Man Page: rmr_realloc_payload





#include <rmr/rmr.h>

extern rmr_mbuf_t* rmr_realloc_payload( rmr_mbuf_t* msg, int new_len, int copy, int clone );


The rmr_realloc_payload function will return a pointer to an RMR message buffer struct (rmr_mbuf_t) which has a payload large enough to accomodate new_len bytes. If necessary, the underlying payload is reallocated, and the bytes from the original payload are copied if the copy parameter is true (1). If the message passed in has a payload large enough, there is no additional memory allocation and copying.

Cloning The Message Buffer

This function can also be used to generate a separate copy of the original message, with the desired payload size, without destroying the original message buffer or the original payload. A standalone copy is made only when the clone parameter is true (1). When cloning, the payload is copied to the cloned message only if the copy parameter is true.

Message Buffer Metadata

The metadata in the original message buffer (message type, subscription ID, and payload length) will be preserved if the copy parameter is true. When this parameter is not true (0), then these values are set to the uninitialised value (-1) for type and ID, and the length is set to 0.


The rmr_realloc_payload function returns a pointer to the message buffer with the payload which is large enough to hold new_len bytes. If the clone option is true, this will be a pointer to the newly cloned message buffer; the original message buffer pointer may still be used to reference that message. It is the calling application’s responsibility to free the memory associateed with both messages using the rmr_free_msg() function.

When the clone option is not used, it is still good practice by the calling application to capture and use this reference as it is possible that the message buffer, and not just the payload buffer, was reallocated. In the event of an error, a nil pointer will be returned and the value of errno will be set to reflect the problem.


These value of errno will reflect the error condition if a nil pointer is returned:


Memory allocation of the new payload failed.


The pointer passed in was nil, or referenced an invalid message, or the required length was not valid.


The following code bit illustrates how this function can be used to reallocate a buffer for a return to sender acknowledgement message which is larger than the message received.

  if( rmr_payload_size( msg ) < ack_sz ) {              // received message too small for ack
    msg = rmr_realloc_payload( msg, ack_sz, 0, 0 );     // reallocate the message with a payload big enough
    if( msg == NULL ) {
      fprintf( stderr, "[ERR] realloc returned a nil pointer: %s\\n", strerror( errno ) );
    } else {
      // populate and send ack message


rmr_alloc_msg(3), rmr_free_msg(3), rmr_init(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_set_stimeout(3), rmr_tokenise(3), rmr_mk_ring(3), rmr_ring_free(3)