[boost] [lexical_cast] Assumed internal double conversion of float?

Hi Boost experts, We are chasing down some test failures with floats on VxWorks using the Dinkum clib and c++lib. We are testing with clang on Intel and Arm targets and GCC on PowerPC targets. An example failure at the bottom of this email. We have determined that boost::detail::lcast_get_precision<float>() is returning 9, well in access of the defined FLT_DIG of 6, or the practical float limit of 6.92. Specifically this code here: bool shl_real_type(float val, char* begin) { using namespace std; const double val_as_double = val; finish = start + #if defined(_MSC_VER) && (_MSC_VER >= 1400) && !defined(__SGI_STL_PORT) && !defined(_STLPORT_VERSION) sprintf_s(begin, CharacterBufferSize, #else sprintf(begin, #endif "%.*g", static_cast<int>(boost::detail::lcast_get_precision<float>()), val_as_double); return finish > start; } Where does boost::detail::lcast_get_precision<float>() come from, and why is it 9 ? Is it perhaps assuming some intermediate double conversion internally? In Dinkum's C library, Plager conservatively does not promote to a double, when converting to a string. Many thanks, Brian Wind River ->cmd "LD_LIBRARY_PATH=/opt/TestTriggerLocalPath/Workspace/vx7-boost/2019-01-13/vsb/app/boost/fsl_imx6_llvm_up_32_vxWorks/usr/root/llvm/bin;/opt/TestTriggerLocalPath/Workspace/vx7-boost/2019-01-13/vsb/app/boost/fsl_imx6_llvm_up_32_vxWorks/usr/lib/common" [vxWorks *]# cd /opt/TestTriggerLocalPath/Workspace/vx7-boost/2019-01-13/vsb/app/boost/fsl_imx6_llvm_up_32_vxWorks/3pp/BOOST/boost_1_68_0/status [vxWorks *]# run -x -t 0x01000000 -u 100000 -- ../bin.v2/libs/lexical_cast/test/lexical_cast_float_types_test.test/clang-vxworks/debug/cross-compile-vxworks/static-only-on/lexical_cast_float_types_test.vxe Launching process '../bin.v2/libs/lexical_cast/test/lexical_cast_float_types_test.test/clang-vxworks/debug/cross-compile-vxworks/static-only-on/lexical_cast_float_types_test.vxe' ... Process '../bin.v2/libs/lexical_cast/test/lexical_cast_float_types_test.test/clang-vxworks/debug/cross-compile-vxworks/static-only-on/lexical_cast_float_types_test.vxe' (process Id = 0x20482f10) launched. Running 3 test cases... unknown location(0): [4;31;49mfatal error: in "lexical_cast float types unit test/test_conversion_from_to_double": boost::exception_detail::clone_implboost::bad_lexical_cast

: bad lexical cast: source type value could not be interpreted as target [0;39;49m ../libs/lexical_cast/test/lexical_cast_float_types_test.cpp(299): [1;36;49mlast checkpoint [0;39;49m unknown location(0): [4;31;49mfatal error: in "lexical_cast float types unit test/test_conversion_from_to_long_double": boost::exception_detail::clone_implboost::bad_lexical_cast : bad lexical cast: source type value could not be interpreted as target [0;39;49m ../libs/lexical_cast/test/lexical_cast_float_types_test.cpp(299): [1;36;49mlast checkpoint [0;39;49m