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 */ next /* at this point you have verified access to the target */ endif 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 endif else call iofunc_open, passing the attr of the file and NULL for dattr endif /* 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 structure /* 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); _IO_SET_CONNECT_RET(ctp, _IO_CONNECT_RET_LINK); 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.