From: Martin Sebor <msebor@gmail.com>
To: Jonathan Wakely <jwakely@redhat.com>
Cc: libstdc++ <libstdc++@gcc.gnu.org>,
gcc-patches <gcc-patches@gcc.gnu.org>
Subject: Re: [patch] libstdc++/67173 Fix filesystem::canonical for Solaris 10.
Date: Wed, 16 Sep 2015 17:38:00 -0000 [thread overview]
Message-ID: <55F9A8A8.3060502@gmail.com> (raw)
In-Reply-To: <20150916144207.GY2631@redhat.com>
> It turns out I was wrong about BSD traditionally supporting
> realpath(path, NULL), it first appeared in these relatively recent
> versions:
>
> FreeBSD 9.0
> OpenBSD 5.0
> NetBSD 6.1
>
> Like Solaris 11, some of them still define _POSIX_VERSION=200112L even
> though they support passing NULL, so we could hardcode the targets
> that are known to support it, but we'll still need a fallback for lots
> of slightly older targets anyway.
>
> So here's a new implementation of canonical() which only uses
> realpath() when this autoconf compile-or-link test passes:
>
> #if _XOPEN_VERSION < 500
> #error
> #elif _XOPEN_VERSION >= 700 || defined(PATH_MAX)
> char *tmp = realpath((const char*)NULL, (char*)NULL);
> #else
> #error
> #endif
>
> i.e. I use realpath() for Issue 7, or for Issue 6 if PATH_MAX is
> defined.
I probably wouldn't code the PATH_MAX test quite the same way.
I would expect it to be mainly Issue 6 implementations that don't
define the macro to want to provide the null extension since there
would otherwise be no safe way to use the function.
I didn't test it at all but I'd be inclined to write the conditional
to look more along these lines:
#if _XOPEN_VERSION >= 700
// Issue 7 and better -- null resolved_name means allocate
char *tmp = realpath (file_name, (char*)NULL);
#elif _XOPEN_VERSION == 600
// Issue 6 -- null resolved_name is implementation-defined
# if FreeBSD 9.0 or OpenBSD 5.0 or NetBSD 6.1
char *tmp = realpath (file_name, (char*)NULL);
# elif PATH_MAX
char *tmp = realpath (file_name, malloc (PATH_MAX));
# else
# error No safe way to call realpath
# endif
#elif _XOPEN_VERSION && PATH_MAX
// SUSv2 -- null resolved_name is an error
char *tmp = realpath (file_name, malloc (PATH_MAX));
#else
# error realpath not supported or no safe way to call it
#endif
>
> Then in filesystem::canonical() if _GLIBCXX_USE_REALPATH is set I use
> it, passing NULL for Issue 7, or allocating a buffer of PATH_MAX
> otherwise. If realpath isn't supported or fails with ENAMETOOLONG then
> I do the path traversal by hand (which can be done entirely using the
> std::experimental::filesystem operations).
FWIW, to work across filesystems with different _PC_PATH_MAX, I
suspect the operations might need to use readlinkat. I.e., they
might need to descend into each individual subdirectory to avoid
forming a temporary pathname that's too long for the file system
being traversed, even though both the initial and the final
pathnames are under the limit. (I haven't actually tested this
but I don't see where GLIBC handles this case so it might not
do the right thing either.)
>
> Only passing NULL for Issue 7 is quite conservative. It means we don't
> do it for targets that support it as an implementation-defined
> extension to Issue 6, which includes Solaris 11, the BSDs and even
> older GNU systems (including RHEL6). But that's OK, we have a fallback
> now so it means no loss of functionality, just efficiency. We can
> tweak the config later for targets known to handle NULL.
Does the config test actually run? If not, I don't see the point
(it doesn't tell us anything the POSIX feature tests macros don't).
If it did run, it would fail since the first argument is null.
>
> The new testcase is not very thorough. I've run a few more involved
> tests that aren't suitable to check in until I figure out a good way
> of running filesystem tests that can create/remove arbitrary files and
> symlinks.
Yeah, writing good tests is always the hard part. If you need help
I can try to put some together in my spare time.
Martin
next prev parent reply other threads:[~2015-09-16 17:36 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-09-11 14:23 Jonathan Wakely
2015-09-11 18:05 ` Martin Sebor
2015-09-12 10:39 ` Jonathan Wakely
2015-09-12 19:49 ` Martin Sebor
2015-09-12 22:00 ` Martin Sebor
2015-09-16 14:52 ` Jonathan Wakely
2015-09-16 16:05 ` Jonathan Wakely
2015-09-16 16:11 ` Jonathan Wakely
2015-09-16 17:38 ` Martin Sebor [this message]
2015-09-16 19:02 ` Jonathan Wakely
2015-09-16 22:17 ` Martin Sebor
2015-09-16 22:23 ` Jonathan Wakely
2015-09-16 23:51 ` Martin Sebor
2015-09-17 11:31 ` Jonathan Wakely
2015-09-17 11:33 ` Jonathan Wakely
2015-09-17 14:38 ` Jonathan Wakely
2015-09-17 15:40 ` Martin Sebor
2015-09-23 12:14 ` Jonathan Wakely
2015-09-16 23:42 ` Jonathan Wakely
2015-09-17 15:36 ` Jonathan Wakely
2015-09-17 19:27 ` Andreas Schwab
2015-09-17 22:23 ` Jonathan Wakely
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