To query the current audio output applications zero out the
reserved array of a struct v4l2_audioout and
call the VIDIOC_G_AUDOUT 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 output.
Audio outputs have no writable properties. Nevertheless, to
select the current audio output applications can initialize the
index field and
reserved array (which in the future may
contain writable properties) of a
v4l2_audioout structure and call the
VIDIOC_S_AUDOUT ioctl. Drivers switch to the
requested output or return the EINVAL error code when the index is out of
bounds. This is a write-only ioctl, it does not return the current
audio output attributes as VIDIOC_G_AUDOUT
does.
Note connectors on a TV card to loop back the received audio signal to a sound card are not audio outputs in this sense.
Table 1. struct v4l2_audioout
| __u32 | index | Identifies the audio output, set by the driver or application. |
| __u8 | name[32] | Name of the audio output, a NUL-terminated ASCII string, for example: "Line Out". This information is intended for the user, preferably the connector label on the device itself. |
| __u32 | capability | Audio capability flags, none defined yet. Drivers must set this field to zero. |
| __u32 | mode | Audio mode, none defined yet. Drivers and
applications (on VIDIOC_S_AUDOUT) must set this
field to zero. |
| __u32 | reserved[2] | Reserved for future extensions. Drivers and applications must set the array to zero. |
On success 0 is returned, on error -1 and the errno variable is set appropriately:
No audio outputs combine with the current video output, or the number of the selected audio output is out of bounds or it does not combine, or there are no audio outputs at all and the ioctl is not supported.
I/O is in progress, the output cannot be switched.