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If :unspecific is the value of a pathname component, the component is considered to be “absent” or to “have no meaning” in the filename being represented by the pathname.
Whether a value of :unspecific is permitted for any component on any given file system accessible to the implementation is implementation-defined. A conforming program must never unconditionally use a :unspecific as the value of a pathname component because such a value is not guaranteed to be permissible in all implementations. However, a conforming program can, if it is careful, successfully manipulate user-supplied data which contains or refers to non-portable pathname components. And certainly a conforming program should be prepared for the possibility that any components of a pathname could be :unspecific.
When reading_1 the value of any pathname component, conforming programs should be prepared for the value to be :unspecific.
When writing_1 the value of any pathname component, the consequences are undefined if :unspecific is given for a pathname in a file system for which it does not make sense.