The resource manager library

Before we get too far into all the issues surrounding resource managers, we have to get acquainted with the QNX Neutrino resource manager library. Note that this "library" actually consists of several distinct pieces:

While you certainly could write resource managers "from scratch" (as was done in the QNX 4 world), that's far more hassle than it's worth.

Just to show you the utility of the library approach, here's the source for a single-threaded version of "/dev/null":

 *  resmgr1.c
 *  /dev/null using the resource manager library

#include <stdio.h>
#include <stdlib.h>
#include <stddef.h>
#include <sys/iofunc.h>
#include <sys/dispatch.h>

main (int argc, char **argv)
    dispatch_t              *dpp;
    resmgr_attr_t           resmgr_attr;
    dispatch_context_t      *ctp;
    resmgr_connect_funcs_t  connect_func;
    resmgr_io_funcs_t       io_func;
    iofunc_attr_t           attr;

    // create the dispatch structure
    if ((dpp = dispatch_create ()) == NULL) {
        perror ("Unable to dispatch_create");
        exit (EXIT_FAILURE);

    // initialize the various data structures
    memset (&resmgr_attr, 0, sizeof (resmgr_attr));
    resmgr_attr.nparts_max = 1;
    resmgr_attr.msg_max_size = 2048;

    // bind default functions into the outcall tables
    iofunc_func_init (_RESMGR_CONNECT_NFUNCS, &connect_func,
                      _RESMGR_IO_NFUNCS, &io_func);
    iofunc_attr_init (&attr, S_IFNAM | 0666, 0, 0);

    // establish a name in the pathname space
    if (resmgr_attach (dpp, &resmgr_attr, "/dev/mynull",
                       _FTYPE_ANY, 0, &connect_func, &io_func,
                       &attr) == -1) {
        perror ("Unable to resmgr_attach");
        exit (EXIT_FAILURE);

    ctp = dispatch_context_alloc (dpp);

    // wait here forever, handling messages
    while (1) {
        if ((ctp = dispatch_block (ctp)) == NULL) {
            perror ("Unable to dispatch_block");
            exit (EXIT_FAILURE);
        dispatch_handler (ctp);

There you have it! A complete /dev/null resource manager implemented in a few function calls!

If you were to write this from scratch, and have it support all the functionality that this one does (e.g., stat() works, chown() and chmod() work, and so on), you'd be looking at many hundreds if not thousands of lines of C code.