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 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
containing 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.
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SYS-PCTL(2) SYS-PCTL(2)
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 imple-
mented 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 sepa-
rately 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 con-
taining a copy of the current environment vari-
ables. 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
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 ALSO
sh-std(1), sys-intro(2)
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