Add the BOOST_CONTEXT_USE_MAP_STACK logic to test_fcontext.cpp

Without this test_fcontext.cpp fails on OpenBSD with a not-on-stack OS error.

test/test_fcontext.cpp

@@ -23,6 +23,12 @@
 #include <boost/context/detail/config.hpp>
 #include <boost/context/detail/fcontext.hpp>
 
+#if defined(BOOST_CONTEXT_USE_MAP_STACK)
+extern "C" {
+#include <sys/mman.h>
+}
+#endif
+
 #define BOOST_CHECK(x) BOOST_TEST(x)
 #define BOOST_CHECK_EQUAL(a, b) BOOST_TEST_EQ(a, b)
 
@@ -44,8 +50,17 @@ class simple_stack_allocator
         BOOST_ASSERT( minimum_stacksize() <= size);
         BOOST_ASSERT( maximum_stacksize() >= size);
 
-        void * limit = malloc( size);
-        if ( ! limit) throw std::bad_alloc();
+#if defined(BOOST_CONTEXT_USE_MAP_STACK)
+        void * limit = ::mmap( 0, size, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANON | MAP_STACK, -1, 0);
+        if ( limit == MAP_FAILED) {
+            throw std::bad_alloc();
+        }
+#else
+        void * limit = std::malloc( size);
+        if ( ! limit) {
+            throw std::bad_alloc();
+        }
+#endif
 
         return static_cast< char * >( limit) + size;
     }
@@ -57,7 +72,11 @@ class simple_stack_allocator
         BOOST_ASSERT( maximum_stacksize() >= size);
 
         void * limit = static_cast< char * >( vp) - size;
+#if defined(BOOST_CONTEXT_USE_MAP_STACK)
+	::munmap( vp, size);
+#else
         free( limit);
+#endif
     }
 };