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.
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
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
returns after that much time, even if there is no
The argument millisec is obsolete (and should not be used), because seconds can be floating point to specify waiting a fractional number of seconds.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
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
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