Attach a message range


#include <sys/iofunc.h>
#include <sys/dispatch.h>

int message_attach( dispatch_t * dpp,
                    message_attr_t * attr,
                    int low,
                    int high,
                    int (* func) (
                           message_context_t * ctp,
                           int code,
                           unsigned flags,
                           void * handle ),
                    void * handle );


The dispatch handle, as returned by a successful call to dispatch_create().
A pointer to a message_attr_t structure that lets you specify additional requirements for the message; see message_attr_t structure,” below.
low, high
The range of messages that you're interested in.
Note: Although these arguments are of type int, message_attach() stores the values internally as short.
The function that you want to call when a message in the given range is received; see Handler function,” below.
An arbitrary handle that you want to associate with data for the defined message range. This handle is passed to func.



Use the -l c option to qcc to link against this library. This library is usually included automatically.


The message_attach() function attaches a handler to the message range defined by the message type [low, high] (i.e., an inclusive message range) for dispatch handle dpp.

  • It's considered a programming error to attach overlapping message or pulse ranges. Message types should be greater than _IO_MAX (defined in <sys/iomsg.h>).
  • Your process needs the PROCMGR_AID_PUBLIC_CHANNEL ability enabled in order to create a public channel (i.e., without _NTO_CHF_PRIVATE set). For more information, see procmgr_ability().

When a message with a type in that range is received, dispatch_handler() calls the user-supplied function func. You can also use the same function with pulse_attach(). By examining ctp->rcvid, func can determine whether a pulse or message was received.

This function is responsible for doing any specific work needed to handle the message pointed to by ctp->msg. The handle passed to the function is the handle initially passed to message_attach().

message_attr_t structure

The attr argument is a pointer to a message_attr_t structure:

typedef struct _message_attr {
   unsigned                 flags;
   unsigned                 nparts_max;
   size_t                   msg_max_size;
   unsigned                 reserved[5];
} message_attr_t;

You can use this structure to specify:

Handler function

The user-supplied function func is called when a message in the defined range is received. This function is passed the message context ctp, in which the message was received, the message type, and the handle (the one passed to message_attach()). Currently, the argument flags is reserved. Your function should return 0; other return values are reserved.

Here's a brief description of the context pointer fields:

The receive ID of the message.
A pointer to the message.
Data from a _msg_info structure.

The message_context_t structure is identical to resmgr_context_t.


Zero on success, or -1 on failure (errno is set).


The message code is out of range.
Insufficient memory to attach message type.
The calling process doesn't have the required permission; see procmgr_ability().


In this example, we create a resource manager where we attach to a private message range and attach a pulse, which is then used as a timer event:

#include <stdio.h>
#include <stddef.h>
#include <stdlib.h>

#define THREAD_POOL_PARAM_T     dispatch_context_t
#include <sys/iofunc.h>
#include <sys/dispatch.h>

static resmgr_connect_funcs_t   connect_func;
static resmgr_io_funcs_t        io_func;
static iofunc_attr_t            attr;

timer_tick(message_context_t *ctp, int code, 
           unsigned flags, void *handle) {

    union sigval       value = ctp->msg->pulse.value;
    /* Do some useful work on every timer firing... */
    printf("received timer event, value %d\n", value.sival_int);
    return 0;

message_handler(message_context_t *ctp, int code,
                unsigned flags, void *handle ) {
    printf("received private message, type %d\n", code);
    return 0;

main(int argc, char **argv) {
    thread_pool_attr_t    pool_attr;
    thread_pool_t         *tpp;
    dispatch_t            *dpp;
    resmgr_attr_t         resmgr_attr;
    int                   id;
    int                   timer_id;
    struct sigevent       event;
    struct _itimer        itime;

    if((dpp = dispatch_create()) == NULL) {
                "%s: Unable to allocate dispatch handle.\n",
        return EXIT_FAILURE;

    memset(&pool_attr, 0, sizeof pool_attr);
    pool_attr.handle = dpp;
    /* We are doing resmgr and pulse-type attaches */
    pool_attr.context_alloc = dispatch_context_alloc;
    pool_attr.block_func = dispatch_block;
    pool_attr.unblock_func = dispatch_unblock;
    pool_attr.handler_func = dispatch_handler;
    pool_attr.context_free = dispatch_context_free;
    pool_attr.lo_water = 2;
    pool_attr.hi_water = 4;
    pool_attr.increment = 1;
    pool_attr.maximum = 50;

    if((tpp = thread_pool_create(&pool_attr, 
                                 POOL_FLAG_EXIT_SELF)) == NULL) {
                "%s: Unable to initialize thread pool.\n",
        return EXIT_FAILURE;

    iofunc_func_init(_RESMGR_CONNECT_NFUNCS, &connect_func,
                     _RESMGR_IO_NFUNCS, &io_func);

    /* You should restrict the permissions according to what you want other
       processes and users to be able to do with your resource manager. */

    iofunc_attr_init(&attr, S_IFNAM | 0660, 0, 0);
    memset(&resmgr_attr, 0, sizeof resmgr_attr);
    resmgr_attr.nparts_max = 1;
    resmgr_attr.msg_max_size = 2048;

    if((id = resmgr_attach(dpp, &resmgr_attr, "/dev/mynull", 
                 _FTYPE_ANY, 0,
                 &connect_func, &io_func, &attr)) == -1) {
        fprintf(stderr, "%s: Unable to attach name.\n", argv[0]);
        return EXIT_FAILURE;

     We want to handle our own private messages, of type 
     0x5000 to 0x5fff
    if(message_attach(dpp, NULL, 0x5000, 0x5fff, 
                      &message_handler, NULL) == -1) {
           "Unable to attach to private message range.\n");
        return EXIT_FAILURE;

    /* Initialize an event structure, and attach a pulse to it */
    if((event.sigev_code = pulse_attach(dpp, 
                              MSG_FLAG_ALLOC_PULSE, 0,
                              &timer_tick, NULL)) == -1) {
        fprintf(stderr, "Unable to attach timer pulse.\n");
        return EXIT_FAILURE;

    /* Connect to our channel */
    if((event.sigev_coid = message_connect(dpp, 
                              MSG_FLAG_SIDE_CHANNEL)) == -1) {
        fprintf(stderr, "Unable to attach to channel.\n");
        return EXIT_FAILURE;

    event.sigev_notify = SIGEV_PULSE;
    event.sigev_priority = -1;
     We could create several timers and use different 
     sigev values for each
    event.sigev_value.sival_int = 0;

    if((timer_id = TimerCreate(CLOCK_MONOTONIC, &event)) == -1) {;
                "Unable to attach channel and connection.\n");
        return EXIT_FAILURE;

    /* And now setup our timer to fire every second */
    itime.nsec = 1000000000;
    itime.interval_nsec = 1000000000;
    TimerSettime(timer_id, 0, &itime, NULL);

    /* Never returns */
    return EXIT_SUCCESS;

For more examples using the dispatch interface, see dispatch_create(), resmgr_attach(), and thread_pool_create().


QNX Neutrino

Cancellation point Yes
Interrupt handler No
Signal handler No
Thread Yes