Commit 22ece83a authored by Eli Zaretskii's avatar Eli Zaretskii

src/w32proc.c: Describe in a comment w32 subprocess implementation.

parent 8478885d
......@@ -787,6 +787,138 @@ alarm (int seconds)
/* Here's an overview of how support for subprocesses and
network/serial streams is implemented on MS-Windows.
The management of both subprocesses and network/serial streams
circles around the child_procs[] array, which can record up to the
grand total of MAX_CHILDREN (= 32) of these. (The reasons for the
32 limitation will become clear below.) Each member of
child_procs[] is a child_process structure, defined on w32.h.
A related data structure is the fd_info[] array, which holds twice
as many members, 64, and records the information about file
descriptors used for communicating with subprocesses and
network/serial devices. Each member of the array is the filedesc
structure, which records the Windows handle for communications,
such as the read end of the pipe to a subprocess, a socket handle,
Both these arrays reference each other: there's a member of
child_process structure that records the file corresponding
descriptor, and there's a member of filedesc structure that holds a
pointer to the corresponding child_process.
Whenever Emacs starts a subprocess or opens a network/serial
stream, the function new_child is called to prepare a new
child_process structure. new_child looks for the first vacant slot
in the child_procs[] array, initializes it, and starts a "reader
thread" that will watch the output of the subprocess/stream and its
status. (If no vacant slot can be found, new_child returns a
failure indication to its caller, and the higher-level Emacs
primitive will then fail with EMFILE or EAGAIN.)
The reader thread started by new_child communicates with the main
(a.k.a. "Lisp") thread via two event objects and a status, all of
them recorded by the members of the child_process structure in
child_procs[]. The event objects serve as semaphores between the
reader thread and the 'select' emulation in sys_select, as follows:
. Initially, the reader thread is waiting for the char_consumed
event to become signaled by sys_select, which is an indication
for the reader thread to go ahead and try reading more stuff
from the subprocess/stream.
. The reader thread then attempts to read by calling a
blocking-read function. When the read call returns, either
successfully or with some failure indication, the reader thread
updates the status of the read accordingly, and signals the 2nd
event object, char_avail, on whose handle sys_select is
waiting. This tells sys_select that the file descriptor
allocated for the subprocess or the the stream is ready to be
read from.
When the subprocess exits or the network/serial stream is closed,
the reader thread sets the status accordingly and exits. It also
exits when the main thread sets the ststus to STATUS_READ_ERROR
and/or the char_avail and char_consumed event handles are NULL;
this is how delete_child, called by Emacs when a subprocess or a
stream is terminated, terminates the reader thread as part of
deleting the child_process object.
The sys_select function emulates the Posix 'pselect' function; it
is needed because the Windows 'select' function supports only
network sockets, while Emacs expects 'pselect' to work for any file
descriptor, including pipes and serial streams.
When sys_select is called, it uses the information in fd_info[]
array to convert the file descriptors which it was asked to watch
into Windows handles. In general, the handle to watch is the
handle of the char_avail event of the child_process structure that
corresponds to the file descriptor. In addition, for subprocesses,
sys_select watches one more handle: the handle for the subprocess,
so that it could emulate the SIGCHLD signal when the subprocess
If file descriptor zero (stdin) doesn't have its bit set in the
'rfds' argument to sys_select, the function always watches for
keyboard interrupts, to be able to return when the user presses
Having collected the handles to watch, sys_select calls
WaitForMultipleObjects to wait for any one of them to become
signaled. Since WaitForMultipleObjects can only watch up to 64
handles, Emacs on Windows is limited to maximum 32 child_process
objects (since a subprocess consumes 2 handles to be watched, see
When any of the handles become signaled, sys_select does whatever
is appropriate for the corresponding child_process object:
. If it's a handle to the char_avail event, sys_select marks the
corresponding bit in 'rfds', and Emacs will then read from that
file descriptor.
. If it's a handle to the process, sys_select calls the SIGCHLD
handler, to inform Emacs of the fact that the subprocess
The waitpid emulation works very similar to sys_select, except that
it only watches handles of subprocesses, and doesn't synchronize
with the reader thread.
Because socket descriptors on Windows are handles, while Emacs
expects them to be file descriptors, all low-level I/O functions,
such as 'read' and 'write', and all socket operations, like
'connect', 'recvfrom', 'accept', etc., are redirected to the
corresponding 'sys_*' functions, which must convert a file
descriptor to a handle using the fd_info[] array, and then invoke
the corresponding Windows API on the handle. Most of these
redirected 'sys_*' functions are implemented on w32.c.
When the file descriptor was produced by functions such as 'open',
the corresponding handle is obtained by calling _get_osfhandle. To
produce a file descriptor for a socket handle, which has no file
descriptor as far as Windows is concerned, the function
socket_to_fd opens the null device; the resulting file descriptor
will never be used directly in any I/O API, but serves as an index
into the fd_info[] array, where the socket handle is stored. The
SOCK_HANDLE macro retrieves the handle when given the file
The function sys_kill emulates the Posix 'kill' functionality to
terminate other processes. It does that by attaching to the
foreground window of the process and sending a Ctrl-C or Ctrl-BREAK
signal to the process; if that doesn't work, then it calls
TerminateProcess to forcibly terminate the process. Note that this
only terminates the immediate process whose PID was passed to
sys_kill; it doesn't terminate the child processes of that process.
This means, for example, that an Emacs subprocess run through a
shell might not be killed, because sys_kill will only terminate the
shell. (In practice, however, such problems are very rare.) */
/* Defined in <process.h> which conflicts with the local copy */
#define _P_NOWAIT 1
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