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AUDIO(4) Device Drivers Manual AUDIO(4)

audio, mixerdevice-independent audio driver layer

audio* at ...

#include <sys/types.h>
#include <sys/ioctl.h>
#include <sys/audioio.h>
#include <string.h>

The audio driver provides support for various audio peripherals. It provides a uniform programming interface layer above different underlying audio hardware drivers. The audio layer provides full-duplex operation if the underlying hardware configuration supports it.

There are three device files available for audio operation: /dev/audio, /dev/audioctl, and /dev/mixer. /dev/audio is used for recording or playback of digital samples. /dev/mixer is used to manipulate volume, recording source, or other audio mixer functions. /dev/audioctl accepts the same ioctl(2) operations as /dev/audio, but no other operations. In contrast to /dev/audio, which has the exclusive open property, /dev/audioctl can be opened at any time and can be used to read the audio device variables while it is in use.

When /dev/audio is opened, it attempts to maintain the previous audio sample format and record/playback mode. In addition, if it is opened read-only (write-only) the device is set to half-duplex record (play) mode with recording (playing) unpaused.

Only one process may hold open a sampling device at a given time (although file descriptors may be shared between processes once the first open completes).

On a half-duplex device, either reads or writes are allowed, but not both. On a full-duplex device, reads and writes may operate concurrently without interference.

If a writing process does not call write(2) frequently enough to provide samples at the pace the hardware consumes them silence is inserted. If a reading process does not call read(2) frequently enough, it will simply miss samples.

The audio device is accessed with read(2) or write(2).

The audio device, like most devices, can be used in poll(2),

The following ioctl(2) commands are supported on the sample devices:

audio_device_t *
This command fetches the current hardware device information into the audio_device_t * argument.
typedef struct audio_device {
        char name[MAX_AUDIO_DEV_LEN];
        char version[MAX_AUDIO_DEV_LEN];
        char config[MAX_AUDIO_DEV_LEN];
} audio_device_t;

struct audio_swpar *
struct audio_swpar *
Set or get audio parameters as encoded in the audio_swpar structure.
struct audio_swpar {
	unsigned int sig;	/* if 1, encoding is signed */
	unsigned int le;	/* if 1, encoding is little-endian */
	unsigned int bits;	/* bits per sample */
	unsigned int bps;	/* bytes per sample */
	unsigned int msb;	/* if 1, bits are msb-aligned */
	unsigned int rate;	/* common play & rec sample rate */
	unsigned int pchan;	/* play channels */
	unsigned int rchan;	/* rec channels */
	unsigned int nblks;	/* number of blocks in play buffer */
	unsigned int round;	/* common frames per block */

When setting the device parameters with AUDIO_SETPAR, the audio_swpar structure should first be initialized with

struct audio_swpar ap;


and then only the values to be changed should be set. This ensures that the software will work with future versions of the driver. The driver will attempt to set the given parameters; if the device doesn't support them, it will choose other parameters. Then the software must call AUDIO_GETPAR to obtain the parameters in use.

The parameters are as follows:

Number of bits per sample: must be between 1 and 32.
Bytes per sample; if specified, it must be large enough to hold all bits. By default it's set to the smallest power of two large enough to hold bits.
If set (i.e. non-zero) then the samples are signed, otherwise they are unsigned.
If set, then the byte order is little endian; if not it is big endian; it's meaningful only if bps > 1.
If set, then the bits are aligned in the packet to the most significant bit (i.e. lower bits are padded), otherwise to the least significant bit (i.e. higher bits are padded). It's meaningful only if bits < bps * 8.
The number of recorded channels; meaningful only if the device is opened for reading.
The number of channels playing; meaningful only if the device is opened for writing.
The sampling frequency in Hz.
The number of blocks in the play buffer.
The audio block size.

Start playback and/or recording immediately. If the device is open for writing (playback), then the play buffer must be filled with the write(2) syscall. The buffer size is obtained by multiplying the nblks, round, and bps parameters obtained with AUDIO_GETPAR.

Stop playback and recording immediately.

struct audio_pos *
Fetch an atomic snapshot of device timing information in the audio_pos structure.
struct audio_pos {
	unsigned int play_pos;	/* total bytes played */
	unsigned int play_xrun;	/* bytes of silence inserted */
	unsigned int rec_pos;	/* total bytes recorded */
	unsigned int rec_xrun;	/* bytes dropped */

The properties have the following meaning:

Total number of bytes played by the device since playback started (a.k.a the device wall clock).
The number of bytes corresponding to silence played because write(2) wasn't called fast enough.
Total number of bytes recorded by the device since recording started (a.k.a the device wall clock).
The number of bytes dropped because read(2) wasn't called fast enough.

struct audio_status *
Fetch the current device status from the audio driver in the audio_status structure. This ioctl(2) is intended for use with diagnostic tools and is of no use to audio programs.
struct audio_status {
#define AUMODE_PLAY	0x01
#define AUMODE_RECORD	0x02
	int mode;	/* current mode */
	int pause;	/* not started yet */
	int active;	/* playing/recording in progress */

The properties have the following meaning:

The current mode determined by open(2) flags.
If set, indicates that AUDIO_STOP was called, and the device is not attempting to start.
If set, indicates that the device is playing and/or recording.

The mixer device, /dev/mixer, may be manipulated with ioctl(2) but does not support read(2) or write(2). It supports the following ioctl(2) commands:

audio_device_t *
This command is the same as described above for the sampling devices.

mixer_ctrl_t *
mixer_ctrl_t *
These commands read the current mixer state or set new mixer state for the specified device dev. type identifies which type of value is supplied in the mixer_ctrl_t * argument.
#define AUDIO_MIXER_ENUM   1
#define AUDIO_MIXER_SET    2
typedef struct mixer_ctrl {
	int dev;			/* input: nth device */
	int type;
	union {
		int ord;		/* enum */
		int mask;		/* set */
		mixer_level_t value;	/* value */
	} un;
} mixer_ctrl_t;

#define AUDIO_MIN_GAIN  0
#define AUDIO_MAX_GAIN  255
typedef struct mixer_level {
	int num_channels;
	u_char level[8];		/* [num_channels] */
} mixer_level_t;

For a mixer value, the value field specifies both the number of channels and the values for each channel. If the channel count does not match the current channel count, the attempt to change the setting may fail (depending on the hardware device driver implementation). For an enumeration value, the ord field should be set to one of the possible values as returned by a prior AUDIO_MIXER_DEVINFO command. The type AUDIO_MIXER_CLASS is only used for classifying particular mixer device types and is not used for AUDIO_MIXER_READ or AUDIO_MIXER_WRITE.

mixer_devinfo_t *
This command is used iteratively to fetch audio mixer device information into the input/output mixer_devinfo_t * argument. To query all the supported devices, start with an index field of 0 and continue with successive devices (1, 2, ...) until the command returns an error.
typedef struct mixer_devinfo {
	int index;		/* input: nth mixer device */
	audio_mixer_name_t label;
	int type;
	int mixer_class;
	int next, prev;
	union {
		struct audio_mixer_enum {
			int num_mem;
			struct {
				audio_mixer_name_t label;
				int ord;
			} member[32];
		} e;
		struct audio_mixer_set {
			int num_mem;
			struct {
				audio_mixer_name_t label;
				int mask;
			} member[32];
		} s;
		struct audio_mixer_value {
			audio_mixer_name_t units;
			int num_channels;
			int delta;
		} v;
	} un;
} mixer_devinfo_t;

The label field identifies the name of this particular mixer control. The index field may be used as the dev field in AUDIO_MIXER_READ and AUDIO_MIXER_WRITE commands. The type field identifies the type of this mixer control. Enumeration types are typically used for on/off style controls (e.g., a mute control) or for input/output device selection (e.g., select recording input source from CD, line in, or microphone). Set types are similar to enumeration types but any combination of the mask bits can be used.

The mixer_class field identifies what class of control this is. This value is set to the index value used to query the class itself. The (arbitrary) value set by the hardware driver may be determined by examining the mixer_class field of the class itself, a mixer of type AUDIO_MIXER_CLASS. For example, a mixer level controlling the input gain on the “line in” circuit would have a mixer_class that matches an input class device with the name “inputs” (AudioCinputs) and would have a label of “line” (AudioNline). Mixer controls which control audio circuitry for a particular audio source (e.g., line-in, CD in, DAC output) are collected under the input class, while those which control all audio sources (e.g., master volume, equalization controls) are under the output class. Hardware devices capable of recording typically also have a record class, for controls that only affect recording, and also a monitor class.

The next and prev may be used by the hardware device driver to provide hints for the next and previous devices in a related set (for example, the line in level control would have the line in mute as its “next” value). If there is no relevant next or previous value, AUDIO_MIXER_LAST is specified.

For AUDIO_MIXER_ENUM mixer control types, the enumeration values and their corresponding names are filled in. For example, a mute control would return appropriate values paired with AudioNon and AudioNoff. For the AUDIO_MIXER_VALUE and AUDIO_MIXER_SET mixer control types, the channel count is returned; the units name specifies what the level controls (typical values are AudioNvolume, AudioNtreble, and AudioNbass).

By convention, all the mixer devices can be distinguished from other mixer controls because they use a name from one of the AudioC* string values.


aucat(1), audioctl(1), cdio(1), mixerctl(1), ioctl(2), sio_open(3), ac97(4), uaudio(4), audio(9)

September 8, 2016 OpenBSD-6.3