The output that an asynchronous subprocess writes to its standard output stream is passed to a function called the filter function. The default filter function simply inserts the output into a buffer, which is called the associated buffer of the process (see Process Buffers). If the process has no buffer then the default filter discards the output.
If the subprocess writes to its standard error stream, by default
the error output is also passed to the process filter function.
Alternatively, you could use the :stderr parameter with a
non-nil value in a call to make-process
(see make-process) to make the destination
of the error output separate from the standard output.
When a subprocess terminates, Emacs reads any pending output, then stops reading output from that subprocess. Therefore, if the subprocess has children that are still live and still producing output, Emacs won’t receive that output.
Output from a subprocess can arrive only while Emacs is waiting: when
reading terminal input (see the function waiting-for-user-input-p),
in sit-for and sleep-for (see Waiting for Elapsed Time or Input), in
accept-process-output (see Accepting Output from Processes), and in
functions which send data to processes (see Sending Input to Processes).
This minimizes the problem of timing errors that usually plague parallel
programming. For example, you can safely create a process and only
then specify its buffer or filter function; no output can arrive
before you finish, if the code in between does not call any primitive
that waits.
On some systems, when Emacs reads the output from a subprocess, the
output data is read in very small blocks, potentially resulting in
very poor performance. This behavior can be remedied to some extent
by setting the variable process-adaptive-read-buffering to a
non-nil value (the default), as it will automatically delay reading
from such processes, thus allowing them to produce more output before
Emacs tries to read it.