[Dev-luatex] file permissions

Reinhard Kotucha reinhard.kotucha at web.de
Sat Feb 5 21:11:52 CET 2011


Hi Taco,
when writing scripts in texlua, I often have to find out whether a
file or directory is readable or writable.  The permission flags I get
from lfs.attributes() are not sufficient.  Though I can determine the
UID and GID of the file owner, I still have to find out my own UID and
GID and whether I'm member of a particular group.  This is difficult,
sometimes impossible (without access to getpwent(3) and getgrent(3)),
and quite system dependent.

In order to test whether a file is readable I currently open this file
in a protected call.  This is also possible for directories when I run
lfs.dir() inside pcall().  It's not very elegant, and I still have
no idea how to test whether a file is writable.  I could try to open
the file in append mode, but I fear that if the file is writable, this
test would change the time stamps.  Not very elegant either.

I'm not aware of a better solution, but maybe I'm missing something.

If not, I propose a new function lfs.access() which takes a file name
and a string as an argument containing any combination of the letters
'r', 'w', or 'x' and returns the result of the system call access(2)
(as a boolean).

For instance, lfs.access(somefile, "rx") would then return the result
of the system call access(somefile, R_OK|X_OK).

On Windows there is a function _access() in the runtime lib which is
similar to access(2) on Unix, though the executable flag isn't
supported.  Maybe it can be ignored.  On the other hand it would be
very convenient if the function returns true if the file is in PATH
and its extension is in PATHEXT (as described in the MSVCRT sources).
This makes the function call more expensive, but I think that
access(2) is relatively expensive on Unix too because of the UID/GID
lookups.

I first considered to propose to extend lfs.isfile() and lfs.isdir()
by an optional argument, but because lfs.isfile() checks whether a
file is a regular file (which is fine), this seems to impose an
unnecessary limitation:

print(lfs.isfile('/dev/null'))
  => false

print(lfs.attributes('/dev/null', 'mode'))
  => char device

Hence, I think that a separate function lfs.access() makes more sense.

WDYT?

Regards,
  Reinhard

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Reinhard Kotucha                                      Phone: +49-511-3373112
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