threads / patch / 42799

patchMakefile: add NEEDS_LIBRT to optionally link with librt

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

## tl;dr

5 messages between Jul 7, 2016 and Jul 11, 2016. Diffs are folded; open one to read it.

replies: 4people: 4as markdown or json

Ronald Wampler· Jul 7, 2016, 20:45 UTC · lore

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(-)

Show changes to Makefile +9 −4

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

Junio C Hamano· Jul 7, 2016, 21:14 UTC · re: Ronald Wampler · lore

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

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.

Eric Wong· Jul 10, 2016, 22:16 UTC · re: Junio C Hamano · lore

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

Junio C Hamano <gitster@pobox.com> wrote:

Show 10 quoted lines

> 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(+)

Show changes to config.mak.uname +2 −0

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

brian m. carlson· Jul 11, 2016, 10:36 UTC · re: Eric Wong · lore

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

On Sun, Jul 10, 2016 at 10:16:44PM +0000, Eric Wong wrote:

Show 7 quoted lines

> 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

Junio C Hamano· Jul 11, 2016, 18:44 UTC · re: brian m. carlson · lore

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

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

Show 12 quoted lines

> 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.