unshare() - Unix, Linux System Call
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unshare() - Unix, Linux System Call

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unshare - disassociate parts of the process execution context


#include <sched.h> 

int unshare(int flags);


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:

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 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.


On success, zero returned. On failure, -1 is returned and errno is set to indicate the error.


EPERM flags specified CLONE_NEWNS but the calling process was not privileged (did not have the CAP_SYS_ADMIN capability).
ENOMEM Cannot allocate sufficient memory to copy parts of caller’s context that need to be unshared.
EINVAL An invalid bit was specified in flags.


The unshare() system call is Linux-specific.


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.


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