Manual Pages  — SNDSTAT

sndstat – nvlist-based PCM audio device enumeration interface

SYNOPSIS DESCRIPTION IOCTLS FILES EXAMPLES SEE ALSO HISTORY AUTHORS

To compile the driver into the kernel, place the following lines in the kernel configuration file: device sound

The ioctl interface provided by /dev/sndstat device allows callers to enumerate PCM audio devices available for use. In other words, it provides means to get the list of all audio devices available to the system.

For ioctl calls that take an argument, the following structure is used:

struct sndstioc_nv_arg {
        size_t nbytes;
        void *buf;
};

Here is an example of an nvlist object with explanations of the common fields:

dsps (NVLIST ARRAY): 1
        from_user (BOOL): FALSE
        nameunit (STRING): [pcm0]
        devnode (STRING): [dsp0]
        desc (STRING): [Generic (0x8086) (Analog Line-out)]
        pchan (NUMBER): 1
        rchan (NUMBER): 0
        info_play (NVLIST):
                min_rate (NUMBER): 48000
                max_rate (NUMBER): 48000
                formats (NUMBER): 16
                min_chn (NUMBER): 2
                max_chn (NUMBER): 2
        provider_info (NVLIST):
                unit (NUMBER): 0
                status (STRING): on hdaa0
                bitperfect (BOOL): FALSE
                pvchan (NUMBER): 1
                pvchanrate (NUMBER): 48000
                pvchanformat (NUMBER): 0x00000010
                rvchan (NUMBER): 0
                rvchanrate (NUMBER): 48000
                rvchanformat (NUMBER): 0x00000010
                channel_info (NVLIST_ARRAY): 1
                        name (STRING): pcm0:virtual_play:dsp0.vp0
                        parentchan (STRING): pcm0:play:dsp0.p0
                        unit (NUMBER): 1
                        caps (NUMBER): 0x073200
                        latency (NUMBER): 2
                        rate (NUMBER): 48000
                        format (NUMBER): 0x201000
                        pid (NUMBER): 1234
                        comm (STRING): mpv
                        interrupts (NUMBER): 0
                        feedcount (NUMBER): 0
                        xruns (NUMBER): 0
                        left_volume (NUMBER): 45
                        right_volume (NUMBER): 45
                        hwbuf_fmt (NUMBER): 0x200010
                        hwbuf_size (NUMBER): 0
                        hwbuf_blksz (NUMBER): 0
                        hwbuf_blkcnt (NUMBER): 0
                        hwbuf_free (NUMBER): 0
                        hwbuf_ready (NUMBER): 0
                        swbuf_fmt (NUMBER): 0x201000
                        swbuf_size (NUMBER): 16384
                        swbuf_blksz (NUMBER): 2048
                        swbuf_blkcnt (NUMBER): 8
                        swbuf_free (NUMBER): 16384
                        swbuf_ready (NUMBER): 0
                        feederchain (STRING):
                                [userland ->
                                feeder_root(0x00201000) ->
                                feeder_format(0x00201000 -> 0x00200010) ->
                                feeder_volume(0x00200010) -> hardware]
        provider (STRING): [sound(4)]

from_user	Whether the PCM audio device node is created by in-kernel audio subsystem or userspace providers.
nameunit	The device identification in the form of subsystem plus a unit number.
devnode	The PCM audio device node relative path in devfs.
desc	The descripton of the PCM audio device.
pchan	The number of playback channels supported by hardware. This can be 0 if this PCM audio device does not support playback at all.
rchan	The number of recording channels supported by hardware. This can be 0 if this PCM audio device does not support recording at all.
info_play	Supported configurations in playback direction. This exists only if this PCM audio device supports playback. There are a number of name/value pairs inside this field:

min_rate
	Minimum supported sampling rate.
max_rate
	Maximum supported sampling rate.
formats	Supported sample formats.
min_chn	Minimum supported number of channels in channel layout
max_chn	Maximum supported number of channels in channel layout

info_rec
	Supported configurations in recording direction. This exists only if this PCM audio device supports recording. There are a number of name/value pairs inside this field:
min_rate
	Minimum supported sampling rate.
max_rate
	Maximum supported sampling rate.
formats	Supported sample formats.
min_chn	Minimum supported number of channels in channel layout
max_chn	Maximum supported number of channels in channel layout

provider_info
	Provider-specific fields. This field may not exist if the PCM audio device is not provided by in-kernel interface. This field will not exist if the provider field is an empty string. For the sound(4) provider, there are a number of name/value pairs inside this field:

unit	Sound card unit.
status	Status string. Usually reports the driver the device is attached on.
bitperfect	Whether the sound card has bit-perfect mode enabled.
pvchan	Number of playback virtual channels.
pvchanrate	Playback virtual channel sample rate.
pvchanformat
	Playback virtual channel format.
rvchan	Number of recording virtual channels.
rvchanrate	Recording virtual channel sample rate.
rvchanformat
	Recording virtual channel format.
channel_info
	Channel information. There are a number of name/value pairs inside this field:
name	Channel name.
parentchan	Parent channel name (e.g., in the case of virtual channels).
unit	Channel unit.
caps	OSS capabilities.
latency	Latency.
rate	Sampling rate.
format	Sampling format.
pid	PID of the process consuming the channel.
comm	Name of the process consuming the channel.
interrupts	Number of interrupts since the channel has been opened.
xruns	Number of overruns/underruns, depending on channel direction.
feedcount	Number of read/written bytes since the channel has been opened.
left_volume	Left volume.
right_volume
	Right volume.
hwbuf_format
	Hardware buffer format.
hwbuf_size	Hardware buffer size.
hwbuf_blksz	Hardware buffer block size.
hwbuf_blkcnt
	Hardware buffer block count.
hwbuf_free	Free space in hardware buffer (in bytes).
hwbuf_ready	Number of bytes ready to be read/written from hardware buffer.
swbuf_format
	Software buffer format.
swbuf_size	Software buffer size.
swbuf_blksz	Software buffer block size.
swbuf_blkcnt
	Software buffer block count.
swbuf_free	Free space in software buffer (in bytes).
swbuf_ready	Number of bytes ready to be read/written from software buffer.
feederchain	Channel feeder chain.

provider	A string specifying the provider of the PCm audio device.

The following ioctls are provided for use:

SNDSTIOC_REFRESH_DEVS	Drop any previously fetched PCM audio devices list snapshots. This ioctl takes no arguments.
SNDSTIOC_GET_DEVS	Generate and/or return PCM audio devices list snapshots to callers. This ioctl takes a pointer to struct, sndstioc_nv_arg as the first and the only argument. Callers need to provide a sufficiently large buffer to hold a serialized nvlist. If there is no existing PCM audio device list snapshot available in the internal structure of the opened sndstat. fd, a new PCM audio device list snapshot will be automatically generated. Callers have to set nbytes to either 0 or the size of buffer provided. In case nbytes is 0, the buffer size required to hold a serialized nvlist stream of current snapshot will be returned in nbytes, and buf will be ignored. Otherwise, if the buffer is not sufficiently large, the ioctl returns success, and nbytes will be set to 0. If the buffer provided is sufficiently large, nbytes will be set to the size of the serialized nvlist written to the provided buffer. Once a PCM audio device list snapshot is returned to user-space successfully, the snapshot stored in the subsystem's internal structure of the given fd will be freed.
SNDSTIOC_ADD_USER_DEVS	Add a list of PCM audio devices provided by callers to /dev/sndstat device. This ioctl takes a pointer to struct, sndstioc_nv_arg as the first and the only argument. Callers have to provide a buffer holding a serialized nvlist. nbytes should be set to the length in bytes of the serialized nvlist. buf should be pointed to a buffer storing the serialized nvlist. Userspace-backed PCM audio device nodes should be listed inside the serialized nvlist.
SNDSTIOC_FLUSH_USER_DEVS
	Flush any PCM audio devices previously added by callers. This ioctl takes no arguments.

/dev/sndstat

The following code enumerates all available PCM audio devices:

#include <sys/types.h>
#include <err.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/nv.h>
#include <sys/sndstat.h>
#include <sysexits.h>
#include <unistd.h>

int
main()
{
        int fd;
        struct sndstioc_nv_arg arg;
        const nvlist_t * const *di;
        size_t i, nitems;
        nvlist_t *nvl;

        /* Open sndstat node in read-only first */
        fd = open("/dev/sndstat", O_RDONLY);

        if (ioctl(fd, SNDSTIOC_REFRESH_DEVS, NULL))
                err(1, "ioctl(fd, SNDSTIOC_REFRESH_DEVS, NULL)");

        /* Get the size of snapshot, when nbytes = 0 */
        arg.nbytes = 0;
        arg.buf = NULL;
        if (ioctl(fd, SNDSTIOC_GET_DEVS, &arg))
                err(1, "ioctl(fd, SNDSTIOC_GET_DEVS, &arg)");

        /* Get snapshot data */
        arg.buf = malloc(arg.nbytes);
        if (arg.buf == NULL)
                err(EX_OSERR, "malloc");
        if (ioctl(fd, SNDSTIOC_GET_DEVS, &arg))
                err(1, "ioctl(fd, SNDSTIOC_GET_DEVS, &arg)");

        /* Deserialize the nvlist stream */
        nvl = nvlist_unpack(arg.buf, arg.nbytes, 0);
        free(arg.buf);

        /* Get DSPs array */
        di = nvlist_get_nvlist_array(nvl, SNDST_DSPS, &nitems);
        for (i = 0; i < nitems; i++) {
                const char *nameunit, *devnode, *desc;

                /*
                 * Examine each device nvlist item
                 */

                nameunit = nvlist_get_string(di[i], SNDST_DSPS_NAMEUNIT);
                devnode = nvlist_get_string(di[i], SNDST_DSPS_DEVNODE);
                desc = nvlist_get_string(di[i], SNDST_DSPS_DESC);
                printf("Name unit: `%s`, Device node: `%s`, Description: `%s`0,
                    nameunit, devnode, desc);
        }

        nvlist_destroy(nvl);
        return (0);
}

The nvlist-based ioctls support for sndstat device first appeared in FreeBSD 13.0 .

This manual page was written by Ka Ho Ng <Mt khng@FreeBSD.org>.