---
name: "mixer"
section: "3"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["freebsd"]
url: "https://new.carta.tech/MIX_ISDEV"
---

# mixer(3)

interface to OSS mixers

Part of [MIX\_ISDEV(3)](/MIX_ISDEV/3.md).

## NAME

**mixer\_open**, **mixer\_close**, **mixer\_get\_dev**, **mixer\_get\_dev\_byname**, **mixer\_add\_ctl**, **mixer\_add\_ctl\_s**, **mixer\_remove\_ctl**, **mixer\_get\_ctl**, **mixer\_get\_ctl\_byname**, **mixer\_set\_vol**, **mixer\_set\_mute**, **mixer\_mod\_recsrc**, **mixer\_get\_dunit**, **mixer\_set\_dunit**, **mixer\_get\_mode**, **mixer\_get\_nmixers**, **mixer\_get\_path**, **MIX\_ISDEV**, **MIX\_ISMUTE**, **MIX\_ISREC**, **MIX\_ISRECSRC**, **MIX\_VOLNORM**, **MIX\_VOLDENORM** – interface to OSS mixers

## LIBRARY

Mixer library (libmixer, -lmixer)

## SYNOPSIS

**#include \<mixer.h\>**

*struct* *mixer* *\**\

**mixer\_open**(*const char \*name*);

*int*\

**mixer\_close**(*struct mixer \*m*);

*struct* *mix\_dev* *\**\

**mixer\_get\_dev**(*struct mixer \*m*, *int devno*);

*struct* *mix\_dev* *\**\

**mixer\_get\_dev\_byname**(*struct mixer \*m*, *name*);

*int*\

**mixer\_add\_ctl**(*struct mix\_dev \*parent*, *int id*, *const char \*name*, *int (\*mod)(struct mix\_dev \*d, void \*p)*, *int (\*print)(struct mix\_dev \*d, void \*p)*);

*int*\

**mixer\_add\_ctl\_s**(*mix\_ctl\_t \*ctl*);

*int*\

**mixer\_remove\_ctl**(*mix\_ctl\_t \*ctl*);

*mix\_ctl\_t* *\**\

**mixer\_get\_ctl**(*struct mix\_dev \*d*, *int id*);

*mix\_ctl\_t* *\**\

**mixer\_get\_ctl\_byname**(*struct mix\_dev \*d*, *const char \*name*);

*int*\

**mixer\_set\_vol**(*struct mixer \*m*, *mix\_volume\_t vol*);

*int*\

**mixer\_set\_mute**(*struct mixer \*m*, *int opt*);

*int*\

**mixer\_mod\_recsrc**(*struct mixer \*m*, *int opt*);

*int*\

**mixer\_get\_dunit**(*void*);

*int*\

**mixer\_set\_dunit**(*struct mixer \*m*, *int unit*);

*int*\

**mixer\_get\_mode**(*int unit*);

*int*\

**mixer\_get\_nmixers**(*void*);

*int*\

**mixer\_get\_path**(*char \* buf*, *size\_t size*, *int unit*);

*int*\

**MIX\_ISDEV**(*struct mixer \*m*, *int devno*);

*int*\

**MIX\_ISMUTE**(*struct mixer \*m*, *int devno*);

*int*\

**MIX\_ISREC**(*struct mixer \*m*, *int devno*);

*int*\

**MIX\_ISRECSRC**(*struct mixer \*m*, *int devno*);

*float*\

**MIX\_VOLNORM**(*int v*);

*int*\

**MIX\_VOLDENORM**(*float v*);

## DESCRIPTION

The **mixer** library allows userspace programs to access and manipulate OSS sound mixers in a simple way.

### 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)](/mixer/8.md), 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:

- MIX\_MUTE

  Mute the device.

- MIX\_UNMUTE

  Unmute the device.

- MIX\_TOGGLEMUTE

  Toggle the device's mute (e.g mute if unmuted and unmute if muted).

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:

- MIX\_ADDRECSRC

  Add device to the recording sources.

- MIX\_REMOVERECSRC

  Remove device from the recording sources.

- MIX\_SETRECSRC

  Set device as the only recording source.

- MIX\_TOGGLERECSRC

  Toggle device from the recording sources.

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)](/sysctl/3.md) controls.

The **mixer\_get\_mode**() function returns the operating mode of the audio device the mixer belongs to. The following values can be OR'ed in case more than one mode is supported:

- MIX\_MODE\_MIXER

  The audio device has a mixer.

- MIX\_MODE\_PLAY

  The audio device supports playback.

- MIX\_MODE\_REC

  The audio device supports recording.

The **mixer\_get\_nmixers**() function returns the maximum mixer unit number. Although this might sound as incorrect behavior, given that one would expect "nmixers" to refer to the total number of active mixers, it is more intuitive for applications that want to loop through all mixer devices (see the EXAMPLES section).

The **mixer\_get\_path**() function writes the path of the mixer device specified in the *unit* argument to the buffer specified in *buf*. *unit* can be either -1, in which case **mixer\_get\_path**() will fetch the path of the default mixer, or between 0 and the maximum mixer unit.

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.

## RETURN VALUES

The **mixer\_open**() function returns the newly created handle on success and NULL on failure.

The **mixer\_close**(), **mixer\_set\_vol**(), **mixer\_set\_mute**(), **mixer\_mod\_recsrc**(), **mixer\_get\_dunut**(), **mixer\_set\_dunit**(), **mixer\_get\_nmixers**(), and **mixer\_get\_path**() functions return 0 or positive values on success and -1 on failure.

The **mixer\_get\_dev**() and **mixer\_get\_dev\_byname**() functions return the selected device on success and NULL on failure.

All functions set the value of *errno* on failure.

## EXAMPLES

### 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);
```

### Loop through all mixer devices in the system

```
struct mixer *m;
char buf[NAME_MAX];
int n;
if ((n = mixer_get_nmixers()) < 0)
	errx(1, "no mixers present in the system");
for (i = 0; i < n; i++) {
	(void)mixer_get_path(buf, sizeof(buf), i);
	if ((m = mixer_open(buf)) == NULL)
		continue;
	...
	(void)mixer_close(m);
}
```

## SEE ALSO

[queue(3)](/queue/3.md), [sysctl(3)](/sysctl/3.md), [sound(4)](/sound/4.md), [mixer(8)](/mixer/8.md) and [errno(2)](/errno/2.md)

## AUTHORS

Christos Margiolis ⟨*christos@FreeBSD.org*⟩