Re: [PATCH v3] filename.7: new manual page

[Florian Weimer <fweimer@redhat.com>]

* 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