Other specifications may reference this specification by specifying the location of a data file as $XDG_DATA_DIRS/subdir/filename. This implies that:
Such file should be installed to $datadir/subdir/filename with $datadir defaulting to /usr/share.
A user specific version of the data file may be created in $XDG_DATA_HOME/subdir/filename, taking into account the default value for $XDG_DATA_HOME if $XDG_DATA_HOME is not set.
Lookups of the data file should search for ./subdir/filename relative to all base directories specified by $XDG_DATA_HOME and $XDG_DATA_DIRS . If an environment variable is either not set or empty, its default value as defined by this specification should be used instead.
Specifications may reference this specification by specifying the location of a configuration file as $XDG_CONFIG_DIRS/subdir/filename. This implies that:
Default configuration files should be installed to $sysconfdir/xdg/subdir/filename with $sysconfdir defaulting to /etc.
A user specific version of the configuration file may be created in $XDG_CONFIG_HOME/subdir/filename, taking into account the default value for $XDG_CONFIG_HOME if $XDG_CONFIG_HOME is not set.
Lookups of the configuration file should search for ./subdir/filename relative to all base directories indicated by $XDG_CONFIG_HOME and $XDG_CONFIG_DIRS . If an environment variable is either not set or empty, its default value as defined by this specification should be used instead.
If, when attempting to write a file, the destination directory is non-existant an attempt should be made to create it with permission 0700. If the destination directory exists already the permissions should not be changed. The application should be prepared to handle the case where the file could not be written, either because the directory was non-existant and could not be created, or for any other reason. In such case it may chose to present an error message to the user.
When attempting to read a file, if for any reason a file in a certain directory is unaccessible, e.g. because the directory is non-existant, the file is non-existant or the user is not authorized to open the file, then the processing of the file in that directory should be skipped. If due to this a required file could not be found at all, the application may chose to present an error message to the user.
A specification that refers to $XDG_DATA_DIRS or $XDG_CONFIG_DIRS should define what the behaviour must be when a file is located under multiple base directories. It could, for example, define that only the file under the most important base directory should be used or, as another example, it could define rules for merging the information from the different files.