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NAME
     tt_message_callback_add - register a callback function

SYNOPSIS
     #include <Tt/tt_c.h>
     Tt_status tt_message_callback_add(Tt_message m,
                                       Tt_message_callback f);

DESCRIPTION
     The tt_message_callback_add() function registers a  callback
     function      to     be     automatically     invoked     by
     tt_message_receive(3) whenever a reply or other state-change
     to this message is returned.

     The callback is defined in  <Tt/tt_c.h>.   If  the  callback
     returns  TT_CALLBACK_CONTINUE,  other callbacks will be run;
     if    no     callback     returns     TT_CALLBACK_PROCESSED,
     tt_message_receive()  returns  the message.  If the callback
     returns  TT_CALLBACK_PROCESSED,  no  further  callbacks  are
     invoked for this event; tt_message_receive() does not return
     the message.

     The m argument is the opaque handle for the message involved
     in  this  operation.  The f argument is the message callback
     to be run.

     The pattern handle will be NULL if the message did not match
     a  dynamic  pattern.   This  is usually the case for message
     callbacks.

RETURN VALUE
     Upon successful  completion,  the  tt_message_callback_add()
     function  returns  the status of the operation as one of the
     following Tt_status values:

        TT_OK   The operation completed successfully.

        TT_ERR_NOMP
                The ttsession(1) process is not running  and  the
                ToolTalk service cannot restart it.

        TT_ERR_POINTER
                The pointer passed does not point to an object of
                the correct type for this operation.

APPLICATION USAGE
     These callbacks are invoked from  tt_message_receive();  the
     program  must, therefore, call tt_message_receive() when the
     file descriptor returned by tt_fd() becomes active.

     The application can use tt_message_callback_add() to  create
     wrappers  for  ToolTalk  messages.   For  example, a library
     routine can construct a request, attach a  callback  to  the
     message,  send  the  message,  and  process the reply in the
     callback.  When the callback returns  TT_CALLBACK_PROCESSED,
     the  message  reply is not returned to the main program; the
     message and reply are, therefore, completely hidden.

SEE ALSO
     tt_c(5), tt_message_receive(3).

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