Filename, Path

A file name, rather than a path name? That means no slashes and no NUL bytes. Slashes are used as directory component separators (with a special exception for a double slash vs. a single slash at the beginning of an absolute path name), so they are not allowed inside file names.

Literally any other byte is allowed.

Note that a file name is defined as an array of bytes, and not characters. Thus, character encodings are irrelevant. Interpretation of a filename as a string of characters is up to applications to do, or not to do, as they see fit.

A path name is a set of zero or more directory components, plus a file name, separated by slashes. An absolute path name begins with a slash. A relative path name does not, and is resolved with respect to the current working directory. If an absolute path name begins with two slash characters, the operating system is allowed to treat it differently from one beginning with a single slash. The exact meaning of that is up to the OS to decide.

The total byte length of a path name may not exceed PATH_MAX, which is usually defined as 4096 on Linux systems.