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UNSHARE(2) Linux Programmer's Manual UNSHARE(2)
NAME
unshare - disassociate parts of the process execution context
SYNOPSIS
#define _GNU_SOURCE
#include <sched.h>
int unshare(int flags);
DESCRIPTION
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
fork(2) or vfork(2), while other parts, such as virtual memory, may be shared by
explicit request when creating a process using clone(2).
The main use of unshare() is to allow a process to control its shared execution
context without creating a new process.
The flags argument is a bit mask that specifies which parts of the execution con-
text should be unshared. This argument is specified by ORing together zero or more
of the following constants:
CLONE_FILES
Reverse the effect of the clone(2) CLONE_FILES flag. Unshare the file
descriptor table, so that the calling process no longer shares its file
descriptors with any other process.
CLONE_FS
Reverse the effect of the clone(2) 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. chroot(2),
chdir(2), or umask(2)
CLONE_NEWNS
This flag has the same effect as the clone(2) 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 CLONE_FS as well.
If flags is specified as zero, then unshare() is a no-op; no changes are made to
the calling process's execution context.
RETURN VALUE
On success, zero returned. On failure, -1 is returned and errno is set to indicate
the error.
ERRORS
EINVAL An invalid bit was specified in flags.
ENOMEM Cannot allocate sufficient memory to copy parts of caller's context that
need to be unshared.
EPERM flags specified CLONE_NEWNS but the calling process was not privileged (did
not have the CAP_SYS_ADMIN capability).
VERSIONS
The unshare() system call was added to Linux in kernel 2.6.16.
CONFORMING TO
The unshare() system call is Linux-specific.
NOTES
Not all of the process attributes that can be shared when a new process is created
using clone(2) can be unshared using unshare(). In particular, as at kernel
2.6.16, unshare() does not implement flags that reverse the effects of CLONE_SIG-
HAND, CLONE_SYSVSEM, CLONE_THREAD, or CLONE_VM. Such functionality may be added in
the future, if required.
SEE ALSO
clone(2), fork(2), vfork(2), Documentation/unshare.txt
COLOPHON
This page is part of release 3.22 of the Linux man-pages project. A description of
the project, and information about reporting bugs, can be found at http://www.ker-
nel.org/doc/man-pages/.
Linux 2008-11-20 UNSHARE(2)
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