lib: xpc: implement writing to local buffers

lib/libxpc/include/xpc/buffer.h

@@ -18,6 +18,13 @@
 		.buf_offset = (offset),                                        \
 		.buf_len = (size),                                             \
 	}
+#define XPC_LOCAL_BUFFER_OUT(ptr, size)                                        \
+	{                                                                      \
+		.buf_flags = XPC_BUFFER_F_OUT,                                 \
+		.buf_ptr = (ptr),                                              \
+		.buf_max = (size),                                             \
+		.buf_len = (size),                                             \
+	}
 
 struct xpc_msg;
 
@@ -49,7 +56,7 @@ typedef struct xpc_buffer {
 			/* only valid if F_OUT is set.
 			 * if F_FREE_ON_DISCARD is set, must be either NULL or
 			 * allocated via xpc_context_alloc */
-			const char *buf_ptr;
+			void *buf_ptr;
 		};
 
 		/* fields that are only valid if F_IN is set */