Re: question on d_ino semantics

[* * <richardvoigt at gmail dot com>]

On 1/20/06, Eric Blake wrote:
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> There are currently two competing uses of d_ino semantics in coreutils:
>
> lib/backupfile.c assumes that if d_ino is ever 0, (captured by the macro
> REAL_DIR_ENTRY, which is always non-zero on platforms without d_ino), that
> readdir() returned an invalid entry and no attempt to dereference that
> name should be attempted.
>
> src/pwd.c assumes that if d_ino is ever 0, (captured by the macro D_INO,
> which is hardcoded to 0 on platforms without d_ino), that an lstat()
> should be attempted and the entry skipped if the stat failed, otherwise
> use st_ino.
>
> Cygwin currently has no d_ino member, because it is prohibitively
> expensive to provide it on WinNT and Win2K machines (the only way to make
> d_ino match st_ino on those two platforms is to perform the equivalent of
> a stat during readdir, but this penalizes the many apps which do not
> follow readdir with stat).  But it is relatively trivial to provide d_ino

In any other language.... this would be fairly easy.  Make d_ino a
postponed call / functor that calls stat if and only if it is read. 
Even with C++ this is possible.  Is there any way for Cygwin to change
d_ino into some type of function call which can perform the processing
*on access*.

Of course, this would be totally incompatible with anything that tries
to take the address of d_ino or write to it, but from the discussion,
that seems to be 0% of callers.

> that matches st_ino on Win9x and WinXP machines.  If cygwin were to
> provide a sentinel value for d_ino in cases where making d_ino match
> st_ino is too expensive, would it be better to make the sentinel be 0 (in
> which case, pwd.c still works but backupfile.c would need to be taught to
> do the lstat before giving up on the entry), or would it be better to make
> the sentinel be -1 (in which case backupfile.c would still work, as it
> does not care what the value of d_ino is so long as it is not 0; but pwd.c
> would need to be taught that -1 also implies that lstat must be used to
> get the real st_ino)?
>
> Which other Unix platforms use 0 as the sentinel for a non-existent entry
> returned by readdir()?  Does anyone know of a standard that describes
> whether d_ino being 0 has special meaning?  POSIX only states that d_ino
> is an optional member (but required in XSI implementations), without any
> details as to whether d_ino must be non-zero.
>
> - --
> Life is short - so eat dessert first!
>
> Eric Blake             ebb9@byu.net
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