Getting Started with QNX Neutrino - QNX Software Systems

© 2009, QNX Software Systems GmbH & Co. KG. Writing a resource manager
ioflag
The open mode; how this resource was opened (e.g. read only). The open
modes (as passed to open() on the client side) correspond to the ioflag
values as follows:
Open mode ioflag value
O_RDONLY _IO_FLAG_RD
O_RDWR _IO_FLAG_RD | _IO_FLAG_WR
O_WRONLY _IO_FLAG_WR
offset
sflag
flags
The current lseek() offset into this resource.
The sharing flag (see ) used with the client’s sopen() function
call. These are the flags SH_COMPAT, SH_DENYRW, SH_DENYWR,
SH_DENYRD, and SH_DENYNO.
System flags. The two flags currently supported are
IOFUNC_OCB_PRIVILEGED, which indicates whether a privileged process
issued the connect message that resulted in this OCB, and
IOFUNC_OCB_MMAP, which indicates whether this OCB is in use by a
mmap() call on the client side. No other flags are defined at this time. You
can use the bits defined by IOFUNC_OCB_FLAGS_PRIVATE for your own
private flags.
If you wish to store additional data along with the “normal” OCB, rest assured that
you can “extend” the OCB. We’ll discuss this in the “Advanced topics” section.
The strange case of the offset member
The offset field is, to say the least, interesting. Have a look at to
see how it’s implemented. Depending on what preprocessor flags you’ve set, you may
get one of six (!) possible layouts for the offset area. But don’t worry too much about
the implementation — there are really only two cases to consider, depending on
whether you want to support 64-bit offsets:
• yes — the offset member is 64 bits
• no (32-bit integers) — the offset member is the lower 32 bits; another member,
offset_hi, contains the upper 32 bits.
For our purposes here, unless we’re specifically going to talk about 32 versus 64 bits,
we’ll just assume that all offsets are 64 bits, of type off_t, and that the platform
knows how to deal with 64-bit quantities.
April 30, 2009 Chapter 5 • Resource Managers 221