docs/rmr_alloc_msg.3.rst

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============================================================================================
Man Page: rmr_alloc_msg
============================================================================================




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



NAME
----

rmr_alloc_msg


SYNOPSIS
--------


::

  #include <rmr/rmr.h>

  rmr_mbuf_t* rmr_alloc_msg( void* ctx, int size );



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

The ``rmr_alloc_msg`` function is used to allocate a buffer
which the user programme can write into and then send through
the RMR library. The buffer is allocated such that sending it
requires no additional copying out of the buffer. If the
value passed in ``size`` is less than or equal to 0, then the
*normal maximum size* supplied on the *rmr_init* call will be
used. When *size* is greater than zero, the message allocated
will have at least the indicated number of bytes in the
payload. There is no maximum size imposed by RMR, however the
underlying system memory management (e.g. malloc) functions
may impose a limit.

The *ctx* parameter is the void context pointer that was
returned by the *rmr_init* function.

The pointer to the message buffer returned is a structure
which has some user application visible fields; the structure
is described in ``rmr.h,`` and is illustrated below.


::

  typedef struct {
      int state;
      int mtype;
      int len;
      unsigned char* payload;
      unsigned char* xaction;
      int sub_id;
      int tp_state;
  } rmr_mbuf_t;


Where:

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

      * - **state**
        -
          Is the current buffer state. Following a call to
          ``rmr_send_msg`` the state indicates whether the buffer was
          successfully sent which determines exactly what the payload
          points to. If the send failed, the payload referenced by the
          buffer is the message that failed to send (allowing the
          application to attempt a retransmission). When the state is
          ``RMR_OK`` the buffer represents an empty buffer that the
          application may fill in in preparation to send.

      * - **mtype**
        -
          When sending a message, the application is expected to set
          this field to the appropriate message type value (as
          determined by the user programme). Upon send this value
          determines how the RMR library will route the message. For a
          buffer which has been received, this field will contain the
          message type that was set by the sending application.

      * - **len**
        -
          The application using a buffer to send a message is expected
          to set the length value to the actual number of bytes that it
          placed into the message. This is likely less than the total
          number of bytes that the message can carry. For a message
          buffer that is passed to the application as the result of a
          receive call, this will be the value that the sending
          application supplied and should indicate the number of bytes
          in the payload which are valid.

      * - **payload**
        -
          The payload is a pointer to the actual received data. The
          user programme may read and write from/to the memory
          referenced by the payload up until the point in time that the
          buffer is used on a ``rmr_send, rmr_call`` or
          ``rmr_reply`` function call. Once the buffer has been passed
          back to a RMR library function the user programme should
          **NOT** make use of the payload pointer.

      * - **xaction**
        -
          The *xaction* field is a pointer to a fixed sized area in the
          message into which the user may write a transaction ID. The
          ID is optional with the exception of when the user
          application uses the ``rmr_call`` function to send a message
          and wait for the reply; the underlying RMR processing expects
          that the matching reply message will also contain the same
          data in the *xaction* field.

      * - **sub_id**
        -
          This value is the subscription ID. It, in combination with
          the message type is used by rmr to determine the target
          endpoint when sending a message. If the application to
          application protocol does not warrant the use of a
          subscription ID, the RMR constant RMR_VOID_SUBID should be
          placed in this field. When an application is forwarding or
          returning a buffer to the sender, it is the application's
          responsibility to set/reset this value.

      * - **tp_state**
        -
          For C applications making use of RMR, the state of a
          transport based failure will often be available via
          ``errno.`` However, some wrapper environments may not have
          direct access to the C-lib ``errno`` value. RMR send and
          receive operations will place the current value of
          ``errno`` into this field which should make it available to
          wrapper functions. User applications are strongly cautioned
          against relying on the value of errno as some transport
          mechanisms may not set this value on all calls. This value
          should also be ignored any time the message status is
          ``RMR_OK.``




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

The function returns a pointer to a ``rmr_mbuf`` structure,
or NULL on error.


ERRORS
------


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

      * - **ENOMEM**
        -
          Unable to allocate memory.




SEE ALSO
--------

rmr_tralloc_msg(3), rmr_call(3), rmr_free_msg(3),
rmr_init(3), rmr_init_trace(3), rmr_get_trace(3),
rmr_get_trlen(3), rmr_payload_size(3), rmr_send_msg(3),
rmr_rcv_msg(3), rmr_rcv_specific(3), rmr_rts_msg(3),
rmr_ready(3), rmr_fib(3), rmr_has_str(3), rmr_tokenise(3),
rmr_mk_ring(3), rmr_ring_free(3), rmr_set_trace(3)