.\" Copyright (C) 2006, Janak Desai <janak@us.ibm.com>
.\" and Copyright (C) 2006, Michael Kerrisk <mtk.manpages@gmail.com>
.\" Licensed under the GPL
.\"
.\" Patch Justification:
.\" unshare system call is needed to implement, using PAM,
.\" per-security_context and/or per-user namespace to provide
.\" polyinstantiated directories. Using unshare and bind mounts, a
.\" PAM module can create private namespace with appropriate
.\" directories(based on user's security context) bind mounted on
.\" public directories such as /tmp, thus providing an instance of
.\" /tmp that is based on user's security context. Without the
.\" unshare system call, namespace separation can only be achieved
.\" by clone, which would require porting and maintaining all commands
.\" such as login, and su, that establish a user session.
.\"
.\" FIXME Document CLONE_NEWIPC, which is new in 2.6.18
.\" FIXME Document CLONE_NEWUTS, which is new in 2.6.19
.\" FIXME Document CLONE_SYSVSEM, which is new in 2.6.26
.\"
.TH UNSHARE 2 2010-09-10 "Linux" "Linux Programmer's Manual"
.SH NAME
unshare \- disassociate parts of the process execution context
.SH SYNOPSIS
.nf
.BR "define _GNU_SOURCE" "             /* See feature_test_macros(7) */"
.\" Actually _BSD_SOURCE || _SVID_SOURCE
.\" See http://sources.redhat.com/bugzilla/show_bug.cgi?id=4749
.B #include <sched.h>
.sp
.BI "int unshare(int " flags );
.fi
.SH DESCRIPTION
.BR unshare ()
allows a process to disassociate parts of its execution
context that are currently being shared with other processes.
Part of the execution context, such as the mount namespace, is shared
implicitly when a new process is created using
.BR fork (2)
or
.BR vfork (2),
while other parts, such as virtual memory, may be
shared by explicit request when creating a process using
.BR clone (2).

The main use of
.BR unshare ()
is to allow a process to control its
shared execution context without creating a new process.

The
.I flags
argument is a bit mask that specifies which parts of
the execution context should be unshared.
This argument is specified by ORing together zero or more
of the following constants:
.TP
.B CLONE_FILES
Reverse the effect of the
.BR clone (2)
.B CLONE_FILES
flag.
Unshare the file descriptor table, so that the calling process
no longer shares its file descriptors with any other process.
.TP
.B CLONE_FS
Reverse the effect of the
.BR clone (2)
.B CLONE_FS
flag.
Unshare file system attributes, so that the calling process
no longer shares its root directory, current directory,
or umask attributes with any other process.
.BR chroot (2),
.BR chdir (2),
or
.BR umask (2)
.TP
.B CLONE_NEWNS
.\" These flag name are inconsistent:
.\" CLONE_NEWNS does the same thing in clone(), but CLONE_VM,
.\" CLONE_FS, and CLONE_FILES reverse the action of the clone()
.\" flags of the same name.
This flag has the
.I same
effect as the
.BR clone (2)
.B CLONE_NEWNS
flag.
Unshare the mount namespace,
so that the calling process has a private copy of
its namespace which is not shared with any other process.
Specifying this flag automatically implies
.B CLONE_FS
as well.
.\" As at 2.6.16, the following forced implications also apply,
.\" although the relevant flags are not yet implemented.
.\" If CLONE_THREAD is set force CLONE_VM.
.\" If CLONE_VM is set, force CLONE_SIGHAND.
.\" If CLONE_SIGHAND is set and signals are also being shared
.\" (i.e., current->signal->count > 1), force CLONE_THREAD.
.\"
.\" FIXME . CLONE_VM is not (yet, as at 2.6.16) implemented.
.\" .TP
.\" .B CLONE_VM
.\" Reverse the effect of the
.\" .BR clone (2)
.\" .B CLONE_VM
.\" flag.
.\" .RB ( CLONE_VM
.\" is also implicitly set by
.\" .BR vfork (2),
.\" and can be reversed using this
.\" .BR unshare ()
.\" flag.)
.\" Unshare virtual memory, so that the calling process no
.\" longer shares its virtual address space with any other process.
.PP
If
.I flags
is specified as zero, then
.BR unshare ()
is a no-op;
no changes are made to the calling process's execution context.
.SH RETURN VALUE
On success, zero returned.
On failure, \-1 is returned and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EINVAL
An invalid bit was specified in
.IR flags .
.TP
.B ENOMEM
Cannot allocate sufficient memory to copy parts of caller's
context that need to be unshared.
.TP
.B EPERM
.I flags
specified
.B CLONE_NEWNS
but the calling process was not privileged (did not have the
.B CAP_SYS_ADMIN
capability).
.SH VERSIONS
The
.BR unshare ()
system call was added to Linux in kernel 2.6.16.
.SH CONFORMING TO
The
.BR unshare ()
system call is Linux-specific.
.SH NOTES
Not all of the process attributes that can be shared when
a new process is created using
.BR clone (2)
can be unshared using
.BR unshare ().
In particular, as at kernel 2.6.16,
.BR unshare ()
does not implement flags that reverse the effects of
.BR CLONE_SIGHAND ,
.\" However, we can do unshare(CLONE_SIGHAND) if CLONE_SIGHAND
.\" was not specified when doing clone(); i.e., unsharing
.\" signal handlers is permitted if we are not actually
.\" sharing signal handlers.   mtk
.BR CLONE_SYSVSEM ,
.BR CLONE_THREAD ,
or
.\" FIXME . check future kernel versions (e.g., 2.6.17)
.\" to see if CLONE_VM gets implemented.
.BR CLONE_VM .
.\" However, as at 2.6.16, we can do unshare(CLONE_VM) if CLONE_VM
.\" was not specified when doing clone(); i.e., unsharing
.\" virtual memory is permitted if we are not actually
.\" sharing virtual memory.   mtk
Such functionality may be added in the future, if required.
.\"
.\"9) Future Work
.\"--------------
.\"The current implementation of unshare does not allow unsharing of
.\"signals and signal handlers. Signals are complex to begin with and
.\"to unshare signals and/or signal handlers of a currently running
.\"process is even more complex. If in the future there is a specific
.\"need to allow unsharing of signals and/or signal handlers, it can
.\"be incrementally added to unshare without affecting legacy
.\"applications using unshare.
.\"
.SH SEE ALSO
.BR clone (2),
.BR fork (2),
.BR vfork (2),
Documentation/unshare.txt
.SH COLOPHON
This page is part of release 3.27 of the Linux
.I man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.