+called the @dfn{filter function} can be called to act on the output. If
+the process has no buffer and no filter function, its output is
+discarded.
+
+ Output from a subprocess can arrive only while Emacs is waiting: when
+reading terminal input, in @code{sit-for} and @code{sleep-for}
+(@pxref{Waiting}), and in @code{accept-process-output} (@pxref{Accepting
+Output}). 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.
+
+ It is impossible to separate the standard output and standard error
+streams of the subprocess, because Emacs normally spawns the subprocess
+inside a pseudo-TTY, and a pseudo-TTY has only one output channel. 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.
+
+ Subprocess output is normally decoded using a coding system before the
+buffer or filter function receives it, much like text read from a file.
+You can use @code{set-process-coding-system} to specify which coding
+system to use (@pxref{Process Information}). Otherwise, the coding
+system comes from @code{coding-system-for-read}, if that is
+non-@code{nil}; or else from the defaulting mechanism (@pxref{Default
+Coding Systems}).
+
+ @strong{Warning:} Coding systems such as @code{undecided} which
+determine the coding system from the data do not work entirely reliably
+with asynchronous subprocess output. This is because Emacs has to
+process asynchronous subprocess output in batches, as it arrives. Emacs
+must try to detect the proper coding system from one batch at a time,
+and this does not always work. Therefore, if at all possible, use a
+coding system which determines both the character code conversion and
+the end of line conversion---that is, one like @code{latin-1-unix},
+rather than @code{undecided} or @code{latin-1}.