Canon EOS C100 Instruction Manual page 47

Recording Video
Locking the Controls in Recording Mode
mode, you can set the Q switch to C (key lock) to
Once in
lock all the physical controls (buttons and switches) on the camcorder,
save for the START/STOP button*. This is useful in preventing settings
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from being changed due to inadvertently pressing one of the buttons.
Return the Q switch to the CAMERA position to reactivate the
controls.
Additionally, the START/STOP button on the handle unit has its own lock
lever to prevent its accidental operation. Turn the lock lever to C position
to lock the START/STOP button**. Return the lock lever to its original
position to enable use of the START/STOP button again.
* With the [w Other Functions] > [Custom Function] > [C START/STOP] setting, you
can choose to lock all buttons, including the START/STOP button.
**You can still use the START/STOP buttons on the camcorder and grip unit.