Mixer Pseudo-Device

• The first purpose is for applications that wish to learn about the list of audio devices on the system, so that they can select (or provide for users to select) an appropriate audio device. The /dev/mixer pseudo-device provides interfaces to enumerate all of the audio devices on the system.
• The second purpose is for mixer panel type applications which need to control master settings for the audio hardware in the system, such as gain levels, balance, port functionality, and other device features.

Ordinary audio applications should not attempt to adjust their playback or record volumes or other device settings using this device. Instead, they should use the SNDCTL_DSP_SETPLAYVOL and SNDCTL_DSP_SETRECVOL ioctls that are documented in dsp(7I).

Sndstat Device

Information IOCTLs

Applications should issue SNDCTL_SYSINFO first to learn what audio devices and mixers are available on the system, and then use SNDCTL_AUDIOINFO or SNDCTL_MIXERINFO to obtain more information about the audio devices or mixers, respectively.

OSS_GETVERSION

The argument is a pointer to an integer, which retrieves the version of the OSS API used. The value is encoded with the major version (currently 4) encoded in the most significant 16 bits, and a minor version encoded in the lower 16 bits.

SNDCTL_SYSINFO

The argument is a pointer to an oss_sysinfo structure, which has the following definition:

typedef struct oss_sysinfo {
   char product[32];     /* E.g. SunOS Audio */
   char version[32];     /* E.g. 4.0a */
   int versionnum;       /* See OSS_GETVERSION */
   char options[128];    /* NOT SUPPORTED */

   int numaudios;        /* # of audio/dsp devices */
   int openedaudio[8];   /* Reserved, always 0 */

   int numsynths;        /* NOT SUPPORTED, always 0 */
   int nummidis;         /* NOT SUPPORTED, always 0 */
   int numtimers;        /* NOT SUPPORTED, always 0 */
   int nummixers;        /* # of mixer devices */

   /* Mask of midi devices are busy */
   int openedmidi[8];

   /* Number of sound cards in the system */
   int numcards;

   /* Number of audio engines in the system */
   int numaudioengines;
   char license[16];         /* E.g. "GPL" or "CDDL" */
   char revision_info[256];  /* Reserved */
   int filler[172];          /* Reserved */
} oss_sysinfo;
    

The important fields here are numaudios, which is used to determine the number of audio devices that can be queried with SNDCTL_AUDIOINFO, nummixers which provides a count of mixers on the system, and numcards which counts to total number of aggregate devices. A card can consist of one or more audio devices and one or more mixers, although more typically there is exactly one audio device and one mixer for each card.

SNDCTL_AUDIOINFO

The argument is a pointer to an oss_audioinfo structure, which has the following structure:

typedef struct oss_audioinfo {
   int dev;             /* Device to query */
   char name[64];       /* Human readable name */
   int busy;            /* reserved */
   int pid;             /* reserved */

   /* PCM_CAP_INPUT, PCM_CAP_OUTPUT */
   int caps;
   int iformats;        /* Supported input formats */
   int oformats;        /* Supported output formats */
   int magic;           /* reserved */
   char cmd[64];        /* reserved */
   int card_number;
   int port_number;     /* reserved */
   int mixer_dev;

   /* Obsolete field.  Replaced by devnode */
   int legacy_device;
   int enabled;         /* reserved */
   int flags;           /* reserved */
   int min_rate;        /* Minimum sample rate */
   int max_rate;        /* Maximum sample rate */
   int min_channels;  /* Minimum number of channels */
   int max_channels;  /* Maximum number of channels */
   int binding;         /* reserved */
   int rate_source;     /* reserved */
   char handle[32];     /* reserved */
   unsigned int nrates;     /* reserved */
   unsigned int rates[20];  /* reserved */
   char song_name[64];  /* reserved */
   char label[16];      /* reserved */
   int latency;         /* reserved */

   /* Device special file name (absolute path) */
   char devnode[32];
   int next_play_engine;    /* reserved */
   int next_rec_engine;     /* reserved */
   int filler[184];         /* reserved */
} oss_audioinfo;
    

In the above structure, all of the fields are reserved except the following: dev, name, card_number, mixer_dev, caps, min_rate, max_rate, min_channels, max_channels, and devnode. The reserved fields are provided for compatibility with other OSS implementations, and available for legacy applications. New applications should not attempt to use these fields.

The dev field should be initialized by the application to the number of the device to query. This is a number between zero (inclusive) and value of numaudios (exclusive) returned by SNDCTL_SYSINFO. Alternatively, when issuing the ioctl against a real mixer or dsp device, the special value -1 can be used to indicate that the query is being made against the device opened. If -1 is used, the field is overwritten with the device number for the current device on successful return.

No other fields are significant upon entry, but a successful return contains details of the device.

The name field is a human readable name representing the device. Applications should not try to interpret it.

The card_number field indicates the number assigned to the aggregate device. This can be used with the SNDCTL_CARDINFO ioctl.

The mixer_dev is the mixer device number for the mixing device associated with the audio device. This can be used with the SNDCTL_MIXERINFO ioctl.

The caps field contains any of the bits PCM_CAP_INPUT, PCM_CAP_OUTPUT, and PCM_CAP_DUPLEX. Indicating whether the device support input, output, and whether input and output can be used simultaneously. All other bits are reserved.

The min_rate and max_rate fields indicate the minimum and maximum sample rates supported by the device. Most applications should try to use the maximum supported rate for the best audio quality and lowest system resource consumption.

The min_channels and max_channels provide an indication of the number of channels (1 for mono, 2 for stereo, 6 for 5.1, etc.) supported by the device.

The devnode field contains the actual full path to the device node for this device, such as /dev/sound/audio810:0dsp. Applications should open this file to access the device.

SNDCTL_CARDINFO

The argument is a pointer to a struct oss_card_info, which has the following definition:

typedef struct oss_card_info {
   int card;
   char shortname[16];
   char longname[128];
   int flags;          /* reserved */
   char hw_info[400];
   int intr_count;     /* reserved */
   int ack_count;      /* reserved */
   int filler[154];
} oss_card_info;
    

This ioctl is used to query for information about the aggregate audio device.

The card field should be initialized by the application to the number of the card to query. This is a number between zero (inclusive) and value of numcards (exclusive) returned by SNDCTL_SYSINFO. Alternatively, when issuing the ioctl against a real mixer or dsp device, the special value -1 can be used to indicate that the query is being made against the device opened. If -1 is used, the field is overwritten with the number for the current hardware device on successful return.

The shortname, longname, and hw_info contain ASCIIZ strings describing the device in more detail. The hw_info member can contain multiple lines of detail, each line ending in a NEWLINE.

The flag, intr_count, and ack_count fields are not used by this implementation.

SNDCTL_MIXERINFO

The argument is a pointer to a struct oss_mixer_info, which has the following definition:

typedef struct oss_mixerinfo {
    int dev;
    char id[16];        /* Reserved */
    char name[32];
    int modify_counter;
    int card_number;
    int port_number;    /* Reserved */
    char handle[32];    /* Reserved */
    int magic;          /* Reserved */
    int enabled;        /* Reserved */
    int caps;           /* Reserved */
    int flags;          /* Reserved */
    int nrext;
    int priority;

    /* Deice special file name (absolute path) */
    char devnode[32];
    int legacy_device;  /* Reserved */
    int filler[245];    /* Reserved */
} oss_mixerinfo;
    

In the above structure, all of the fields are reserved except the following: dev, name, modify_counter, card_number, nrext, priority, and devnode. The reserved fields are provided for compatibility with other OSS implementations, and available for legacy applications. New applications should not attempt to use these fields.

The dev field should be initialized by the application to the number of the device to query. This is a number between zero inclusive and value of nummixers (exclusive) returned by SNDCTL_SYSINFO, or by SNDCTL_MIX_NRMIX. Alternatively, when issuing the ioctl against a real mixer or dsp device, the special value -1 can be used to indicate that the query is being made against the device opened. If -1 is used, the field is overwritten with the mixer number for the current open file on successful return.

No other fields are significant upon entry, but on successful return contains details of the device.

The name field is a human readable name representing the device. Applications should not try to interpret it.

The modify_counter is changed by the mixer framework each time the settings for the various controls or extensions of the device are changed. Applications can poll this value to learn if any other changes need to be searched for.

The card_number field is the number of the aggregate audio device this mixer is located on. It can be used with the SNDCTL_CARDINFO ioctl.

The nrext field is the number of mixer extensions available on this mixer. See the SNDCTL_MIX_NREXT description.

The priority is used by the framework to assign a preference that applications can use in choosing a device. Higher values are preferable. Mixers with priorities less than -1 should never be selected by default.

The devnode field contains the actual full path to the device node for the physical mixer, such as /dev/sound/audio810:0mixer. Applications should open this file to access the mixer settings.

Mixer Extension IOCTLs

Those ioctls should only be used by dedicated mixer applications or desktop volume controls, and not by typical ordinary audio applications such as media players. Ordinary applications that wish to adjust their own volume settings should use the SNDCTL_DSP_SETPLAYVOL or SNDCTL_DSP_SETRECVOL ioctls for that purpose. See dsp(7I) for more information. Ordinary applications should never attempt to change master port selection or hardware settings such as monitor gain settings.

The ioctls in this section can only be used to access the mixer device that is associated with the current file descriptor.

Applications should not assume that a single /dev/mixer node is able to access any physical settings. Instead, they should use the ioctl SNDCTL_MIXERINFO to determine the device path for the real mixer device, and issue ioctls on a file descriptor opened against the corresponding devnode field.

When a dev member is specified in each of the following ioctls, the application should specify -1, although for compatibility the mixer allows the application to specify the mixer device number.

SNDCTL_MIX_NRMIX

The argument is a pointer to an integer, which receives the number of mixer devices in the system. Each can be queried by using its number with the SNDCTL_MIXERINFO ioctl. The same information is available using the SNDCTL_SYSINFO ioctl.

SNDCTL_MIX_NREXT

The argument is a pointer to an integer. On entry, the integer should contain the special value -1. On return the argument receives the number of mixer extensions (or mixer controls) supported by the mixer device. More details about each extension can be obtained by SNDCTL_MIX_EXTINFO ioctl.

SNDCTL_MIX_EXTINFO

The argument is a pointer to an oss_mixext structure which is defined as follows:

typedef struct oss_mixext {
   int dev;            /* Mixer device number */
   int ctrl;           /* Extension number */
   int type;           /* Entry type */
   int maxvalue;
   int minvalue;
   int flags;
   char id[16];  /* Mnemonic ID (internal use) */
   int parent;   /* Entry# of parent (-1 if root) */
   int dummy;          /* NOT SUPPORTED */
   int timestamp;
   char data[64];      /* Reserved */

   /* Mask of allowed enum values */
   unsigned char enum_present[32];
   int control_no;     /* Reserved */
   unsigned int desc;  /* NOT SUPPORTED */
   char extname[32];
   int update_counter;
   int filler[7];      /* Reserved */
} oss_mixext;
    

On entry, the dev field should be initialized to the value -1, and the ctrl field should be initialized with the number of the extension being accessed. Between 0, inclusive, and the value returned by SNDCTL_MIX_NREXT, exclusive.

Mixer extensions are organized as a logical tree, starting with a root node. The root node always has a ctrl value of zero. The structure of the tree can be determined by looking at the parent field, which contains the extension number of the parent extension, or -1 if the extension is the root extension.

The type indicates the type of extension used. This implementation supports the following values:

MIXT_DEVROOT	Root node for extension tree
MIXT_GROUP	Logical grouping of controls
MXIT_ONOFF	Boolean value, 0 = off, 1 = on.
MIXT_ENUM	Enumerated value, 0 to maxvalue.
MIXT_MONOSLIDER	Monophonic slider, 0 to 255.
MIXT_STEREOSLIDER	Stereophonic slider, 0 to 255 (encoded as lower two bytes in value.)
MIXT_MARKER	Place holder, can ignore.

The flags field is a bit array. This implementation makes use of the following possible bits:

MIXF_READABLE	Extension's value is readable.
MIXF_WRITEABLE	Extension's value is modifiable.
MIXF_POLL	Extension can self-update.
MIXF_PCMVOL	Extension is for master PCM playback volume.
MIXF_MAINVOL	Extension is for a typical analog volume
MIXF_RECVOL	Extension is for master record gain.
MIXF_MONVOL	Extension is for a monitor source's gain.

The id field contains an ASCIIZ identifier for the extension.

The timestamp field is set when the extension tree is first initialized. Applications must use the same timestamp value when attempting to change the values. A change in the timestamp indicates a change a in the structure of the extension tree.

The enum_present field is a bit mask of possible enumeration values. If a bit is present in the enum_present mask, then the corresponding enumeration value is legal. The mask is in little endian order.

The desc field provides information about scoping, which can be useful as layout hints to applications. The following hints are available:

MIXEXT_SCOPE_MASK	Mask of possible scope values.
MIXEXT_SCOPE_INPUT	Extension is an input control.
MIXEXT_SCOPE_OUTPUT	Extension is an output control.
MIXEXT_SCOPE_MONITOR	Extension relates to input monitoring.
MIXEXT_SCOPE_OTHER	scoping hint provided.

The extname is the full name of the extension.

The update_counter is incremented each time the control's value is changed.

SNDCTL_MIX_ENUMINFO

The argument is a pointer to an oss_mixer_enuminfo structure, which is defined as follows:

typedef struct oss_mixer_enuminfo {
   int dev;
   int ctrl;
   int nvalues;
   int version;
   short strindex[255];
   char strings[3000];
} oss_mixer_enuminfo;
    

On entry, the dev field should be initialized to the value -1, and the ctrl field should be initialized with the number of the extension being accessed. Between 0, inclusive, and the value returned by SNDCTL_MIX_NREXT, exclusive.

On return the nvalues field contains the number of values, and strindex contains an array of indices into the strings member, each index pointing to an ASCIIZ describing the enumeration value.

SNDCTL_MIX_READ

 

SNDCTL_MIX_WRITE

The argument is a pointer to an oss_mixer_value structure, defined as follows:

typedef struct oss_mixer_value {
   int dev;
   int ctrl;
   int value;

   /* Reserved for future use.  Initialize to 0 */
   int flags;

   /* Must be set to oss_mixext.timestamp */
   int timestamp;

   /* Reserved for future use.  Initialize to 0 */
   int filler[8];
} oss_mixer_value;

    

On entry, the dev field should be initialized to the value -1, and the ctrl field should be initialized with the number of the extension being accessed. Between 0, inclusive, and the value returned by SNDCTL_MIX_NREXT, exclusive. Additionally, the timestamp member must be initialized to the same value as was supplied in the oss_mixext structure used with SNDCTL_MIX_EXTINFO.

For SNDCTL_MIX_WRITE, the application should supply the new value for the extension. For SNDCTL_MIX_READ, the mixer returns the extensions current value in value.

Compatibility IOCTLs

These ioctls can affect the software volume levels associated with the calling process. They have no effect on the physical hardware levels or settings. They should not be used in new applications.