--- old/src/os/posix/vm/os_posix.cpp	2015-06-15 21:33:53.433100971 +0200
+++ new/src/os/posix/vm/os_posix.cpp	2015-06-15 21:33:53.277095730 +0200
@@ -1015,3 +1015,61 @@
     }
   }
 }
+
+#define assert_with_errno(cond, msg)                                             \
+  do {                                                                           \
+    int err = errno;                                                             \
+    assert(cond, err_msg("%s; error='%s' (errno=%d)", msg, strerror(err), err)); \
+} while (false)
+
+// POSIX unamed semaphores are not supported on OS X.
+#ifndef __APPLE__
+
+Semaphore::Semaphore(uint value, uint /* max */) {
+  int ret = sem_init(&_semaphore, 0, value);
+
+  assert_with_errno(ret == 0, "Failed to initialize semaphore");
+}
+
+Semaphore::~Semaphore() {
+  sem_destroy(&_semaphore);
+}
+
+void Semaphore::signal() {
+  int ret = sem_post(&_semaphore);
+
+  assert_with_errno(ret == 0, "sem_post failed");
+}
+
+void Semaphore::signal(uint count) {
+  for (uint i = 0; i < count; i++) {
+    signal();
+  }
+}
+
+void Semaphore::wait() {
+  while (sem_wait(&_semaphore) == -1 && errno == EINTR) {
+    // Retry if the wait was interrupted by a signal.
+  }
+}
+
+bool os::PosixSemaphore::trywait() {
+  return sem_trywait(&_semaphore) == 0;
+}
+
+bool os::PosixSemaphore::timedwait(const struct timespec ts) {
+  while (1) {
+    int result = sem_timedwait(&_semaphore, &ts);
+    if (result == 0) {
+      return true;
+    } else if (errno == EINTR) {
+      continue;
+    } else if (errno == ETIMEDOUT) {
+      return false;
+    } else {
+      return false;
+    }
+  }
+}
+
+#endif // __APPLE__