SYS-PCTL(2)SYS-PCTL(2)NAME
pctl - process control
SYNOPSIS
include "sys.m";
sys := load Sys Sys->PATH;
pctl: fn(flags: int, movefd: list of int): int;
DESCRIPTION
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 working directory, and so on.
Pctl controls this sharing, allowing 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 mes‐
sage 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 descrip‐
tors are closed except those listed in movefd, which are pre‐
served 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 containing a
copy of the current set of file descriptors. The file descrip‐
tors listed in movefd are closed in the old group. After this
call, changes the process makes to its set of open file descrip‐
tors 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 directory, of the new name
space. The current directory is unaffected by this call.
FORKNS Place the process in a new file name space group containing 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 processes.
NODEVS Set the current file name space group to prevent subsequent
access to the roots of file trees implemented by a device driver
(ie, the use of path names beginning with #, as described by
intro(3)). Even after NODEVS the following devices can be
attached, which are either private or can be separately con‐
trolled: 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 containing a copy
of the current environment variables. After this call, changes
the process makes to its environment will not affect other pro‐
cesses.
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 NEWPGRP|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 ALSOsh-std(1), sys-intro(2)SYS-PCTL(2)
