To query the current audio input applications zero out the
reserved array of a struct v4l2_audio
and call the VIDIOC_G_AUDIO ioctl with a pointer
to this structure. Drivers fill the rest of the structure or return an
EINVAL error code when the device has no audio inputs, or none which combine
with the current video input.
Audio inputs have one writable property, the audio mode. To
select the current audio input and change the
audio mode, applications initialize the
index and mode
fields, and the
reserved array of a
v4l2_audio structure and call the
VIDIOC_S_AUDIO ioctl. Drivers may switch to a
different audio mode if the request cannot be satisfied. However, this
is a write-only ioctl, it does not return the actual new audio
mode.
Table 1. struct v4l2_audio
| __u32 | index | Identifies the audio input, set by the driver or application. |
| __u8 | name[32] | Name of the audio input, a NUL-terminated ASCII string, for example: "Line In". This information is intended for the user, preferably the connector label on the device itself. |
| __u32 | capability | Audio capability flags, see Table 2. |
| __u32 | mode | Audio mode flags set by drivers and applications (on
VIDIOC_S_AUDIO ioctl), see Table 3. |
| __u32 | reserved[2] | Reserved for future extensions. Drivers and applications must set the array to zero. |
Table 2. Audio Capability Flags
V4L2_AUDCAP_STEREO | 0x00001 | This is a stereo input. The flag is intended to automatically disable stereo recording etc. when the signal is always monaural. The API provides no means to detect if stereo is received, unless the audio input belongs to a tuner. |
V4L2_AUDCAP_AVL | 0x00002 | Automatic Volume Level mode is supported. |
On success 0 is returned, on error -1 and the errno variable is set appropriately:
No audio inputs combine with the current video input, or the number of the selected audio input is out of bounds or it does not combine, or there are no audio inputs at all and the ioctl is not supported.
I/O is in progress, the input cannot be switched.