update os layer

os_linux.cpp

@@ -0,0 +1,21 @@
+#ifndef OS_IMPL_LINUX_CPP
+#define OS_IMPL_LINUX_CPP
+
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include "os.h"
+
+void *os_alloc(size_t capacity) {
+    return mmap(0, capacity, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+}
+
+void os_reserve(void *ptr) {
+}
+
+void os_decommit(void *ptr) {
+}
+
+void os_free(void *ptr) {
+}
+
+#endif