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36.9.4 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 just-this-one

This function allows Emacs to read pending output from processes. The output is 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, even if there is no subprocess output.

The argument millisec is obsolete (and should not be used), because seconds can be floating point to specify waiting a fractional number of seconds. If seconds is 0, the function accepts whatever output is pending but does not wait.

If process is a process, and the argument just-this-one is non-nil, only output from that process is handled, suspending output from other processes until some output has been received from that process or the timeout expires. If just-this-one is an integer, also inhibit running timers. This feature is generally not recommended, but may be necessary for specific applications, such as speech synthesis.

The function accept-process-output returns non-nil if it got output from process, or from any process if process is nil. It returns nil if the timeout expired before output arrived.