void _clock_init(void);

This is a function that you can re-implement in an implementation-specific way. It is called from the library initialization code, so you do not have to call it from your application code.

The initialization that _clock_init() applies enables clock() to return the time that has elapsed since the program was started.

An example of how you might re-implement _clock_init() might be to set the timer to zero. However, if your implementation of clock() relies on a system timer that cannot be reset, then _clock_init() could instead read the time at startup (when called from the library initialization code), with clock() subsequently subtracting the time that was read at initialization, from the current value of the timer. In both cases, some form of initialization is required of _clock_init().