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Sidecar files, also known as buddy files or connected files, are computer files that store data (often metadata) which is not supported by the format of 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 and file managers have no knowledge of these relationships, and might allow the user to rename or move one of the files thereby breaking the relationship.

A file system level solution for the same problem are forks, 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, these resource forks will not show up as separate files, and all applications inherit support for resource forks.[forks, 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, these resource forks will not show up as separate files, and all applications inherit support for resource forks.[These can then be manipulated with usual file system tools: because the support is built into the operating system, these resource forks will 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.

Mac OS and OS X are notable examples of operating systems with support for forks, in the HFS file system. However, this causes problems with exchanging over ISO 9660 format CD-ROM, FAT format MS-DOS disks, and over internet email, and requires the use of sidecar files to store this information. Microsoft NTFS supports Alternate Data Streams which are similar.