= pctl: process control

    include "sys.m";
    sys := load Sys Sys->PATH;

    pctl: fn(flags: int, movefd: list of int): int;

A newly spawned Limbo process (also known as a thread) shares
with its parent process a number of resources and properties,
such as file name space, open file descriptors, current work‐
ing directory, and so on.  Pctl controls this sharing, allow‐
ing  a  process  to  gain a copy of a resource rather than to
share it, to start with a null resource, and so on.

The set of processes sharing a property are called  a  group;
for  example  the  set  of processes sharing a name space are
called a name space group.  Each process is  a  member  of  a
process  group, typically the set of threads functioning as a
single program.  All the members of a process  group  may  be
terminated  at  once using the killgrp control message in the
prog(3) device.

A call to pctl affects the calling  process  and,  indirectly
according  to  flags,  any other processes sharing properties
with it.  The flags to pctl are:

NEWFD  Give the process a new  file  descriptor  group.   All
       file  descriptors  are  closed  except those listed in
       movefd, which are preserved in the new  group.   After
       this  call,  changes  the  process makes to its set of
       open file descriptors will not  be  visible  to  other
       processes.

FORKFD Place  the process in a new file descriptor group con‐
       taining a copy of the current set of file descriptors.
       The  file  descriptors  listed in movefd are closed in
       the old group.  After this call, changes  the  process
       makes  to its set of open file descriptors will not be
       visible to other processes.

NEWNS  Place the process in a new file name  space  group  in
       which  the  current  directory is made the root direc‐
       tory, of the new name space.  The current directory is
       unaffected by this call.

FORKNS Place  the process in a new file name space group con‐
       taining a copy of the current name space.  After  this
       call,  changes  the  process  makes to its name space,
       including chdir calls,  will  not  affect  other  pro‐
       cesses.

NODEVS Set  the current file name space group to prevent sub‐
       sequent access to the roots of file trees  implemented
       by  a  device driver (ie, the use of path names begin‐
       ning with #, as described by  intro(3)).   Even  after
       NODEVS  the  following  devices can be attached, which
       are either private or can  be  separately  controlled:
       pipe(3), env(3), srv(3) and ssl(3).

NEWENV Place  the  process  in  a new empty environment group
       containing no environment variables.  After this call,
       changes  the process makes to its environment will not
       affect other processes.

FORKENV
       Place the process in a new environment group  contain‐
       ing  a  copy  of  the  current  environment variables.
       After this call, changes  the  process  makes  to  its
       environment will not affect other processes.

NEWPGRP
       Establish a new process group with a group id equal to
       that of the pid of the calling process.

The Inferno shell sh(1) uses FORKFD when starting a  command;
its  pctl  built-in  (see  sh-std(1))  can  invoke  the other
effects when needed.  The window  manager  wm(1)  uses  NEWP‐
GRP|FORKFD  when  starting  a  window manager application.  A
network server  might  use  NEWPGRP|FORKNS|FORKFD|FORKENV  to
insulate itself completely from services it starts.

The  return value of pctl is the numerical process id of  the
calling process, which can be used for example to access  its
prog(3) files.

== see ALSO
sh-std(1), sys-intro(2)