unshare() - Unix, Linux System Call
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NAME
unshare - disassociate parts of the process execution context
SYNOPSIS
#include <sched.h>
int unshare(int flags);
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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 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 context should be unshared.
This argument is specified by ORing together zero or more
of the following constants:
Tag | Description |
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.
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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)
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CLONE_NEWNS |
This flag has the
same effect as the
clone(2)
CLONE_NEWNS flag.
Unshare the 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.
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If
flags is specified as zero, then
unshare() is a no-op;
no changes are made to the calling processs execution context.
RETURN VALUE
On success, zero returned. On failure, -1 is returned and
errno is set to indicate the error.
ERRORS
Tag | Description |
EPERM |
flags specified
CLONE_NEWNS but the calling process was not privileged (did not have the
CAP_SYS_ADMIN capability).
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ENOMEM |
Cannot allocate sufficient memory to copy parts of callers
context that need to be unshared.
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EINVAL |
An invalid bit was specified in
flags. |
CONFORMING TO
The unshare() system call is Linux-specific.
NOTES
The unshare() system call was added to Linux in kernel 2.6.16.
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_SIGHAND, CLONE_SYSVSEM, CLONE_THREAD, or
CLONE_VM. Such functionality may be added in the future, if required.
SEE ALSO
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