Getting Started with QNX Neutrino - QNX Software Systems

Advanced topics
© 2009, QNX Software Systems GmbH & Co. KG.
Directory Function File Function Message (resmgr)
closedir() close() _IO_CLOSE_DUP
opendir() open() _IO_CONNECT
readdir() read() _IO_READ
rewinddir() lseek() _IO_LSEEK
seekdir() lseek() _IO_LSEEK
telldir() tell() _IO_LSEEK
If we assume for a moment that the opendir() and closedir() functions will be handled
automatically for us, we can focus on just the _IO_READ and _IO_LSEEK messages
and related functions.
Offsets
The _IO_LSEEK message and related function is used to “seek” (or “move”) within a
file. It does the exact same thing within a directory; you can move to the “first”
directory entry (by explicitly giving an offset to seekdir() or by calling rewinddir()), or
any arbitrary entry (by using seekdir()), or you can find out the current location in the
directory entry list (by using telldir()).
The “trick” with directories, however, is that the seek offsets are entirely up to you to
define and manage. This means that you may decide to call your directory entry
offsets “0,” “1,” “2” and so on, or you may instead call them “0,” “64,” “128” and so
on. The only important thing here is that the offsets must be consistent in both the
io_lseek() handler as well as the io_read() handler functions.
In the example below, we’ll assume that we’re using the simple “0,” “1,” “2” . . .
approach. (You might use the “0,” “64,” “128” . . . approach if those numbers
correspond to, for example, some kind of on-media offsets. Your choice.)
Contents
So now all that’s left is to “simply” fill in the struct dirent with the “contents” of
our directory. Here’s what the struct dirent looks like (from ):
struct dirent {
ino_t d_ino;
off_t d_offset;
uint16_t d_reclen;
uint16_t d_namelen;
char d_name [1];
};
Here’s a quick explanation of the various members:
d_ino
The “inode” — a mountpoint-unique serial number that cannot be
zero (zero traditionally indicates that the entry corresponding to this
inode is free/empty).
274 Chapter 5 • Resource Managers April 30, 2009