataN_len>
97 The \fIdata\fR block contains multiple chunks of data with a total length of
100 |<------------------------data_len------------------------>|
101 |<data1><data2><data3><data4><data5><data6><data7><data...>|
103 To select one data element, we need the \fIdata_offset\fR and the
104 \fIdata_len\fR from the \fIindex\fR. With those, we can count
105 \fIdata_offset\fR bytes from the start of the \fIdata\fR block,
106 and then cut out the next \fIdata_len\fR bytes. Then we got our
109 |<-----dataN_offset----->|<--dataN_len->|
110 |<data1data2data3data...>|<data-we-want>|
112 Let's say we have an xpak with two chunks of data. The first has the name
113 "file1" with the contents "ddDddDdd" and the second has the name "file2" with
114 the contents "jjJjjJjj". There is no \fI"STOP"\fR or \fIxpak_offset\fR as
115 this xpak is not part of a binpkg.
117 Here is the hexdump output (we will break it down line by line below):
118 00 58 50 41 4b 50 41 43 4b 00 00 00 20 00 00 00 10 |XPAKPACK... ....|
119 10 00 00 00 04 66 69 6c 31 00 00 00 00 00 00 00 08 |....fil1........|
120 20 00 00 00 04 66 69 6c 32 00 00 00 08 00 00 00 08 |....fil2........|
121 30 64 64 44 64 64 44 64 64 6a 6a 4a 6a 6a 4a 6a 6a |ddDddDddjjJjjJjj|
122 40 58 50 41 4b 53 54 4f 50 |XPAKSTOP|
124 The \fIindex_len\fR is 32 and the \fIdata_len\fR 16 (as there are 16 bytes:
125 "ddDddDdd" and "jjJjjJjj").
126 |<------"XPAKPACK"----->|| 32 | 16 |
127 00 58 50 41 4b 50 41 43 4b 00 00 00 20 00 00 00 10
129 Now we have the first index element with a \fIname_len\fR of 4, followed
130 by the name string "fil1", followed by the data1 offset of 0 and a data1
131 len of 8 (since data1 has 8 bytes: "ddDddDdd").
132 | 4 |<--"fil1"->||data1_off:0|data1_len:8|
133 10 00 00 00 04 66 69 6c 31 00 00 00 00 00 00 00 08
135 Now we have the second index element with a \fIname_len\fR of 4, followed
136 by the name string "fil2", followed by the data2 offset of 8 and a data2
137 len of 8 (since data2 has 8 bytes: "jjJjjJjj").
138 | 4 |<--"fil2"->||data2_off:8|data2_len:8|
139 20 00 00 00 04 66 69 6c 32 00 00 00 08 00 00 00 08
141 |<------"XPAKSTOP"----->|
142 40 58 50 41 4b 53 54 4f 50
145 Lars Hartmann <lars@chaotika.org>
146 Mike Frysinger <vapier@gentoo.org>