Writing the entire function yourself

Sometimes a default function will be of no help for your particular resource manager.

For example, iofunc_read_default() and iofunc_write_default() functions implement /dev/null; they do all the work of returning 0 bytes (EOF) or swallowing all the message bytes (respectively). You'll want to do something in those handlers (unless your resource manager doesn't support the _IO_READ or _IO_WRITE messages).

Note that even in such cases, there are still helper functions you can use, such as iofunc_read_verify() and iofunc_write_verify().

Here's a sample skeleton for a typical filesystem, in pseudo-code, to illustrate the steps that need to be taken to handle an open request for a file:

if the open request is for a path (i.e., multiple 
  directory levels)
    call iofunc_client_info_ext to get information 
      about client
    for each directory component
        call iofunc_check_access to check execute 
          permission for access
         recall that execute permission on a 
         directory is really the "search" 
         permission for that directory
     at this point you have verified access 
     to the target

if O_CREAT is set and the file doesn't exist
    call iofunc_open, passing the attribute of the
      parent as dattr
    if the iofunc_open succeeds,
        do the work to create the new inode, 
          or whatever
    call iofunc_open, passing the attr of the file
     and NULL for dattr

 at this point, check for things like o_trunc, 
 etc. -- things that you have to do for the attr

call iofunc_ocb_attach
return EOK

For a device (i.e., resmgr_attach() didn't specify that the managed resource is a directory), the following steps apply:

 at startup time (i.e., in the main() of the 
 resource manager)
call iofunc_attr_init to initialize an attribute 

/* in the io_open message handler: */
call iofunc_open, passing in the attribute of the 
  device and NULL for dattr

call iofunc_ocb_attach
return EOK

A resource manager's response to an open() request isn't always a yes-or-no answer. It's possible to return a connect message indicating that the server would like some other action taken. For example, if the open occurs on a path that represents a symbolic link to some other path, the server could respond using the _IO_SET_CONNECT_RET() macro and the _IO_CONNECT_RET_LINK value.

For example, an open handler that only redirects pathnames might look something like:

io_open(resmgr_context_t *ctp, io_open_t *msg, 
        iofunc_attr_t *dattr, void *extra) {
    char *newpath;

    /* Do all the error/access checking ... */

    /* Lookup the redirected path and store 
       the new path in 'newpath' */
    newpath = get_a_new_path(msg->connect.path);

    len = strlen(newpath) + 1;

    msg->link_reply.eflag = msg->connect.eflag;
    msg->link_reply.nentries = 0;
    msg->link_reply.path_len = len;
    strcpy((char *)(msg->link_reply + 1), newpath);

    len += sizeof(msg->link_reply);

    return(_RESMGR_PTR(ctp, &msg->link_reply, len));

In this example, we use the macro _IO_SET_CONNECT_RET() (defined in <sys/iomsg.h>) to set the ctp->status field to _IO_CONNECT_RET_LINK. This value indicates to the resource-manager framework that the return value isn't actually a simple return code, but a new request to be processed.

The path for this new request follows directly after the link_reply structure and is path_len bytes long. The final few lines of the code just stuff an IOV with the reply message (and the new path to be queried) and return to the resource-manager framework.