zisofs is a compression format for files on ISO9660 file system. It has limited support across operating systems, mainly Linux kernel. Typically a directory tree is first preprocessed by mkzftree (from the zisofs-tools package before being turned into an ISO9660 image by mkisofs, genisoimage or similar tool. The data is zlib compressed.
The specification here describes the structure of a file that has been preprocessed by mkzftree, not of a full ISO9660 ziso. Data is not decompressed, as blocks with length 0 have a special meaning. Decompression and deconstruction of this data should be done outside of Kaitai Struct.
This page hosts a formal specification of zisofs using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.
digraph {
rankdir=LR;
node [shape=plaintext];
subgraph cluster__zisofs {
label="Zisofs";
graph[style=dotted];
zisofs__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="header_pos">0</TD><TD PORT="header_size">16</TD><TD>Header</TD><TD PORT="header_type">header</TD></TR>
<TR><TD PORT="block_pointers_pos">16</TD><TD PORT="block_pointers_size">4</TD><TD>u4le</TD><TD PORT="block_pointers_type">block_pointers</TD></TR>
<TR><TD COLSPAN="4" PORT="block_pointers__repeat">repeat (header.num_blocks + 1) times</TD></TR>
</TABLE>>];
zisofs__inst__blocks [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="blocks_pos">...</TD><TD PORT="blocks_size">0</TD><TD>Block</TD><TD PORT="blocks_type">blocks</TD></TR>
<TR><TD COLSPAN="4" PORT="blocks__repeat">repeat header.num_blocks times</TD></TR>
</TABLE>>];
subgraph cluster__header {
label="Zisofs::Header";
graph[style=dotted];
header__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="magic_pos">0</TD><TD PORT="magic_size">8</TD><TD></TD><TD PORT="magic_type">magic</TD></TR>
<TR><TD PORT="uncompressed_size_pos">8</TD><TD PORT="uncompressed_size_size">4</TD><TD>u4le</TD><TD PORT="uncompressed_size_type">uncompressed_size</TD></TR>
<TR><TD PORT="len_header_pos">12</TD><TD PORT="len_header_size">1</TD><TD>u1</TD><TD PORT="len_header_type">len_header</TD></TR>
<TR><TD PORT="block_size_log2_pos">13</TD><TD PORT="block_size_log2_size">1</TD><TD>u1</TD><TD PORT="block_size_log2_type">block_size_log2</TD></TR>
<TR><TD PORT="reserved_pos">14</TD><TD PORT="reserved_size">2</TD><TD></TD><TD PORT="reserved_type">reserved</TD></TR>
</TABLE>>];
header__inst__block_size [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">id</TD><TD BGCOLOR="#E0FFE0">value</TD></TR>
<TR><TD>block_size</TD><TD>(1 << block_size_log2)</TD></TR>
</TABLE>>];
header__inst__num_blocks [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">id</TD><TD BGCOLOR="#E0FFE0">value</TD></TR>
<TR><TD>num_blocks</TD><TD>((uncompressed_size / block_size) + ((uncompressed_size % block_size) != 0 ? 1 : 0))</TD></TR>
</TABLE>>];
}
subgraph cluster__block {
label="Zisofs::Block";
graph[style=dotted];
block__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
</TABLE>>];
block__inst__len_data [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">id</TD><TD BGCOLOR="#E0FFE0">value</TD></TR>
<TR><TD>len_data</TD><TD>(ofs_end - ofs_start)</TD></TR>
</TABLE>>];
block__inst__data [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="data_pos">ofs_start</TD><TD PORT="data_size">len_data</TD><TD></TD><TD PORT="data_type">data</TD></TR>
</TABLE>>];
}
}
zisofs__seq:header_type -> header__seq [style=bold];
header__inst__num_blocks:num_blocks_type -> zisofs__seq:block_pointers__repeat [color="#404040"];
zisofs__inst__blocks:blocks_type -> block__seq [style=bold];
header__inst__num_blocks:num_blocks_type -> zisofs__inst__blocks:blocks__repeat [color="#404040"];
header__seq:block_size_log2_type -> header__inst__block_size [color="#404040"];
header__seq:uncompressed_size_type -> header__inst__num_blocks [color="#404040"];
header__inst__block_size:block_size_type -> header__inst__num_blocks [color="#404040"];
header__seq:uncompressed_size_type -> header__inst__num_blocks [color="#404040"];
header__inst__block_size:block_size_type -> header__inst__num_blocks [color="#404040"];
block__params:ofs_end_type -> block__inst__len_data [color="#404040"];
block__params:ofs_start_type -> block__inst__len_data [color="#404040"];
block__params:ofs_start_type -> block__inst__data:data_pos [color="#404040"];
block__inst__len_data:len_data_type -> block__inst__data:data_size [color="#404040"];
}