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unshare - disassociate parts of the process execution context
#include <sched.h> |
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. |
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 processs execution context.
Tag | Description |
---|---|
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 callers 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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