boost: context needs thread if gcc <= 4.6

mutex is available on gcc 4.7.x, the issue is that boost does not correctly detect it because -std=c++11 is missing.
Keep thread select for gcc version lower or equal to 4.6

---
Changes v1 -> v2 (after review of Thomas Petazzoni):
- Update first patch to keep selection of boost-thread but lower gcc version from 4.8 to 4.7
- Update second patch to add -std=c++11 only if gcc >= 4.7

 package/boost/Config.in | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 965d198ebf..627cf2f356 100644
@@ -83,7 +83,7 @@ config BR2_PACKAGE_BOOST_CONTEXT
 	bool "boost-context"
 	depends on BR2_PACKAGE_BOOST_CONTEXT_ARCH_SUPPORTS
 	depends on !BR2_TOOLCHAIN_HAS_GCC_BUG_64735
-	select BR2_PACKAGE_BOOST_THREAD if !BR2_TOOLCHAIN_GCC_AT_LEAST_4_8
+	select BR2_PACKAGE_BOOST_THREAD if !BR2_TOOLCHAIN_GCC_AT_LEAST_4_7
 	help
 	  C++11 context switching library.
 

2.14.1

Without this flag, mutex is not always correctly detected with gcc 4.x or 5.x. As a result, boost_thread is wrongly selected by boost_context.

Fixes:
- http://autobuild.buildroot.net/results/fb046c04fe18bec973d120e4ab33971f32ba5769
- http://autobuild.buildroot.net/results/99f1a255b78a973faeb8bf3b94d78efc54426a8b

---
Changes v1 -> v2 (after review of Thomas Petazzoni):
- Update first patch to keep selection of boost-thread but lower gcc version from 4.8 to 4.7
- Update second patch to add -std=c++11 only if gcc >= 4.7

 package/boost/boost.mk | 5 +++++
 1 file changed, 5 insertions(+)

index 14f6162003..f4a44fa123 100644
@@ -61,6 +61,11 @@ else
 BOOST_FLAGS += --without-icu
 endif
 
+# boost-context needs C++11
+ifeq ($(BR2_PACKAGE_BOOST_CONTEXT)$(BR2_TOOLCHAIN_GCC_AT_LEAST_4_7),yy)
+BOOST_TARGET_CXXFLAGS += -std=c++11
+endif
+
 ifeq ($(BR2_PACKAGE_BOOST_IOSTREAMS),y)
 BOOST_DEPENDENCIES += bzip2 zlib
 endif

2.14.1

Hello,

Where did you get the information that mutex is available since gcc 4.7 ? Is it by testing gcc 4.6 and 4.7 ?

I couldn't get this information by looking at
https://gcc.gnu.org/projects/cxx-status.html#cxx11.

Also, I am wondering if the two patches should really be separate. Aren't they in fact both intimately related, where std::mutex will only be visible in gcc >= 4.7 if std=c++11 is passed ?

To me, they are basically one single patch:

- Below gcc 4.7, we don't have std::mutex, so boost-thread must be used
- Above gcc 4.7, we have std::mutex, but we need to pass std=c++11 for std::mutex to be visible/usable

Thanks,

Thomas

Thomas Petazzoni, CTO, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com

Dear Thomas,

Le mar. 21 ao?t 2018 ? 11:27, Thomas Petazzoni <[email protected]> a ?crit :

I tested with 4.7. but std::mutex is very old, it exists since gcc 4.4 (see history of
https://github.com/gcc-mirror/gcc/blob/gcc-4_7_0-release/libstdc%2B%2B-v3/include/std/mutex
).
As a result, it probably also works on gcc 4.4, 4.5 and 4.6 but we'll have to add std=c++0x instead of std=c++11. However, it seems that the current best practice in buildroot is to add a dependency on gcc 4.7 (poppler), 4.8 (mongodb) or 4.9 (upmpdcli) for C+11 packages (depending on the C++11 features required by the package). We're also adding std=c++11 for 6 C++11 packages (flatbuffers, libcrossguid, libmediaart, libv4l, mosh, ...) but we never add std=c++0x (except for host-kodi).

OK I will merge both patches.

Best Regards,

Fabrice
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