Add support for BOOST_SYSTEM_DISABLE_THREADS (refs #92)

include/boost/system/detail/config.hpp

@@ -12,8 +12,14 @@
 #include <boost/config/workaround.hpp>
 
 // BOOST_SYSTEM_HAS_SYSTEM_ERROR
+//
+// The macro BOOST_SYSTEM_DISABLE_THREADS can be defined on configurations
+// that provide <system_error> and <atomic>, but not <mutex>, such as the
+// single-threaded libstdc++.
+//
+// https://github.com/boostorg/system/issues/92
 
-#if !defined(BOOST_NO_CXX11_HDR_SYSTEM_ERROR) && !defined(BOOST_NO_CXX11_HDR_ATOMIC) && !defined(BOOST_NO_CXX11_HDR_MUTEX)
+#if !defined(BOOST_NO_CXX11_HDR_SYSTEM_ERROR) && !defined(BOOST_NO_CXX11_HDR_ATOMIC) && ( !defined(BOOST_NO_CXX11_HDR_MUTEX) || defined(BOOST_SYSTEM_DISABLE_THREADS) )
 # define BOOST_SYSTEM_HAS_SYSTEM_ERROR
 #endif
 

include/boost/system/detail/error_category_impl.hpp

@@ -98,14 +98,19 @@ inline char const * error_category::message( int ev, char * buffer, std::size_t
 #if defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
 
 #include <boost/system/detail/std_category_impl.hpp>
-#include <mutex>
 #include <new>
 
+#if !defined(BOOST_SYSTEM_DISABLE_THREADS)
+# include <mutex>
+#endif
+
 namespace boost
 {
 namespace system
 {
 
+#if !defined(BOOST_SYSTEM_DISABLE_THREADS)
+
 namespace detail
 {
 
@@ -118,6 +123,8 @@ template<class T> std::mutex stdcat_mx_holder<T>::mx_;
 
 } // namespace detail
 
+#endif
+
 inline void error_category::init_stdcat() const
 {
     static_assert( sizeof( stdcat_ ) >= sizeof( boost::system::detail::std_category ), "sizeof(stdcat_) is not enough for std_category" );
@@ -130,7 +137,9 @@ inline void error_category::init_stdcat() const
 
 #endif
 
+#if !defined(BOOST_SYSTEM_DISABLE_THREADS)
     std::lock_guard<std::mutex> lk( boost::system::detail::stdcat_mx_holder<>::mx_ );
+#endif
 
     if( sc_init_.load( std::memory_order_acquire ) == 0 )
     {