Getting Started with QNX Neutrino - QNX Software Systems

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my_read_dir()
In my_read_dir() is where the fun begins. From a high level perspective, we allocate a
buffer that’s going to hold the result of this operation (called reply_msg). We then use
dp to “walk” along the output buffer, stuffing struct dirent entries as we go along.
The helper routine dirent_size() is used to determine if we have sufficient room in the
output buffer to stuff the next entry; the helper routine dirent_fill() is used to perform
the stuffing. (Note that these routines are not part of the resource manager library;
they’re discussed and documented below.)
On first glance this code may look inefficient; we’re using sprintf() to create a
two-byte filename (the filename character and a NUL terminator) into a buffer that’s
_POSIX_PATH_MAX (256) bytes long. This was done to keep the code as generic as
possible.
Finally, notice that we use the OCB’s offset member to indicate to us which particular
filename we’re generating the struct dirent for at any given time. This means that
we also have to update the offset field whenever we return data.
The return of data to the client is accomplished in the “usual” way, via MsgReply().
Note that the status field of MsgReply() is used to indicate the number of bytes that
were sent to the client.
static int
my_read_dir (resmgr_context_t *ctp, io_read_t *msg,
iofunc_ocb_t *ocb)
{
int nbytes;
int nleft;
struct dirent *dp;
char *reply_msg;
char fname [_POSIX_PATH_MAX];
// allocate a buffer for the reply
reply_msg = calloc (1, msg -> i.nbytes);
if (reply_msg == NULL) {
return (ENOMEM);
}
// assign output buffer
dp = (struct dirent *) reply_msg;
// we have "nleft" bytes left
nleft = msg -> i.nbytes;
while (ocb -> offset < NUM_ENTS) {
// create the filename
sprintf (fname, "%c", ocb -> offset + ’a’);
// see how big the result is
nbytes = dirent_size (fname);
// do we have room for it?
if (nleft - nbytes >= 0) {
// fill the dirent, and advance the dirent pointer
dp = dirent_fill (dp, ocb -> offset + 1,
280 Chapter 5 • Resource Managers April 30, 2009