Mixer

A mixer is described by the following structure:

struct mixer {
	TAILQ_HEAD(mix_devhead, mix_dev) devs;	/* device list */
	struct mix_dev *dev;			/* selected device */
	oss_mixerinfo mi;			/* mixer info */
	oss_card_info ci;			/* audio card info */
	char name[NAME_MAX];			/* mixer name (e.g /dev/mixer0) */
	int fd;					/* file descriptor */
	int unit;				/* audio card unit */
	int ndev;				/* number of devices */
	int devmask;				/* supported devices */
#define MIX_MUTE		0x01
#define MIX_UNMUTE		0x02
#define MIX_TOGGLEMUTE		0x04
	int mutemask;				/* muted devices */
	int recmask;				/* recording devices */
#define MIX_ADDRECSRC		0x01
#define MIX_REMOVERECSRC	0x02
#define MIX_SETRECSRC		0x04
#define MIX_TOGGLERECSRC	0x08
	int recsrc;				/* recording sources */
#define MIX_MODE_MIXER		0x01
#define MIX_MODE_PLAY		0x02
#define MIX_MODE_REC		0x04
	int mode;				/* dev.pcm.X.mode sysctl */
	int f_default;				/* default mixer flag */
};

The fields are follows:

devs

A tail queue structure containing all supported mixer devices.

dev

A pointer to the currently selected device. The device is one of the elements in devs.

mi

OSS information about the mixer. Look at the definition of the oss_mixerinfo structure in <sys/soundcard.h> to see its fields.

ci

OSS audio card information. This structure is also defined in <sys/soundcard.h>.

name

Path to the mixer (e.g /dev/mixer0).

fd

File descriptor returned when the mixer is opened in mixer_open().

unit

Audio card unit. Since each mixer device maps to a pcmX device, unit is always equal to the number of that pcmX device. For example, if the audio device's number is 0 (i.e pcm0), then unit is 0 as well. This number is useful when checking if the mixer's audio card is the default one.

ndev

Number of devices in devs.

devmask

Bit mask containing all supported devices for the mixer. For example, if device 10 is supported, then the 10th bit in the mask will be set. By default, mixer_open() stores only the supported devices in devs, so it is very unlikely this mask will be needed.

mutemask

Bit mask containing all muted devices. The logic is the same as with devmask.

recmask

Bit mask containing all recording devices. Again, same logic as with the other masks.

recsrc

Bit mask containing all recording sources. Yes, same logic again.

mode

Bit mask containing the supported modes for this audio device. It holds the value of the dev.pcm.X.mode sysctl.

f_default

Flag which tells whether the mixer's audio card is the default one.

Mixer device

Each mixer device stored in a mixer is described as follows:

struct mix_dev {
	struct mixer *parent_mixer;		/* parent mixer */
	char name[NAME_MAX];			/* device name (e.g "vol") */
	int devno;				/* device number */
	struct mix_volume {
#define MIX_VOLMIN		0.0f
#define MIX_VOLMAX		1.0f
#define MIX_VOLNORM(v)		((v) / 100.0f)
#define MIX_VOLDENORM(v)	((int)((v) * 100.0f + 0.5f))
		float left;			/* left volume */
		float right;			/* right volume */
	} vol;
	int nctl;				/* number of controls */
	TAILQ_HEAD(mix_ctlhead, mix_ctl) ctls;	/* control list */
	TAILQ_ENTRY(mix_dev) devs;
};

The fields are follows:

parent_mixer

Pointer to the mixer the device is attached to.

name

Device name given by the OSS API. Devices can have one of the following names:

vol, bass, treble, synth, pcm, speaker, line, mic, cd, mix, pcm2, rec, igain, ogain, line1, line2, line3, dig1, dig2, dig3, phin, phout, video, radio, and monitor.

devno

Device's index in the SOUND_MIXER_NRDEVICES macro defined in <sys/soundcard.h>. This number is used to check against the masks defined in the mixer structure.

left right

Left and right-ear volumes. Although the OSS API stores volumes in integers from 0-100, we normalize them to 32-bit floating point numbers. However, the volumes can be denormalized using the MIX_VOLDENORM macro if needed.

nctl

Number of user-defined mixer controls associated with the device.

ctls

A tail queue containing user-defined mixer controls.

User-defined mixer controls

Each mixer device can have user-defined controls. The control structure is defined as follows:

struct mix_ctl {
	struct mix_dev *parent_dev;		/* parent device */
	int id;					/* control id */
	char name[NAME_MAX];			/* control name */
	int (*mod)(struct mix_dev *, void *);	/* modify control values */
	int (*print)(struct mix_dev *, void *);	/* print control */
	TAILQ_ENTRY(mix_ctl) ctls;
};

The fields are follows:

parent_dev

Pointer to the device the control is attached to.

id

Control ID assigned by the caller. Even though the library will report it, care has to be taken to not give a control the same ID in case the caller has to choose controls using their ID.

name

Control name. As with id, the caller has to make sure the same name is not used more than once.

mod

Function pointer to a control modification function. As in mixer(8), each mixer control's values can be modified. For example, if we have a volume control, the mod function will be responsible for handling volume changes.

print

Function pointer to a control print function.

Opening and closing the mixer

The application must first call the mixer_open() function to obtain a handle to the device, which is used as an argument in most other functions and macros. The parameter name specifies the path to the mixer. OSS mixers are stored under /dev/mixerN where N is the number of the mixer device. Each device maps to an actual pcm audio card, so /dev/mixer0 is the mixer for pcm0, and so on. If name is NULL or /dev/mixer, mixer_open() opens the default mixer (hw.snd.default_unit).

The mixer_close() function frees resources and closes the mixer device. It is a good practice to always call it when the application is done using the mixer.

Manipulating the mixer

The mixer_get_dev() and mixer_get_dev_byname() functions select a mixer device, either by its number or by its name respectively. The mixer structure keeps a list of all the devices, but only one can be manipulated at a time. Each time a new device is to be manipulated, one of the two functions has to be called.

The mixer_set_vol() function changes the volume of the selected mixer device. The vol parameter is a structure that stores the left and right volumes of a given device. The allowed volume values are between MIX_VOLMIN (0.0) and MIX_VOLMAX (1.0).

The mixer_set_mute() function modifies the mute of a selected device. The opt parameter has to be one of the following options:

The mixer_mod_recsrc() function modifies a recording device. The selected device has to be a recording device, otherwise the function will fail. The opt parameter has to be one of the following options:

The mixer_get_dunit() and mixer_set_dunit() functions get and set the default audio card in the system. Although this is not really a mixer feature, it is useful to have instead of having to use the sysctl(3) controls.

The mixer_get_mode() function returns the playback/recording mode of the audio device the mixer belongs to. The available values are the following:

The mixer_get_nmixers() function returns the total number of mixer devices in the system.

The MIX_ISDEV() macro checks if a device is actually a valid device for a given mixer. It is very unlikely that this macro will ever be needed since the library stores only valid devices by default.

The MIX_ISMUTE() macro checks if a device is muted.

The MIX_ISREC() macro checks if a device is a recording device.

The MIX_ISRECSRC() macro checks if a device is a recording source.

The MIX_VOLNORM() macro normalizes a value to 32-bit floating point number. It is used to normalize the volumes read from the OSS API.

The MIX_VOLDENORM() macro denormalizes the left and right volumes stores in the mix_dev structure.

Defining and using mixer controls

The mix_add_ctl() function creates a control and attaches it to the device specified in the parent argument.

The mix_add_ctl_s() function does the same thing as with mix_add_ctl() but the caller passes a mix_ctl_t * structure instead of each field as a separate argument.

The mixer_remove_ctl() functions removes a control from the device its attached to.

The mixer_get_ctl() function searches for a control in the device specified in the d argument and returns a pointer to it. The search is done using the control's ID.

The mixer_get_ctl_byname() function is the same as with mixer_get_ctl() but the search is done using the control's name.

Change the volume of a device

struct mixer *m;
mix_volume_t vol;
char *mix_name, *dev_name;

mix_name = ...;
if ((m = mixer_open(mix_name)) == NULL)
	err(1, "mixer_open: %s", mix_name);

dev_name = ...;
if ((m->dev = mixer_get_dev_byname(m, dev_name)) < 0)
	err(1, "unknown device: %s", dev_name);

vol.left = ...;
vol.right = ....;
if (mixer_set_vol(m, vol) < 0)
	warn("cannot change volume");

(void)mixer_close(m);

Mute all unmuted devices

struct mixer *m;
struct mix_dev *dp;

if ((m = mixer_open(NULL)) == NULL)	/* Open the default mixer. */
	err(1, "mixer_open");
TAILQ_FOREACH(dp, &m->devs, devs) {
	m->dev = dp;			/* Select device. */
	if (M_ISMUTE(m, dp->devno))
		continue;
	if (mixer_set_mute(m, MIX_MUTE) < 0)
		warn("cannot mute device: %s", dp->name);
}

(void)mixer_close(m);

Print all recording sources' names and volumes

struct mixer *m;
struct mix_dev *dp;

char *mix_name, *dev_name;

mix_name = ...;
if ((m = mixer_open(mix_name)) == NULL)
	err(1, "mixer_open: %s", mix_name);

TAILQ_FOREACH(dp, &m->devs, devs) {
	if (M_ISRECSRC(m, dp->devno))
		printf("%s\t%.2f:%.2f\n",
		    dp->name, dp->vol.left, dp->vol.right);
}

(void)mixer_close(m);