show errno on FiberError ref #8711

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42270 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

cont.c

@@ -568,9 +568,10 @@ fiber_machine_stack_alloc(size_t size)
 	void *page;
 	STACK_GROW_DIR_DETECTION;
 
+	errno = 0;
 	ptr = mmap(NULL, size, PROT_READ | PROT_WRITE, FIBER_STACK_FLAGS, -1, 0);
 	if (ptr == MAP_FAILED) {
-	    rb_raise(rb_eFiberError, "can't alloc machine stack to fiber");
+	    rb_raise(rb_eFiberError, "can't alloc machine stack to fiber errno: %d", errno);
 	}
 
 	/* guard page setup */