posix: wrap mmap() for anonymous maps

Automatically munmap() when the result is destroyed.

core/posix.cc

@@ -3,6 +3,17 @@
  */
 
 #include "posix.hh"
+#include <sys/mman.h>
+
+void mmap_deleter::operator()(void* ptr) const {
+    ::munmap(ptr, _size);
+}
+
+mmap_area mmap_anonymous(void* addr, size_t length, int prot, int flags) {
+    auto ret = ::mmap(addr, length, prot, flags | MAP_ANONYMOUS, -1, 0);
+    throw_system_error_on(ret == MAP_FAILED);
+    return mmap_area(reinterpret_cast<char*>(ret), mmap_deleter{length});
+}
 
 void* posix_thread::start_routine(void* arg) {
     auto pfunc = reinterpret_cast<std::function<void ()>*>(arg);

core/posix.hh

@@ -185,6 +185,15 @@ private:
     file_desc(int fd) : _fd(fd) {}
  };
 
+struct mmap_deleter {
+    size_t _size;
+    void operator()(void* ptr) const;
+};
+
+using mmap_area = std::unique_ptr<char[], mmap_deleter>;
+
+mmap_area mmap_anonymous(void* addr, size_t length, int prot, int flags);
+
 class posix_thread {
     // must allocate, since this class is moveable
     std::unique_ptr<std::function<void ()>> _func;