Getting Started with QNX Neutrino - QNX Software Systems

Examples
© 2009, QNX Software Systems GmbH & Co. KG.
the sound card should convert to analog audio), and you may decide that you need to
change the number of channels from 1 (mono) to 2 (stereo), or the sampling rate from
the CD-standard (44.1 kHz) to the DAT-standard (48 kHz). The devctl() function is the
appropriate way to do this. When you write a resource manager, you may find that you
don’t need any devctl() support at all and that you can perform all the functionality
needed simply through the standard read() and write() functions. You may, on the
other hand, find that you need to mix devctl() calls with the read() and write() calls, or
indeed that your device uses only devctl() functions and does not use read() or write().
The devctl() function takes these arguments:
fd
dcmd
dev_data_ptr
nbytes
dev_info_ptr
The file descriptor of the resource manager that you’re sending the
devctl() to.
The command itself — a combination of two bits worth of
direction, and 30 bits worth of command (see discussion below).
A pointer to a data area that can be sent to, received from, or both.
The size of the dev_data_ptr data area.
An extra information variable that can be set by the resource
manager.
The top two bits in the dcmd encode the direction of data transfer, if any. For details,
see the description in the I/O reference section (under io_devctl()).
When the _IO_DEVCTL message is received by the resource manager, it’s handled by
your io_devctl() function. Here is a very simple example, which we’ll assume is used
to set the number of channels and the sampling rate for the audio device we discussed
above:
/*
* io_devctl1.c
*/
#include
#include
#include
#include
#include
#include
#define DCMD_AUDIO_SET_CHANNEL_MONO 1
#define DCMD_AUDIO_SET_CHANNEL_STEREO 2
#define DCMD_AUDIO_SET_SAMPLE_RATE_CD 3
#define DCMD_AUDIO_SET_SAMPLE_RATE_DAT 4
int
io_devctl (resmgr_context_t *ctp, io_devctl_t *msg,
iofunc_ocb_t *ocb)
{
int sts;
// 1) see if it’s a standard POSIX-supported devctl()
264 Chapter 5 • Resource Managers April 30, 2009