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(elisp)Accepting Output

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Accepting Output from Processes
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   Output from asynchronous subprocesses normally arrives only while
Emacs is waiting for some sort of external event, such as elapsed time
or terminal input.  Occasionally it is useful in a Lisp program to
explicitly permit output to arrive at a specific point, or even to wait
until output arrives from a process.

 - Function: accept-process-output &optional process seconds millisec
     This function allows Emacs to read pending output from processes.
     The output is inserted in the associated buffers or given to their
     filter functions.  If PROCESS is non-`nil' then this function does
     not return until some output has been received from PROCESS.

     The arguments SECONDS and MILLISEC let you specify timeout
     periods.  The former specifies a period measured in seconds and the
     latter specifies one measured in milliseconds.  The two time
     periods thus specified are added together, and
     `accept-process-output' returns after that much time whether or
     not there has been any subprocess output.

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

     Not all operating systems support waiting periods other than
     multiples of a second; on those that do not, you get an error if
     you specify nonzero MILLISEC.

     The function `accept-process-output' returns non-`nil' if it did
     get some output, or `nil' if the timeout expired before output
     arrived.