Getting Started with QNX Neutrino - QNX Software Systems

© 2009, QNX Software Systems GmbH & Co. KG. Examples
Step 4
Step 5
Step 6
Step 7
Step 8
A simple io_devctl() example
Here you’d do whatever you want with the data — I’ve just called a made-up function
called process_data() and passed it the buffer and size.
This step is crucial! Forgetting to do it is easy, and will lead to “memory leaks.”
Notice how we also took care to free the memory in the case of a failure in step 3.
We’re using the macro _IO_SET_WRITE_NBYTES() (see the entry for
iofunc_write_verify() in the Neutrino Library Reference) to store the number of bytes
we’ve written, which will then be passed back to the client as the return value from the
client’s write(). It’s important to note that you should return the actual number of
bytes! The client is depending on this.
Now we do similar housekeeping for stat(), lseek(), and further write() functions as we
did for the io_read() routine (and again, we modify the offset in the ocb only in the
case of this not being a _IO_XTYPE_OFFSET type of message). Since we’re writing to
the device, however, we use the IOFUNC_ATTR_MTIME constant instead of the
IOFUNC_ATTR_ATIME constant. The MTIME flag means “modification” time, and a
write() to a resource certainly “modifies” it.
The last step is simple: we return the constant EOK, which tells the resource manager
library that it should reply to the client. This ends our processing. The resource
manager will use the number of bytes that we stashed away with the
_IO_SET_WRITE_NBYTES() macro in the reply and the client will unblock; the
client’s C library write() function will return the number of bytes that were written by
our device.
The client’s devctl() call is formally defined as:
#include
#include
#include
int
devctl (int fd,
int dcmd,
void *dev_data_ptr,
size_t nbytes,
int *dev_info_ptr);
We should first understand this function before we look at the resource manager side
of things. The devctl() function is used for “out of band” or “control” operations. For
example, you may be writing data to a sound card (the actual digital audio samples that
April 30, 2009 Chapter 5 • Resource Managers 263