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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. If
Emacs uses a pseudo-TTY (pty) for communication with the subprocess,
then it is impossible to separate the standard output and standard
error streams of the subprocess, because a pseudo-TTY has only one
output channel. In that case, if you want to keep the output to those
streams separate, you should redirect one of them to a file—for
example, by using an appropriate shell command via
start-process-shell-command or a similar 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; in that case,
Emacs will use pipes for communicating with the subprocess.
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), in
accept-process-output (see Accepting Output), and in
functions which send data to processes (see 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-bufferingto a non-nilvalue (the default), as it will automatically delay reading from such processes, thus allowing them to produce more output before Emacs tries to read it.