int unshare(int flags);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
Since glibc 2.14:
Before glibc 2.14:
_BSD_SOURCE || _SVID_SOURCE
/* _GNU_SOURCE also suffices */
unshare() allows a process to disassociate parts of its execution con-
text 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 exe-
cution context should be unshared. This argument is specified by ORing
together zero or more of the following constants:
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.
Reverse the effect of the clone(2) CLONE_FS flag. Unshare
filesystem attributes, so that the calling process no longer
shares its root directory (chroot(2)), current directory
(chdir(2)), or umask (umask(2)) attributes with any other
CLONE_NEWIPC (since Linux 2.6.19)
This flag has the same effect as the clone(2) CLONE_NEWIPC flag.
Unshare the System V IPC namespace, so that the calling process
has a private copy of the System V IPC namespace which is not
shared with any other process. Specifying this flag automati-
cally implies CLONE_SYSVSEM as well. Use of CLONE_NEWIPC
requires the CAP_SYS_ADMIN capability.
CLONE_NEWNET (since Linux 2.6.24)
This flag has the same effect as the clone(2) CLONE_NEWNET flag.
Unshare the network namespace, so that the calling process is
moved into a new network namespace which is not shared with any
Unshare the UTS IPC namespace, so that the calling process has a
private copy of the UTS namespace which is not shared with any
other process. Use of CLONE_NEWUTS requires the CAP_SYS_ADMIN
CLONE_SYSVSEM (since Linux 2.6.26)
This flag reverses the effect of the clone(2) CLONE_SYSVSEM
flag. Unshare System V semaphore undo values, so that the call-
ing process has a private copy which is not shared with any
other process. Use of CLONE_SYSVSEM requires the CAP_SYS_ADMIN
If flags is specified as zero, then unshare() is a no-op; no changes
are made to the calling process's execution context.
On success, zero returned. On failure, -1 is returned and errno is set
to indicate the error.
EINVAL An invalid bit was specified in flags.
ENOMEM Cannot allocate sufficient memory to copy parts of caller's con-
text that need to be unshared.
EPERM The calling process did not have the required privileges for
The unshare() system call was added to Linux in kernel 2.6.16.
The unshare() system call is Linux-specific.
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 particu-
lar, as at kernel 3.8, unshare() does not implement flags that reverse
the effects of CLONE_SIGHAND, CLONE_THREAD, or CLONE_VM. Such func-
tionality may be added in the future, if required.
clone(2), fork(2), kcmp(2), setns(2), vfork(2)
Documentation/unshare.txt in the Linux kernel source tree
This page is part of release 3.54 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2013-04-17 UNSHARE(2)
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