From 16645be01bf267fe71a8301d0161c3958041d3d0 Mon Sep 17 00:00:00 2001
From: "Eric S. Raymond" <esr@thyrsus.com>
Date: Thu, 26 Feb 2015 15:04:31 -0500
Subject: memory_barrier() now uses a standardized C11 lockless-concurrency
 primitive.

All regression tests pass.
---
 SConstruct |  5 +++++
 build.txt  | 19 ++++++++++++-------
 compiler.h | 10 +++++-----
 3 files changed, 22 insertions(+), 12 deletions(-)

diff --git a/SConstruct b/SConstruct
index 9006f5c6..a942cfba 100644
--- a/SConstruct
+++ b/SConstruct
@@ -634,6 +634,11 @@ else:
     else:
         confdefs.append("/* #undef HAVE_TERMIOS_H */\n")
 
+    if config.CheckHeader("stdatomic.h"):
+        confdefs.append("#define HAVE_STDATOMIC_H 1\n")
+    else:
+        confdefs.append("/* #undef HAVE_STDATOMIC_H */\n")
+
     # endian.h is required for rtcm104v2 unless the compiler defines
     # __ORDER_BIG_ENDIAN__, __ORDER_LITTLE_ENDIAN__ and __BYTE_ORDER__
     if config.CheckCompilerDefines("__ORDER_BIG_ENDIAN__") \
diff --git a/build.txt b/build.txt
index 41f0c7db..811b6adc 100644
--- a/build.txt
+++ b/build.txt
@@ -58,10 +58,17 @@ arrange this by putting symlink in a local directory on $PATH.
 === C compiler ===
 
 C99 conformance is required in the compiler. The C code depends on one
-non-C99 feature (supported by GCC, clang, and pretty much any C
-compiler that also speaks C++): anonymous unions.  We could eliminate
-these, but the cost would be source-level interface breakage if we
-have to move structure members in and out of unions.
+C11 feature (supported by GCC, clang, and pretty much any C compiler
+that also speaks C++): anonymous unions.  We could eliminate these,
+but the cost would be source-level interface breakage if we have to
+move structure members in and out of unions.
+
+Some portions of the code using shared-memory segments are improved by
+the C11 stdatomic.h features for lockless concurrency.  These are: the
+SHM export mode in shmexport.c, the code for writing clock corrections
+to NTP in ntpshm.c, and the code for reading NTP corrections in
+ntpshmread.c. These features have been supported in GCC since 4.7 and
+clang since 3.1.
 
 GPSD is normally built and tested with GCC. Do not compile with a version
 older than 4.1.1; there are several known issues with older versions,
@@ -71,9 +78,7 @@ generation, (c)  the option -Wno-missing-field-initializers is
 unavailable, leading to a flood of warnings (this is due to generated
 code and cannot be fixed).
 
-The shared-memory interface relies on one GCCism, but the code is
-otherwise pretty compiler-agnostic.  It is reported that clang
-produces a gpsd that passes all regression tests.
+clang produces a gpsd that passes all regression tests.
 
 === Python ===
 
diff --git a/compiler.h b/compiler.h
index bd73808a..f46b05f0 100644
--- a/compiler.h
+++ b/compiler.h
@@ -7,6 +7,8 @@
 #ifndef _GPSD_COMPILER_H_
 #define _GPSD_COMPILER_H_
 
+#include "gpsd_config.h"	/* is STD_ATOMIC_H defined? */
+
 /* Macro for declaring function with printf-like arguments. */
 # if __GNUC__ >= 3 || (__GNUC__ == 2 && __GNUC_MINOR__ >= 7)
 #define PRINTF_FUNC(format_index, arg_index) \
@@ -32,11 +34,9 @@
 static /*@unused@*/ inline void memory_barrier(void)
 {
 #ifndef S_SPLINT_S
-#if (__GNUC__ > 4) || (__GNUC__ == 4 && __GNUC_MINOR__ >= 7)
-       __atomic_thread_fence(__ATOMIC_SEQ_CST);
-#elif (__GNUC__ == 4 && __GNUC_MINOR__ >= 4)
-       __sync_synchronize();
-#endif
+#ifdef STD_ATOMIC_H
+    atomic_thread_fence(memory_order_seq_cst);
+#endif /* STD_ATOMIC_H */
 #endif /* S_SPLINT_S */
 }
 
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