* Thaddeus H. Black: > +.TH FILENAME 7 2021-10-18 "Linux" "Linux Programmer's Manual" > +.SH NAME > +filename \- requirements and conventions for the naming of files > +.SH DESCRIPTION > +This manual page sets forth requirements for > +and delineates conventions regarding filenames > +on a Linux system, > +where a > +.I filename > +is either (as the word suggests) the name of a regular file > +or the name of another object held by the system's filesystem > +such as a directory, symbolic link, named pipe or device. Maybe add: “A pathname contains zero or more filenames.” > +.SS Legal filenames > +A filename on a Linux system can consist > +of almost any sequence of UTF-8 characters > +or, indeed, almost any sequence of bytes. > +The exceptions are as follows. > +.TP > +.B Reserved characters > +.RS > +The following characters are reserved. > +.TP > +.B / > +The solidus is reserved to separate pathname components > +as for example in > +.IR /usr/share/doc , > +each component being itself a filename. > +For this reason, no filename may include a solidus. > +More precisely, > +no filename may include the byte that, > +in ASCII and UTF-8, > +exclusively represents the solidus. What does this mean? I think only byte 0x2f is reserved. The UTF-8 comment is misleading. A historic/overlong encoding of / in multiple UTF-8 bytes is *not* reserved. > +.B \e0 > +The null character is reserved for the filesystem to append > +to terminate a filename's representation in memory. > +For this reason, no filename may include a null character. > +More precisely, > +no filename may include the byte that, > +in ASCII and UTF-8, > +exclusively represents the null character. See above. > +.B Reserved names > +.RS > +The following names are reserved. > +.TP > +.B . > +The filename consisting of a single full stop > +is reserved to represent the current directory. > +.TP > +.B .. > +The filename consisting of two full stops > +is reserved to represent the parent directory. > +.TP > +(empty) > +The empty filename, > +consisting of no bytes at all > +(except a terminating null byte), > +is not allowed. This conflicts with the presentation of / as a separator in pathnames, I think: The pathname "/usr/" contains two empty filenames. > +.TP > +.B Long names > +.RS > +No filename may exceed\~255 bytes in length, > +or\~256 bytes after counting the terminating null byte. This is not correct for Linux. Despite the definition of NAME_MAX, filenames can be longer than 255 bytes. NTFS and CIFS have a limit of 255 UTF-16 characters, which translates to about 768 bytes in the UTF-8 encoding used by Linux. Thanks, Florian
