Merge pull request #50 from nmcclatchey/winver_check

Descriptive error on insufficient Windows version.

mingw.condition_variable.h

@@ -39,6 +39,10 @@
 #include "mingw.mutex.h"
 #include "mingw.shared_mutex.h"
 
+#if !defined(_WIN32_WINNT) || (_WIN32_WINNT < 0x0501)
+#error To use the MinGW-std-threads library, you will need to define the macro _WIN32_WINNT to be 0x0501 (Windows XP) or higher.
+#endif
+
 namespace mingw_stdthread
 {
 #if defined(__MINGW32__ ) && !defined(_GLIBCXX_HAS_GTHREADS)
@@ -280,7 +284,8 @@ class condition_variable
 
     bool wait_impl (unique_lock<xp::mutex> & lock, DWORD time)
     {
-        static_assert(std::is_same<typename xp::mutex::native_handle_type, PCRITICAL_SECTION>::value,
+        using mutex_handle_type = typename xp::mutex::native_handle_type;
+        static_assert(std::is_same<mutex_handle_type, PCRITICAL_SECTION>::value,
                       "Native Win32 condition variable requires std::mutex to \
 use native Win32 critical section objects.");
         xp::mutex * pmutex = lock.release();
@@ -298,7 +303,13 @@ use native Win32 critical section objects.");
         before_wait(pmutex);
         BOOL success = SleepConditionVariableSRW( native_handle(),
                                                   pmutex->native_handle(),
-                                                  time, 0);
+                                                  time,
+//    CONDITION_VARIABLE_LOCKMODE_SHARED has a value not specified by
+//  Microsoft's Dev Center, but is known to be (convertible to) a ULONG. To
+//  ensure that the value passed to this function is not equal to Microsoft's
+//  constant, we can either use a static_assert, or simply generate an
+//  appropriate value.
+                                           !CONDITION_VARIABLE_LOCKMODE_SHARED);
         after_wait(pmutex);
         return success;
     }
@@ -414,10 +425,6 @@ class condition_variable_any
 //    Some shared_mutex functionality is available even in Vista, but it's not
 //  until Windows 7 that a full implementation is natively possible. The class
 //  itself is defined, with missing features, at the Vista feature level.
-    static_assert(CONDITION_VARIABLE_LOCKMODE_SHARED != 0, "The flag \
-CONDITION_VARIABLE_LOCKMODE_SHARED is not defined as expected. The value for \
-exclusive mode is unknown (not specified by Microsoft Dev Center), but assumed \
-to be 0. There is a conflict with CONDITION_VARIABLE_LOCKMODE_SHARED.");
     bool wait_impl (unique_lock<native_shared_mutex> & lock, DWORD time)
     {
         native_shared_mutex * pmutex = lock.release();

mingw.mutex.h

@@ -48,6 +48,10 @@
 //  Need for the implementation of invoke
 #include "mingw.thread.h"
 
+#if !defined(_WIN32_WINNT) || (_WIN32_WINNT < 0x0501)
+#error To use the MinGW-std-threads library, you will need to define the macro _WIN32_WINNT to be 0x0501 (Windows XP) or higher.
+#endif
+
 namespace mingw_stdthread
 {
 //    The _NonRecursive class has mechanisms that do not play nice with direct
@@ -161,6 +165,7 @@ class mutex
     mutex & operator= (const mutex&) = delete;
     void lock (void)
     {
+//  Note: Undefined behavior if called recursively.
 #if STDMUTEX_RECURSION_CHECKS
         DWORD self = mOwnerThread.checkOwnerBeforeLock();
 #endif
@@ -216,6 +221,9 @@ class mutex
     mutex & operator= (const mutex&) = delete;
     ~mutex() noexcept
     {
+//    Undefined behavior if the mutex is held (locked) by any thread.
+//    Undefined behavior if a thread terminates while holding ownership of the
+//  mutex.
         DeleteCriticalSection(&mHandle);
     }
     void lock (void)

mingw.shared_mutex.h

@@ -55,6 +55,10 @@
 //  Might be able to use native Slim Reader-Writer (SRW) locks.
 #ifdef _WIN32
 #include <windows.h>
+
+#if !defined(_WIN32_WINNT) || (_WIN32_WINNT < 0x0501)
+#error To use the MinGW-std-threads library, you will need to define the macro _WIN32_WINNT to be 0x0501 (Windows XP) or higher.
+#endif
 #endif
 
 namespace mingw_stdthread

mingw.thread.h

@@ -50,6 +50,10 @@
 #include <cstdio>
 #endif
 
+#if !defined(_WIN32_WINNT) || (_WIN32_WINNT < 0x0501)
+#error To use the MinGW-std-threads library, you will need to define the macro _WIN32_WINNT to be 0x0501 (Windows XP) or higher.
+#endif
+
 //  Instead of INVALID_HANDLE_VALUE, _beginthreadex returns 0.
 namespace mingw_stdthread
 {
@@ -220,7 +224,14 @@ class thread
     static unsigned int _hardware_concurrency_helper() noexcept
     {
         SYSTEM_INFO sysinfo;
+//    This is one of the few functions used by the library which has a nearly-
+//  equivalent function defined in earlier versions of Windows. Include the
+//  workaround, just as a reminder that it does exist.
+#if defined(_WIN32_WINNT) && (_WIN32_WINNT >= 0x0501)
         ::GetNativeSystemInfo(&sysinfo);
+#else
+        ::GetSystemInfo(&sysinfo);
+#endif
         return sysinfo.dwNumberOfProcessors;
     }
 public:
@@ -259,6 +270,11 @@ class thread
     }
 
     bool joinable() const {return mHandle != kInvalidHandle;}
+
+//    Note: Due to lack of synchronization, this function has a race condition
+//  if called concurrently, which leads to undefined behavior. The same applies
+//  to all other member functions of this class, but this one is mentioned
+//  explicitly.
     void join()
     {
         using namespace std;