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     40 >Video for Linux Two API Specification: Revision 0.24</TH
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     76 NAME="APP-PRI"
     77 >1.3. Application Priority</A
     78 ></H1
     79 ><P
     80 >When multiple applications share a device it may be
     81 desirable to assign them different priorities. Contrary to the
     82 traditional "rm -rf /" school of thought a video recording application
     83 could for example block other applications from changing video
     84 controls or switching the current TV channel. Another objective is to
     85 permit low priority applications working in background, which can be
     86 preempted by user controlled applications and automatically regain
     87 control of the device at a later time.</P
     88 ><P
     89 >Since these features cannot be implemented entirely in user
     90 space V4L2 defines the <A
     91 HREF="r11946.htm"
     92 ><CODE
     93 CLASS="CONSTANT"
     94 >VIDIOC_G_PRIORITY</CODE
     95 ></A
     96 > and <A
     97 HREF="r11946.htm"
     98 ><CODE
     99 CLASS="CONSTANT"
    100 >VIDIOC_S_PRIORITY</CODE
    101 ></A
    102 >
    103 ioctls to request and query the access priority associate with a file
    104 descriptor. Opening a device assigns a medium priority, compatible
    105 with earlier versions of V4L2 and drivers not supporting these ioctls.
    106 Applications requiring a different priority will usually call
    107 <CODE
    108 CLASS="CONSTANT"
    109 >VIDIOC_S_PRIORITY</CODE
    110 > after verifying the device with
    111 the <A
    112 HREF="r13105.htm"
    113 ><CODE
    114 CLASS="CONSTANT"
    115 >VIDIOC_QUERYCAP</CODE
    116 ></A
    117 > ioctl.</P
    118 ><P
    119 >Ioctls changing driver properties, such as <A
    120 HREF="r11217.htm"
    121 ><CODE
    122 CLASS="CONSTANT"
    123 >VIDIOC_S_INPUT</CODE
    124 ></A
    125 >,
    126 return an <SPAN
    127 CLASS="ERRORCODE"
    128 >EBUSY</SPAN
    129 > error code after another application obtained higher priority.
    130 An event mechanism to notify applications about asynchronous property
    131 changes has been proposed but not added yet.</P
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