On Tue, Jun 30, 2026 at 7:48 PM Jonathan Wakely <[email protected]> wrote:
> Although the systemd docs say that /etc/localtime should be a symlink to
> one of the zoneinfo files, some systems make it a symlink to another
> path, where that second path is a symlink to a zoneinfo file (e.g. if
> /etc is mounted read-only then /etc/localtime can be a symlink to
> another symlink on a writable disk, so that the system timezone can be
> altered by re-pointing the symlink on the writable disk).
>
> In that case, using readlink would only tell us the location of the
> second symlink, not which zoneinfo file it points to. Therefore, we
> would not be able to extract a valid time zone name from the path, and
> chrono::current_zone() would fail.
>
> To support multiple symlinks we could recursively keep resolving
> symlinks with readlink until we reach a path from which we can extract a
> zone name. Alternatively, we can just use realpath to resolve all
> symlinks to a physical file (which is what HowardHinnant/date does).
> This means we only need one system call and don't need the extra
> complexity of calling readlink in a loop.
>
> The realpath system call also removes redunant slashes, so we can remove
> the code that did that manually.
>
> The possible downsides of this approach that I'm aware of are:
>
> - When /etc/localtime is a symlink to /invalid/Europe/London but that
> file doesn't exist. With the previous implementation we would have
> resolved that symlink to the zone "Europe/London" as long as that name
> is known to the current chrono::tzdb object. With this change, we
> won't get a valid zone name and current_zone() will fail. I'm not sure
> how realistic this case is. It might be plausible if libstdc++ is
> using the embedded static copy of tzdata.zi and there are no zoneinfo
> files on disk at all. In that case the system might still use
> /etc/localtime to name a zone, even though the symlink is dangling.
> We could fall back to filesystem::weakly_canonical for this case, but
> this patch leaves that for a future change, if it turns out to be
> needed by any users.
>
> - When /etc/localtime is a symlink to /usr/share/zoneinfo/Foo/Bar where
> "Foo/Bar" is a valid zone in the chrono::tzdb object, but the Bar file
> is another symlink to ./Baz where "Foo/Bar" is also a valid zone.
> With the previous implementation current_zone() would have returned
> the "Foo/Bar" zone. With this change it would return "Foo/Baz". I
> don't think it's realistic to have two zones which are distinct zones
> (not a Zone and a Link to it) but where one of them is defined on-disk
> using a symlink to the other.
>
> libstdc++-v3/ChangeLog:
>
> PR libstdc++/125467
> * src/c++20/tzdb.cc (tzdb::current_zone): Use realpath to
> resolve the /etc/localtime symlink instead of readlink.
> ---
>
> v2: Make the type of 'str' always std::string_view. Check str !=
> "/etc/localtime" so that we don't bother trying to extract a zone name
> from the symlink target if it isn't even a symlink.
>
> Tested x86_64-linux.
>
LGTM with very small suggestion.
>
> libstdc++-v3/src/c++20/tzdb.cc | 76 +++++++++++++---------------------
> 1 file changed, 28 insertions(+), 48 deletions(-)
>
> diff --git a/libstdc++-v3/src/c++20/tzdb.cc
> b/libstdc++-v3/src/c++20/tzdb.cc
> index 9e601fc176f3..c658e0c9cd37 100644
> --- a/libstdc++-v3/src/c++20/tzdb.cc
> +++ b/libstdc++-v3/src/c++20/tzdb.cc
> @@ -41,8 +41,13 @@
> # include <ext/concurrence.h> // __gnu_cxx::__mutex
> #endif
>
> -#if defined(_GLIBCXX_HAVE_READLINK) && defined(_GLIBCXX_HAVE_UNISTD_H)
> -# include <unistd.h> // readlink
> +#ifdef _GLIBCXX_HAVE_UNISTD_H
> +# include <unistd.h> // _XOPEN_VERSION
> +#endif
> +#if defined _GLIBCXX_USE_REALPATH && _XOPEN_VERSION >= 700
> +# include <stdlib.h> // malloc, free, realpath
> +#else
> +# include <filesystem> // filesystem::canonicalize
> #endif
>
> #ifdef _AIX
> @@ -2098,58 +2103,33 @@ constinit tzdb_list::_Node::NumLeapSeconds
> tzdb_list::_Node::num_leap_seconds;
> // to have a way to force a re-read.
>
> #if !defined(_AIX) && !defined(_GLIBCXX_HAVE_WINDOWS_H)
> -#if defined(_GLIBCXX_HAVE_READLINK) && defined(_GLIBCXX_HAVE_UNISTD_H)
> - string_view str;
> - char buf[128]; // strlen("../usr/share/zoneinfo/...") is usually < 55
> - string dynbuf;
> // /etc/localtime should be a symlink that ends with a zone name,
> // e.g. /etc/localtime -> /usr/share/zoneinfo/Europe/London
> //
> https://www.freedesktop.org/software/systemd/man/latest/localtime.html
> // This should work on GNU/Linux, macOS, NetBSD, and OpenBSD.
> - // Some FreeBSD systems also use a symlink for /etc/localtime.
> - // Use readlink directly to avoid std::filesystem overhead.
> - if (auto n = ::readlink("/etc/localtime", buf, sizeof(buf)); n > 0)
> + // Some FreeBSD systems also use a symlink for /etc/localtime (since
> 15.0).
> +
> + // N.B. we do not support dangling symlinks here. If that becomes
> necessary
> + // then after realpath fails we could fallback to using
> + //
> filesystem::weakly_canonical(filesystem::read_symlink("etc/localtime")).
> +
> +#if defined _GLIBCXX_USE_REALPATH && _XOPEN_VERSION >= 700
> + unique_ptr<char[], void(*)(void*)> cbuf{ nullptr, &::free };
> + string_view str;
>
This is very subjective and not necessary, but I would prefer this
string_view to be
declared before #if here. I find it easier to follow that way.
> + // Use realpath directly to avoid std::filesystem overhead.
> + // We use realpath not readlink to resolve multiple levels of
> symlinks.
> + if (char* p = ::realpath("/etc/localtime", nullptr))
> {
> - if (static_cast<size_t>(n) < sizeof(buf))
> - str = string_view(buf, n);
> - else [[unlikely]]
> - {
> - // We read the symlink but it didn't fit in buf[], use dynbuf.
> - do
> - {
> - n *= 2;
> - dynbuf.__resize_and_overwrite(n, [](char* p, size_t len) {
> - auto n2 = ::readlink("/etc/localtime", p, len);
> - if (n2 == -1) // symlink removed or replaced by file?!
> - __throw_runtime_error("tzdb: error reading
> /etc/localtime");
> - const size_t r = n2;
> - return r < len ? r : 0;
> - });
> - }
> - while (dynbuf.empty());
> - str = dynbuf;
> - }
> + cbuf.reset(p);
> + str = p;
> }
> +#else
> + string sbuf = std::filesystem::canonical("/etc/localtime").string();
> + string_view str = sbuf;
>
And this will become an assignment.
> +#endif
>
> - if (!str.empty())
> + if (!str.empty() && str != "/etc/localtime")
> {
> - // Remove any redundant slashes so we can match zone names.
> - // e.g. /usr/share/zoneinfo/Europe//London is a valid symlink,
> - // but won't match against "Europe/London".
> - if (auto pos = str.rfind("//"); pos != str.npos) [[unlikely]]
> - {
> - if (str.data() != dynbuf.data())
> - dynbuf = str;
> - string::size_type spos = pos;
> - do
> - {
> - dynbuf.erase(spos, 1);
> - spos = dynbuf.rfind("//", spos);
> - }
> - while (spos != dynbuf.npos);
> - str = dynbuf;
> - }
> -
> // Check the trailing components of the path against known zone
> names.
> // Valid IANA times zones can have one, two, or three parts, e.g.
> // "UTC", "Europe/London", and "America/Indiana/Indianapolis".
> @@ -2175,10 +2155,10 @@ constinit tzdb_list::_Node::NumLeapSeconds
> tzdb_list::_Node::num_leap_seconds;
> str.substr(pos + 1)))
> return tz;
> }
> -#endif
> +
> // Otherwise, look for a file naming the time zone.
> string_view files[] {
> - "/etc/timezone", // Debian derivates
> + "/etc/timezone", // Debian derivates, non-systemd Gentoo
> "/var/db/zoneinfo", // FreeBSD
> };
> for (auto f : files)
> --
> 2.54.0
>
>
