[PATCH] Makefile: add NEEDS_LIBRT to optionally link with librt

[PATCH] Makefile: add NEEDS_LIBRT to optionally link with librt

[Ronald Wampler]

We unconditionally link with librt, when HAVE_CLOCK_GETTIME is defined.
But clock_gettime() has been available in most libc implementations for
some time now (e.g., for glibc since version 2.17) and no longer
requires linking with librt. Furthermore, commit a6c3c63 (configure.ac:
check for clock_gettime() and CLOCK_MONOTONIC) will automatically
determined which library (libc or librt) is required for linking when
checking for clock_gettime().

The assumption to unconditionally link with librt was OK, since either
almost every Unix-like system provides a version of librt for backwards
compatibility or other systems, namely Windows or OS X, never provided
clock_gettime(). However, in the latest release of OS X (macOS Sierra),
this function has been added to OS X libc version. As a result, when
running the configuration script, HAVE_CLOCK_GETTIME is set and since
librt is not present, it causes a linker error.

This patches requires those not building via the configuration scripts
to define NEEDS_LIBRT in addition to HAVE_CLOCK_GETTIME, if needed.

Signed-off-by: Ronald Wampler <rdwampler@gmail.com>
---
I am not sure if this the correction solution. Another option I
considered was to wrap the EXTLIBS += -lrt is an ifndef NO_RT and only
defining NO_RT for Mac OS X in config.mak.uname.

 Makefile | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/Makefile b/Makefile
index de5a030..32f503e 100644
--- a/Makefile
+++ b/Makefile
@@ -351,9 +351,12 @@ all::
 # Define GMTIME_UNRELIABLE_ERRORS if your gmtime() function does not
 # return NULL when it receives a bogus time_t.
 #
-# Define HAVE_CLOCK_GETTIME if your platform has clock_gettime in librt.
+# Define HAVE_CLOCK_GETTIME if your platform has clock_gettime.
 #
-# Define HAVE_CLOCK_MONOTONIC if your platform has CLOCK_MONOTONIC in librt.
+# Define HAVE_CLOCK_MONOTONIC if your platform has CLOCK_MONOTONIC.
+#
+# Define NEEDS_LIBRT if your platform requires linking with librt (glibc version
+# before 2.17) for clock_gettime and CLOCK_MONOTONIC.
 #
 # Define USE_PARENS_AROUND_GETTEXT_N to "yes" if your compiler happily
 # compiles the following initialization:
@@ -1465,13 +1468,16 @@ endif

 ifdef HAVE_CLOCK_GETTIME
 	BASIC_CFLAGS += -DHAVE_CLOCK_GETTIME
-	EXTLIBS += -lrt
 endif

 ifdef HAVE_CLOCK_MONOTONIC
 	BASIC_CFLAGS += -DHAVE_CLOCK_MONOTONIC
 endif

+ifdef NEEDS_LIBRT
+	EXTLIBS += -lrt
+endif
+
 ifdef HAVE_BSD_SYSCTL
 	BASIC_CFLAGS += -DHAVE_BSD_SYSCTL
 endif
--
2.8.2.874.gcf4c2cf

Re: [PATCH] Makefile: add NEEDS_LIBRT to optionally link with librt

[Junio C Hamano]

Ronald Wampler <rdwampler@gmail.com> writes:

> I am not sure if this the correction solution. Another option I
> considered was to wrap the EXTLIBS += -lrt is an ifndef NO_RT and only
> defining NO_RT for Mac OS X in config.mak.uname.

That alternative would make the resulting code noisier/uglier with
nested ifdef, I would imagine, but it would be of less impact to the
existing users.  But my gut feeling is that the patch you sent is
probably a better solution for the longer term.

Thanks.

[PATCH] config.mak.uname: define NEEDS_LIBRT under Linux, for now

[Eric Wong]

Junio C Hamano <gitster@pobox.com> wrote:
> Ronald Wampler <rdwampler@gmail.com> writes:
> 
> > I am not sure if this the correction solution. Another option I
> > considered was to wrap the EXTLIBS += -lrt is an ifndef NO_RT and only
> > defining NO_RT for Mac OS X in config.mak.uname.
> 
> That alternative would make the resulting code noisier/uglier with
> nested ifdef, I would imagine, but it would be of less impact to the
> existing users.  But my gut feeling is that the patch you sent is
> probably a better solution for the longer term.

That change broke my Debian wheezy LTS system, which isn't too
out-of-date.  I think having excessive linkage on newer systems
is preferable to breaking the out-of-the-box experience.

I don't know about other platforms, but I think the following
will help users on older GNU/Linux systems for the next few
years:

-------8<------
Subject: [PATCH] config.mak.uname: define NEEDS_LIBRT under Linux, for now

My Debian wheezy LTS system is still on glibc 2.13; and LTS
distros may use older glibc, still, so lets not unnecessarily
break things out-of-the-box.

We seem to assume Linux is using glibc in our Makefiles anyways,
so I don't think this will introduce new breakage for users of
alternative libc implementations.

Signed-off-by: Eric Wong <e@80x24.org>
---
 config.mak.uname | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/config.mak.uname b/config.mak.uname
index a88f139..22958a8 100644
--- a/config.mak.uname
+++ b/config.mak.uname
@@ -36,6 +36,8 @@ ifeq ($(uname_S),Linux)
 	HAVE_DEV_TTY = YesPlease
 	HAVE_CLOCK_GETTIME = YesPlease
 	HAVE_CLOCK_MONOTONIC = YesPlease
+	# -lrt is needed for clock_gettime on glibc <= 2.16
+	NEEDS_LIBRT = YesPlease
 	HAVE_GETDELIM = YesPlease
 	SANE_TEXT_GREP=-a
 endif
-- 
EW

Re: [PATCH] config.mak.uname: define NEEDS_LIBRT under Linux, for now

[brian m. carlson]

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On Sun, Jul 10, 2016 at 10:16:44PM +0000, Eric Wong wrote:
> My Debian wheezy LTS system is still on glibc 2.13; and LTS
> distros may use older glibc, still, so lets not unnecessarily
> break things out-of-the-box.
> 
> We seem to assume Linux is using glibc in our Makefiles anyways,
> so I don't think this will introduce new breakage for users of
> alternative libc implementations.

This is a good idea.  If it's broken with Debian wheezy, it will also be
broken for RHEL/CentOS 6.  People who really want the slimmest linkage
can use the -Wl,--as-needed flag.
-- 
brian m. carlson / brian with sandals: Houston, Texas, US
+1 832 623 2791 | https://www.crustytoothpaste.net/~bmc | My opinion only
OpenPGP: https://keybase.io/bk2204

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Re: [PATCH] config.mak.uname: define NEEDS_LIBRT under Linux, for now

[Junio C Hamano]

"brian m. carlson" <sandals@crustytoothpaste.net> writes:

> On Sun, Jul 10, 2016 at 10:16:44PM +0000, Eric Wong wrote:
>> My Debian wheezy LTS system is still on glibc 2.13; and LTS
>> distros may use older glibc, still, so lets not unnecessarily
>> break things out-of-the-box.
>> 
>> We seem to assume Linux is using glibc in our Makefiles anyways,
>> so I don't think this will introduce new breakage for users of
>> alternative libc implementations.
>
> This is a good idea.  If it's broken with Debian wheezy, it will also be
> broken for RHEL/CentOS 6.  People who really want the slimmest linkage
> can use the -Wl,--as-needed flag.

Thanks all, will apply.