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GNU Emacs Lisp Reference Manual

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37.9.3 Accepting Output from Processes

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.

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