chrono: check _POSIX_TIMERS before using clock_gettime

[LittleFox94]

Summary

clock_gettime is documented to be available when _POSIX_TIMERS is defined. Add a check for this

[ldionne]

LGTM generally speaking, but is this triggered by a specific failure you ran into? If so, on what configuration? We should have a way to test this change.

Also, next time, please include context in your Phabricator diff.

libcxx/src/chrono.cpp

15

From https://linux.die.net/man/3/clock_gettime, it says you need to include <unistd.h> to get the macro definition.

[LittleFox94]

In D79305#2017632, @ldionne wrote:

LGTM generally speaking, but is this triggered by a specific failure you ran into? If so, on what configuration? We should have a way to test this change.

built libc++ for my hobby OS but this looked general enough to include upstream. The
__APPLE__ check is probably not required with the new one in place.

Also, next time, please include context in your Phabricator diff.

OK, will do. Phabricator was a bit confusing, first time usage ^^'

[LittleFox94]

Included unistd.h and removed the check for __APPLE__, since _POSIX_TIMERS should work there, too

[ldionne]

Please make a corresponding change at libcxx/src/filesystem/operations.cpp:40, where the #if has been copied.

@EricWF What would you say about having a header in src/ that contains these additional configuration checks only used for building the dylib? Either that, or we should move these checks to __config. The fact that we duplicated the check in src/chrono.cpp and src/filesystem/operations.cpp is bad.

libcxx/src/chrono.cpp

13

<unistd.h> is not supported on Windows AFAICT, so you'll need to guard this with an appropriate #if.

16

It turns out the __APPLE__ check is still required. See libcxx/src/include/apple_availability.h:33 & friends. Thus, I would use:

#if !defined(__APPLE__) && defined(_POSIX_TIMERS)

18

Please remove the comment at the closing #endif. For one-liner #ifs like that, it's just an occasion to get the actual condition and the comment out of sync. I know you were just being consistent with surrounding code, but it's bad that we've been doing it.

[LittleFox94]

Do not include unistd.h on _WIN32, re-add __APPLE__ check and change _POSIX_TIMER check to check for > 0

[LittleFox94]

libcxx/src/chrono.cpp

16

I extend the check with _POSIX_TIMERS > 0, I have seen a #define _POSIX_TIMERS (-1) in a apple unistd.h and doc says the symbol _POSIX_TIMERS is defined in <unistd.h> to a value greater than 0

[LittleFox94]

Also fix duplicated check in filesystem/operations.cpp

[ldionne]

LGTM. Do you need help committing? If so, please specify the author info to use in Git as "First Last <[e-mail]>" so I can give you proper credits. Thanks!

[LittleFox94]

Cannot commit myself

"Mara Sophie Grosch <[e-mail]>"

Thank you very much for your time and comments :)

[ldionne]

Thanks! Committed as

commit 59b3102739c5988c9fc7ea77888de1bc8ce9c8ab
Author: Mara Sophie Grosch <[e-mail]>
Date:   Thu May 7 12:10:33 2020 -0400

    [libc++] chrono: check _POSIX_TIMERS before using clock_gettime

    clock_gettime is documented to be available when _POSIX_TIMERS is
    defined. Add a check for this.

    Differential Revision: https://reviews.llvm.org/D79305

libcxx/src/chrono.cpp

 //===------------------------- chrono.cpp ---------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "chrono"
 #include "cerrno"        // errno
 #include "system_error"  // __throw_system_error
 #include <time.h>        // clock_gettime, CLOCK_MONOTONIC and CLOCK_REALTIME
+#include <unistd.h>
 #include "include/apple_availability.h"
 
-#if !defined(__APPLE__)
+#if _POSIX_TIMERS > 0
 #define _LIBCPP_USE_CLOCK_GETTIME
-#endif // __APPLE__
+#endif // _POSIX_TIMERS
 
 #if defined(_LIBCPP_WIN32API)
 #define WIN32_LEAN_AND_MEAN
 #define VC_EXTRA_LEAN
 #include <windows.h>
 #if _WIN32_WINNT >= _WIN32_WINNT_WIN8
 #include <winapifamily.h>
 #endif