Sidecar file
 | This articleneeds additional citations forverification.(February 2023) |
Sidecar files,also known asbuddy filesorconnected files,arecomputer filesthat storedata(oftenmetadata) which is not supported by theformatof a source file.
There may be one or more sidecar files for each source file. There may also be "metadata databases" where one database contains metadata for several source files.
In most cases the relationship between the source file and the sidecar file is based on the file name; sidecar files have the same base name as the source file, but with a different extension. The problem with this system is that most operating systems andfile managershave no knowledge of these relationships, and might allow the user to rename or move one of the files thereby breaking the relationship.
Examples[edit]
- Amiga Hunk metadata
- InAmigaOS,a file with a.infoextensioncontains metadata for a companionAmiga Hunkexecutable file.
- Extensible Metadata Platform
- Extensible Metadata Platform(XMP) metadata is stored in a sidecar file when either afile formatdoes not support embedded XMP metadata or if the workflow requires this.
- DxO sidecar
- Similar to the XMP format, DOP sidecars store metadata and edits made through DxO PhotoLab[1]and other DxO tools[2]
- Connected Web Files and Folders
- Afile systemobject that associated two or more files. The file system treats connected files as a unit for purposes of moving, copying, and deleting. Some versions ofInternet ExplorerandMicrosoft Wordcan save an HTML and its hyperlinked assets as such a unit.
- THM
- Many digital cameras will store a.thm(thumbnail) file alongside a recorded movie, with the same base filename as the movie file. These thumbnail files areJFIF-encoded image files. This system allows for quickly displaying a still preview of the movie, and storing camera data which is not supported by the AVI file format.
- INF
- Acorn filesystems support metadata such as load and execution addresses that may not be natively supported on other filesystems. A.inffile is used to store this metadata in text format, stored in a file with the same base filename, e.g. Menu andMenu.inf,Build.srcandBuild.src.inf.
- JPEG + WAV
- Some digital cameras allow for voice/audio annotations with photos. These are then stored asWAVaudio files alongside theJPEGphoto file, with the same base filename.
- PDF + annotations
PDF viewers which allow the reader to annotate documents with comments and drawings may store these in a sidecar file, such asXournal's.xojfiles.
- RunPacker
- The MS-DOS-basedself-extracting archivegeneratorRunPackerrelied heavily on sidecar files because the proprietary package formatPFA(packfilearchive) used in it did not natively storefile attributesortimestamps.To address this limitation, abackward-compatiblesystem ofauxiliaryfiles(as called in official documentation) was implemented to store aforementioned data in volumes without changing their format. These were added to the PFA volume the same way as ordinary files, except they were marked by special characters in their name, and software made aware of this system (sometimes calledePFA,Extended PFA) processed themtransparently.Older programs relying on the PFA format would simply read or extract the file along with all others found in the volume. The ePFA format was designed with further extensibility in mind, and has been actually extended with several features over time.
- Meta Information Encapsulation (MIE)
- Meta Information Encapsulationsidecar files. The MIE format is an extensible, dedicated meta information format part ofExifTool.MIE files can be used to encapsulate meta information from many sources and bundle it together with any type of file.
A variation of this are copies of the source file which contain largely the same information, but in a different format or from a previous version:
- Exif
- Since many JPEG editing software used to destroyExifmetadata stored in digital photos, some photo cataloging applications can extract the Exif data and store that in an.exffile, so that the metadata can later be re-inserted into the JPEG file.
- Raw + JPEG
- Many digital cameras allow to store both uncompressedrawdata and aJFIF-encoded image file when shooting in raw mode. This allows for faster previewing the photo, and support by applications that do not support the (often undocumented) raw format.
- TIF + TFW or JPG + JGW
- Aerial photos may be supplied with aWorld Filethat determines the location, size and rotation of the image.
Alternatives[edit]
Rather than storing data separately, it can be stored as part of the main file. This is particularly done forcontainer files,which allow certain types of data to be stored in them. Instead of separate files on the file system, multiple files can be combined into anarchive file,which keeps them together, but requires that software processes the archive file, rather than individual files. This is a generic solution, as archive files can contain arbitrary files from the file system.
Forks[edit]
A file system level solution for the same problem areforks,which allow multiple pieces of data to be associated with a single file. Sidecar files can be seen as "forks for file systems without native support for forks".
These can then be manipulated with usual file system tools: because the support is built into the operating system, theseresource forkswill not show up as separate files, and all applications inherit support for resource forks.[clarification needed]However, forks cannot be copied to file systems without support for forks, or transmitted over a channel that does not support forks. For interchange forks are generally instead stored as sidecar file.
Theclassic Mac OSandmacOSare notable examples of operating systems with support for forks, in theHFSfile system. However, this causes problems with exchanging overISO 9660formatCD-ROM,FATformatMS-DOSdisks, and over internet email, and requires the use of sidecar files to store this information. MicrosoftNTFSsupports Alternate Data Streams which are similar.