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(elisp)Waiting

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Waiting for Elapsed Time or Input
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   The wait functions are designed to wait for a certain amount of time
to pass or until there is input.  For example, you may wish to pause in
the middle of a computation to allow the user time to view the display.
`sit-for' pauses and updates the screen, and returns immediately if
input comes in, while `sleep-for' pauses without updating the screen.

 - Function: sit-for seconds &optional millisec nodisp
     This function performs redisplay (provided there is no pending
     input from the user), then waits SECONDS seconds, or until input is
     available.  The value is `t' if `sit-for' waited the full time
     with no input arriving (see `input-pending-p' in Note: Event Input
     Misc).  Otherwise, the value is `nil'.

     The argument SECONDS need not be an integer.  If it is a floating
     point number, `sit-for' waits for a fractional number of seconds.
     Some systems support only a whole number of seconds; on these
     systems, SECONDS is rounded down.

     The optional argument MILLISEC specifies an additional waiting
     period measured in milliseconds.  This adds to the period
     specified by SECONDS.  If the system doesn't support waiting
     fractions of a second, you get an error if you specify nonzero
     MILLISEC.

     The expression `(sit-for 0)' is a convenient way to request a
     redisplay, without any delay.  Note: Forcing Redisplay.

     If NODISP is non-`nil', then `sit-for' does not redisplay, but it
     still returns as soon as input is available (or when the timeout
     elapses).

     Iconifying or deiconifying a frame makes `sit-for' return, because
     that generates an event.  Note: Misc Events.

     The usual purpose of `sit-for' is to give the user time to read
     text that you display.

 - Function: sleep-for seconds &optional millisec
     This function simply pauses for SECONDS seconds without updating
     the display.  It pays no attention to available input.  It returns
     `nil'.

     The argument SECONDS need not be an integer.  If it is a floating
     point number, `sleep-for' waits for a fractional number of seconds.
     Some systems support only a whole number of seconds; on these
     systems, SECONDS is rounded down.

     The optional argument MILLISEC specifies an additional waiting
     period measured in milliseconds.  This adds to the period
     specified by SECONDS.  If the system doesn't support waiting
     fractions of a second, you get an error if you specify nonzero
     MILLISEC.

     Use `sleep-for' when you wish to guarantee a delay.

   Note: Time of Day, for functions to get the current time.