Man Page: rmr_alloc_msg





#include <rmr/rmr.h>

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


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;



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.


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.


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.


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.


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.


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.


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.


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



Unable to allocate memory.


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)