Makefile: add NEEDS_LIBRT to optionally link with librt

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.

---
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(-)

index de5a030..32f503e 100644
@@ -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

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.

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.

---
 config.mak.uname | 2 ++
 1 file changed, 2 insertions(+)

index a88f139..22958a8 100644
@@ -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

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

Thanks all, will apply.