AppleSingle / AppleDouble: GraphViz block diagram (.dot) source

AppleSingle and AppleDouble files are used by certain Mac applications (e.g. Finder) to store Mac-specific file attributes on filesystems that do not support that.

Syntactically, both formats are the same, the only difference is how they are being used:

  • AppleSingle means that only one file will be created on external filesystem that will hold both the data (AKA "data fork" in Apple terminology), and the attributes (AKA "resource fork").
  • AppleDouble means that two files will be created: a normal file that keeps the data ("data fork") is kept separately from an auxiliary file that contains attributes ("resource fork"), which is kept with the same name, but starting with an extra dot and underscore ._ to keep it hidden.

In modern practice (Mac OS X), Finder only uses AppleDouble to keep compatibility with other OSes, as virtually nobody outside of Mac understands how to access data in AppleSingle container.

KS implementation details

License: CC0-1.0

References

This page hosts a formal specification of AppleSingle / AppleDouble using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.

GraphViz block diagram source

apple_single_double.dot

Download 

digraph {
	rankdir=LR;
	node [shape=plaintext];
	subgraph cluster__apple_single_double {
		label="AppleSingleDouble";
		graph[style=dotted];

		apple_single_double__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">4</TD><TD>u4be→FileType</TD><TD PORT="magic_type">magic</TD></TR>
			<TR><TD PORT="version_pos">4</TD><TD PORT="version_size">4</TD><TD>u4be</TD><TD PORT="version_type">version</TD></TR>
			<TR><TD PORT="reserved_pos">8</TD><TD PORT="reserved_size">16</TD><TD></TD><TD PORT="reserved_type">reserved</TD></TR>
			<TR><TD PORT="num_entries_pos">24</TD><TD PORT="num_entries_size">2</TD><TD>u2be</TD><TD PORT="num_entries_type">num_entries</TD></TR>
			<TR><TD PORT="entries_pos">26</TD><TD PORT="entries_size">12</TD><TD>Entry</TD><TD PORT="entries_type">entries</TD></TR>
			<TR><TD COLSPAN="4" PORT="entries__repeat">repeat num_entries times</TD></TR>
		</TABLE>>];
		subgraph cluster__entry {
			label="AppleSingleDouble::Entry";
			graph[style=dotted];

			entry__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="type_pos">0</TD><TD PORT="type_size">4</TD><TD>u4be→Types</TD><TD PORT="type_type">type</TD></TR>
				<TR><TD PORT="ofs_body_pos">4</TD><TD PORT="ofs_body_size">4</TD><TD>u4be</TD><TD PORT="ofs_body_type">ofs_body</TD></TR>
				<TR><TD PORT="len_body_pos">8</TD><TD PORT="len_body_size">4</TD><TD>u4be</TD><TD PORT="len_body_type">len_body</TD></TR>
			</TABLE>>];
			entry__inst__body [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="body_pos">ofs_body</TD><TD PORT="body_size">...</TD><TD>switch (type)</TD><TD PORT="body_type">body</TD></TR>
			</TABLE>>];
entry__inst__body_body_switch [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
	<TR><TD BGCOLOR="#F0F2E4">case</TD><TD BGCOLOR="#F0F2E4">type</TD></TR>
	<TR><TD>:types_finder_info</TD><TD PORT="case0">FinderInfo</TD></TR>
</TABLE>>];
		}
		subgraph cluster__finder_info {
			label="AppleSingleDouble::FinderInfo";
			graph[style=dotted];

			finder_info__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="file_type_pos">0</TD><TD PORT="file_type_size">4</TD><TD></TD><TD PORT="file_type_type">file_type</TD></TR>
				<TR><TD PORT="file_creator_pos">4</TD><TD PORT="file_creator_size">4</TD><TD></TD><TD PORT="file_creator_type">file_creator</TD></TR>
				<TR><TD PORT="flags_pos">8</TD><TD PORT="flags_size">2</TD><TD>u2be</TD><TD PORT="flags_type">flags</TD></TR>
				<TR><TD PORT="location_pos">10</TD><TD PORT="location_size">4</TD><TD>Point</TD><TD PORT="location_type">location</TD></TR>
				<TR><TD PORT="folder_id_pos">14</TD><TD PORT="folder_id_size">2</TD><TD>u2be</TD><TD PORT="folder_id_type">folder_id</TD></TR>
			</TABLE>>];
		}
		subgraph cluster__point {
			label="AppleSingleDouble::Point";
			graph[style=dotted];

			point__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="x_pos">0</TD><TD PORT="x_size">2</TD><TD>u2be</TD><TD PORT="x_type">x</TD></TR>
				<TR><TD PORT="y_pos">2</TD><TD PORT="y_size">2</TD><TD>u2be</TD><TD PORT="y_type">y</TD></TR>
			</TABLE>>];
		}
	}
	apple_single_double__seq:entries_type -> entry__seq [style=bold];
	apple_single_double__seq:num_entries_type -> apple_single_double__seq:entries__repeat [color="#404040"];
	entry__seq:ofs_body_type -> entry__inst__body:body_pos [color="#404040"];
	entry__inst__body:body_type -> entry__inst__body_body_switch [style=bold];
	entry__inst__body_body_switch:case0 -> finder_info__seq [style=bold];
	entry__seq:type_type -> entry__inst__body:body_type [color="#404040"];
	finder_info__seq:location_type -> point__seq [style=bold];
}