SNDSTAT(4)

NAME
sndstat - nvlist-based PCM audio device enumeration interface

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

device sound

DESCRIPTION
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.

IOCTLS
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 (1) (0x1)
        rchan (NUMBER): 0 (0) (0x0)
        info_play (NVLIST):
            min_rate (NUMBER): 48000 (48000) (0xbb80)
            max_rate (NUMBER): 48000 (48000) (0xbb80)
            formats (NUMBER): 16 (16) (0x10)
            min_chn (NUMBER): 2 (2) (0x2)
            max_chn (NUMBER): 2 (2) (0x2)
        provider_info (NVLIST):
            unit (NUMBER): 0 (0) (0x0)
            bitperfect (BOOL): FALSE
            pvchan (NUMBER): 1 (1) (0x1)
            rvchan (NUMBER): 0 (0) (0x0)
        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.

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.

FILES
/dev/sndstat

EXAMPLES
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`,
    		    nameunit, devnode, desc);
    	}
    	nvlist_destroy(nvl);
    	return (0);
    }

SEE ALSO
sound(4), nv(9)

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

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