libipm: Add support for receiving file descriptors

libipm/libipm.h

@@ -300,8 +300,8 @@ libipm_msg_in_peek_type(struct trans *trans);
  *   x  | int64_t *  | Signed (two's complement) 64-bit integer
  *   t  | uint64_t * | Unsigned 64-bit integer
  *   s  | char **    | NULL-terminated string
+ *   h  | int *      | File descriptor
  *   d  |   -        | (reserved)
- *   h  |   -        | (reserved)
  *   o  |   -        | (reserved)
  *   g  |   -        | (reserved)
  *
@@ -315,6 +315,9 @@ libipm_msg_in_peek_type(struct trans *trans);
  * returned. This pointer will only be valid until the next call to
  * libipm_msg_in_reset()
  *
+ * The 'h' type can only be used where the underlying transport is a
+ * UNIX domain socket.
+ *
  * For the 'B' type, pass in the address of an initialised descriptor
  * containing the address and size of the object to copy the data
  * to. The size in the descriptor must match the size of the object
@@ -330,6 +333,9 @@ libipm_msg_in_parse(struct trans *trans, const char *format, ...);
  * If the LIBIPM_E_MSG_IN_ERASE_AFTER_USE flag is set for the transport,
  * the entire buffer is erased, and the flag is cleared
  *
+ * Any file descriptors received from the other end but not parsed
+ * by the application are closed.
+ *
  * @param trans libipm transport
  */
 void