docs/rmr_send_msg.3.rst

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============================================================================================
Man Page: rmr_send_msg
============================================================================================




RMR LIBRARY FUNCTIONS
=====================



NAME
----

rmr_send_msg


SYNOPSIS
--------


::

  #include <rmr/rmr.h>

  rmr_mbuf_t* rmr_send_msg( void* vctx, rmr_mbuf_t* msg );



DESCRIPTION
-----------

The ``rmr_send_msg`` function accepts a message buffer from
the user application and attempts to send it. The destination
of the message is selected based on the message type
specified in the message buffer, and the matching information
in the routing tables which are currently in use by the RMR
library. This may actually result in the sending of the
message to multiple destinations which could degrade expected
overall performance of the user application. (Limiting
excessive sending of messages is the responsibility of the
application(s) responsible for building the routing table
used by the RMR library, and not the responsibility of the
library.)


Retries
-------

The send operations in RMR will retry *soft* send failures
until one of three conditions occurs:


* The message is sent without error

* The underlying transport reports a *hard* failure

* The maximum number of retry loops has been attempted


A retry loop consists of approximately 1000 send attempts
**without** any intervening calls to *sleep()* or *usleep().*
The number of retry loops defaults to 1, thus a maximum of
1000 send attempts is performed before returning to the user
application. This value can be set at any point after RMR
initialisation using the *rmr_set_stimeout()* function
allowing the user application to completely disable retires
(set to 0), or to increase the number of retry loops.


Transport Level Blocking
------------------------

The underlying transport mechanism used to send messages is
configured in *non-blocking* mode. This means that if a
message cannot be sent immediately the transport mechanism
will **not** pause with the assumption that the inability to
send will clear quickly (within a few milliseconds). This
means that when the retry loop is completely disabled (set to
0), that the failure to accept a message for sending by the
underlying mechanisms (software or hardware) will be reported
immediately to the user application.

It should be noted that depending on the underlying transport
mechanism being used, it is extremely likely that retry
conditions will happen during normal operations. These are
completely out of RMR's control, and there is nothing that
RMR can do to avoid or mitigate these other than by allowing
RMR to retry the send operation, and even then it is possible
(e.g., during connection reattempts), that a single retry
loop is not enough to guarantee a successful send.


RETURN VALUE
------------

On success, a new message buffer, with an empty payload, is
returned for the application to use for the next send. The
state in this buffer will reflect the overall send operation
state and will be ``RMR_OK`` when the send was successful.

When the message cannot be successfully sent this function
will return the unsent (original) message buffer with the
state set to indicate the reason for failure. The value of
*errno* may also be set to reflect a more detailed failure
reason if it is known.

In the event of extreme failure, a nil pointer is returned.
In this case the value of ``errno`` might be of some use, for
documentation, but there will be little that the user
application can do other than to move on.

**CAUTION:** In some cases it is extremely likely that the
message returned by the send function does **not** reference
the same memory structure. Thus is important for the user
programme to capture the new pointer for future use or to be
passed to ``rmr_free().`` If you are experiencing either
double free errors or segment faults in either
``rmr_free()`` or ``rmr_send_msg(),`` ensure that the return
value from this function is being captured and used.


ERRORS
------

The following values may be passed back in the *state* field
of the returned message buffer.


    .. list-table::
      :widths: auto
      :header-rows: 0
      :class: borderless

      * - **RMR_RETRY**
        -
          The message could not be sent, but the underlying transport
          mechanism indicates that the failure is temporary. If the
          send operation is tried again it might be successful.

      * - **RMR_SEND_FAILED**
        -
          The send operation was not successful and the underlying
          transport mechanism indicates a permanent (hard) failure;
          retrying the send is not possible.

      * - **RMR_ERR_BADARG**
        -
          The message buffer pointer did not refer to a valid message.

      * - **RMR_ERR_NOHDR**
        -
          The header in the message buffer was not valid or corrupted.

      * - **RMR_ERR_NOENDPT**
        -
          The message type in the message buffer did not map to a known
          endpoint.



The following values may be assigned to ``errno`` on failure.

    .. list-table::
      :widths: auto
      :header-rows: 0
      :class: borderless

      * - **INVAL**
        -
          Parameter(s) passed to the function were not valid, or the
          underlying message processing environment was unable to
          interpret the message.

      * - **ENOKEY**
        -
          The header information in the message buffer was invalid.

      * - **ENXIO**
        -
          No known endpoint for the message could be found.

      * - **EMSGSIZE**
        -
          The underlying transport refused to accept the message
          because of a size value issue (message was not attempted to
          be sent).

      * - **EFAULT**
        -
          The message referenced by the message buffer is corrupt (nil
          pointer or bad internal length).

      * - **EBADF**
        -
          Internal RMR error; information provided to the message
          transport environment was not valid.

      * - **ENOTSUP**
        -
          Sending was not supported by the underlying message
          transport.

      * - **EFSM**
        -
          The device is not in a state that can accept the message.

      * - **EAGAIN**
        -
          The device is not able to accept a message for sending. The
          user application should attempt to resend.

      * - **EINTR**
        -
          The operation was interrupted by delivery of a signal before
          the message was sent.

      * - **ETIMEDOUT**
        -
          The underlying message environment timed out during the send
          process.

      * - **ETERM**
        -
          The underlying message environment is in a shutdown state.




EXAMPLE
-------

The following is a simple example of how the
``rmr_send_msg`` function is called. In this example, the
send message buffer is saved between calls and reused
eliminating alloc/free cycles.


::

      static rmr_mbuf_t*  send_msg = NULL;        // message to send; reused on each call
      msg_t*  send_pm;                            // payload for send
      msg_t*  pm;                                 // our message format in the received payload

      if( send_msg  == NULL ) {
          send_msg = rmr_alloc_msg( mr, MAX_SIZE ); // new buffer to send
      }

      // reference payload and fill in message type
      pm = (msg_t*) send_msg->payload;
      send_msg->mtype = MT_ANSWER;

      msg->len = generate_data( pm );       // something that fills the payload in
      msg = rmr_send_msg( mr, send_msg );   // ensure new pointer used after send
      if( ! msg ) {
          return ERROR;
      } else {
          if( msg->state != RMR_OK ) {
              // check for RMR_ERR_RETRY, and resend if needed
              // else return error
          }
      }
      return OK;




SEE ALSO
--------

rmr_alloc_msg(3), rmr_call(3), rmr_free_msg(3), rmr_init(3),
rmr_payload_size(3), rmr_rcv_msg(3), rmr_rcv_specific(3),
rmr_rts_msg(3), rmr_ready(3), rmr_mk_ring(3),
rmr_ring_free(3), rmr_torcv_rcv(3), rmr_wh_send_msg(3)