Add native_handle_init

HIDL requires file descriptors to be wrapped in native_handle_t.  We want
a low overhead way to do that when the number of file descriptors is known
at compile time.  Instead of

  // wrap an fd in native_handle_t
  native_handle_t* fd_handle = native_handle_create(1, 0);
  if (!fd_handle) {
    // clean up and return error
  }
  fd_handle->data[0] = fd;

  hidl_cb(..., fd_handle);

  native_handle_delete(fd_handle);

this change adds native_handle_init to allow for

  // wrap an fd in native_handle_t
  NATIVE_HANDLE_DECLARE_STORAGE(fd_storage, 1, 0);
  native_handle_t* fd_handle = native_handle_init(fd_storage, 1, 0);
  fd_handle->data[0] = fd;

  hidl_cb(..., fd_handle);

Test: make libcutils
Bug: 32021609
Change-Id: If1fd07482243d37492fdea57c602a1b13c8953cc

1 files changed, 15 insertions, 0 deletions

diff --git a/include/cutils/native_handle.h b/include/cutils/native_handle.h
index 813dadc7c..7d6a98802 100644
--- a/include/cutils/native_handle.h
+++ b/include/cutils/native_handle.h
@@ -17,10 +17,17 @@
 #ifndef NATIVE_HANDLE_H_
 #define NATIVE_HANDLE_H_
 
+#include <stdalign.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif
 
+/* Declare a char array for use with native_handle_init */
+#define NATIVE_HANDLE_DECLARE_STORAGE(name, maxFds, maxInts) \
+    alignas(native_handle_t) char name[                            \
+      sizeof(native_handle_t) + sizeof(int) * (maxFds + maxInts)]
+
 typedef struct native_handle
 {
     int version;        /* sizeof(native_handle_t) */
@@ -46,6 +53,14 @@ typedef struct native_handle
  */
 int native_handle_close(const native_handle_t* h);
 
+/*
+ * native_handle_init
+ *
+ * Initializes a native_handle_t from storage.  storage must be declared with
+ * NATIVE_HANDLE_DECLARE_STORAGE.  numFds and numInts must not respectively
+ * exceed maxFds and maxInts used to declare the storage.
+ */
+native_handle_t* native_handle_init(char* storage, int numFds, int numInts);
 
 /*
  * native_handle_create