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22 Mnemonic rmr_mt_call_man.xfm
23 Abstract The manual page for the rmr multi-threaded call function.
24 Author E. Scott Daniels
29 .im &{lib}/man/setup.im
33 &h1(RMR Library Functions)
42 extern rmr_mbuf_t* rmr_mt_call( void* vctx, rmr_mbuf_t* msg, int id, int timeout );
47 The &cw(rmr_mt_call) function sends the user application message to a remote
48 endpoint, and waits for a corresponding response message before returning
49 control to the user application.
50 The user application supplies a completed message buffer, as it would for
51 a &cw(rmr_send_msg) call, but unlike with a send, the buffer returned will have
52 the response from the application that received the message.
53 The thread invoking the &ital(rmr_mt_call()) will block until a message arrives
54 or until &ital(timeout) milliseconds has passed; which ever comes first.
55 Using a timeout value of zero (0) will cause the thread to block without a timeout.
58 The &ital(id) supplied as the third parameter is an integer in the range of 2 through
60 This is a caller defined "thread number" and is used to match the response message
61 with the correct user application thread.
62 If the ID value is not in the proper range, the attempt to make the call will fail.
65 Messages which are received while waiting for the response are queued on a &ital(normal)
66 receive queue and will be delivered to the user application with the next invocation
67 of &ital(rmr_mt_rcv()) or &ital(rmr_rvv_msg().)
68 by RMR, and are returned to the user application when &cw(rmr_rcv_msg) is
70 These messages are returned in the order received, one per call to &cw(rmr_rcv_msg.)
72 &h3(The Transaction ID)
73 The user application is responsible for setting the value of the transaction ID field
74 before invoking &ital(rmr_mt_call.)
75 The transaction ID is a &cw(RMR_MAX_XID) byte field that is used to match the
76 response message when it arrives.
77 RMR will compare &bold(all) of the bytes in the field, so the caller must ensure
78 that they are set correctly to avoid missing the response message.
79 The application which returns the response message is also expected to ensure that
80 the return buffer has the matching transaction ID. This can be done transparently if
81 the application uses the &ital(rmr_rts_msg()) function and does not adjust the
84 .** pull in common retry text
85 .im &{lib}/man/retry.im
88 The &cw(rmr_mt_call) function returns a pointer to a message buffer with the state set to reflect
89 the overall state of call processing.
90 If the state is &cw(RMR_OK) then the buffer contains the response message; otherwise
91 the state indicates the error encountered while attempting to send the message.
94 If no response message is received when the timeout period has expired, a nil pointer
95 will be returned (NULL).
98 These values are reflected in the state field of the returned message.
101 &beg_dlist(.75i : ^&bold_font )
102 &di(RMR_OK) The call was successful and the message buffer references the response message.
105 &di(RMR_ERR_BADARG) An argument passed to the function was invalid.
108 &di(RMR_ERR_CALLFAILED) The call failed and the value of &ital(errno,) as described below,
109 should be checked for the specific reason.
112 &di(RMR_ERR_NOENDPT) An endpoint associated with the message type could not be found in the
116 &di(RMR_ERR_RETRY) The underlying transport mechanism was unable to accept the message
117 for sending. The user application can retry the call operation if appropriate to
123 The global "variable" &ital(errno) will be set to one of the following values if the
124 overall call processing was not successful.
127 &beg_dlist(.75i : ^&bold_font )
128 &di(ETIMEDOUT) Too many messages were queued before receiving the expected response
131 &di(ENOBUFS) The queued message ring is full, messages were dropped
134 &di(EINVAL) A parameter was not valid
137 &di(EAGAIN) The underlying message system wsa interrupted or the device was busy;
138 the message was &bold(not) sent, and user application should call
139 this function with the message again.
143 The following code bit shows one way of using the &cw(rmr_mt_call) function, and illustrates
144 how the transaction ID must be set.
148 int retries_left = 5; // max retries on dev not available
149 static rmr_mbuf_t* mbuf = NULL; // response msg
150 msg_t* pm; // private message (payload)
152 // get a send buffer and reference the payload
153 mbuf = rmr_alloc_msg( mr, RMR_MAX_RCV_BYTES );
154 pm = (msg_t*) mbuf->payload;
156 // generate an xaction ID and fill in payload with data and msg type
157 rmr_bytes2xact( mbuf, xid, RMR_MAX_XID );
158 snprintf( pm->req, sizeof( pm->req ), "{ \"req\": \"num users\"}" );
159 mbuf->mtype = MT_USR_RESP;
161 msg = rmr_mt_call( mr, msg, my_id, 100 ); // wait up to 100ms
162 if( ! msg ) { // probably a timeout and no msg received
163 return NULL; // let errno trickle up
166 if( mbuf->state != RMR_OK ) {
167 while( retries_left-- > 0 && // loop as long as eagain
168 mbuf->state == RMR_ERR_RETRY &&
169 (msg = rmr_mt_call( mr, msg )) != NULL &&
170 mbuf->state != RMR_OK ) {
172 usleep( retry_delay );
175 if( mbuf == NULL || mbuf->state != RMR_OK ) {
176 rmr_free_msg( mbuf ); // safe if nil
181 // do something with mbuf