* 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