Man Page: rmr_send_msg





#include <rmr/rmr.h>

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


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.)


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.


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.


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


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.


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


The message buffer pointer did not refer to a valid message.


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


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

The following values may be assigned to errno on failure.


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


The header information in the message buffer was invalid.


No known endpoint for the message could be found.


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


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


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


Sending was not supported by the underlying message transport.


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


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


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


The underlying message environment timed out during the send process.


The underlying message environment is in a shutdown state.


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;


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)