# unshare

## NAME



           unshare - disassociate parts of the process execution context
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#define _GNU_SOURCE**
           **#include <sched.h>**
    
           **int unshare(int** _flags_**);**
    


## DESCRIPTION



           **unshare**() allows a process (or thread) to disassociate parts of
           its execution context that are currently being shared with other
           processes (or threads).  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 or thread 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
                  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 process.
    
           **CLONE_NEWCGROUP**(since Linux 4.6)
                  This flag has the same effect as the clone(2)
                  **CLONE_NEWCGROUP** flag.  Unshare the cgroup namespace.  Use
                  of **CLONE_NEWCGROUP** requires the **CAP_SYS_ADMIN** capability.
    
           **CLONE_NEWIPC**(since Linux 2.6.19)
                  This flag has the same effect as the clone(2) **CLONE_NEWIPC**
                  flag.  Unshare the IPC namespace, so that the calling
                  process has a private copy of the IPC namespace which is
                  not shared with any other process.  Specifying this flag
                  automatically 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 previously existing process.  Use of
                  **CLONE_NEWNET** requires the **CAP_SYS_ADMIN** capability.
    
           **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.  Use of **CLONE_NEWNS**
                  requires the **CAP_SYS_ADMIN** capability.  For further
                  information, see mount_namespaces(7).
    
           **CLONE_NEWPID**(since Linux 3.8)
                  This flag has the same effect as the clone(2) **CLONE_NEWPID**
                  flag.  Unshare the PID namespace, so that the calling
                  process has a new PID namespace for its children which is
                  not shared with any previously existing process.  The
                  calling process is _not_ moved into the new namespace.  The
                  first child created by the calling process will have the
                  process ID 1 and will assume the role of init(1) in the new
                  namespace.  **CLONE_NEWPID** automatically implies **CLONE_THREAD**
                  as well.  Use of **CLONE_NEWPID** requires the **CAP_SYS_ADMIN**
                  capability.  For further information, see
                  pid_namespaces(7).
    
           **CLONE_NEWTIME**(since Linux 5.6)
                  Unshare the time namespace, so that the calling process has
                  a new time namespace for its children which is not shared
                  with any previously existing process.  The calling process
                  is _not_ moved into the new namespace.  Use of **CLONE_NEWTIME**
                  requires the **CAP_SYS_ADMIN** capability.  For further
                  information, see time_namespaces(7).
    
           **CLONE_NEWUSER**(since Linux 3.8)
                  This flag has the same effect as the clone(2) **CLONE_NEWUSER**
                  flag.  Unshare the user namespace, so that the calling
                  process is moved into a new user namespace which is not
                  shared with any previously existing process.  As with the
                  child process created by clone(2) with the **CLONE_NEWUSER**
                  flag, the caller obtains a full set of capabilities in the
                  new namespace.
    
                  **CLONE_NEWUSER** requires that the calling process is not
                  threaded; specifying **CLONE_NEWUSER** automatically implies
                  **CLONE_THREAD**.  Since Linux 3.9, **CLONE_NEWUSER** also
                  automatically implies **CLONE_FS**.  **CLONE_NEWUSER** requires
                  that the user ID and group ID of the calling process are
                  mapped to user IDs and group IDs in the user namespace of
                  the calling process at the time of the call.
    
                  For further information on user namespaces, see
                  user_namespaces(7).
    
           **CLONE_NEWUTS**(since Linux 2.6.19)
                  This flag has the same effect as the clone(2) **CLONE_NEWUTS**
                  flag.  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** capability.
    
           **CLONE_SYSVSEM**(since Linux 2.6.26)
                  This flag reverses the effect of the clone(2) **CLONE_SYSVSEM**
                  flag.  Unshare System V semaphore adjustment (_semadj_)
                  values, so that the calling process has a new empty _semadj_
                  list that is not shared with any other process.  If this is
                  the last process that has a reference to the process's
                  current _semadj_ list, then the adjustments in that list are
                  applied to the corresponding semaphores, as described in
                  semop(2).
    
           In addition, **CLONE_THREAD** , **CLONE_SIGHAND** , and **CLONE_VM** can be
           specified in _flags_ if the caller is single threaded (i.e., it is
           not sharing its address space with another process or thread).  In
           this case, these flags have no effect.  (Note also that specifying
           **CLONE_THREAD** automatically implies **CLONE_VM** , and specifying
           **CLONE_VM** automatically implies **CLONE_SIGHAND**.)  If the process is
           multithreaded, then the use of these flags results in an error.
    
           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_.
    
           **EINVAL CLONE_THREAD** , **CLONE_SIGHAND** , or **CLONE_VM** was specified in
                  _flags_ , and the caller is multithreaded.
    
           **EINVAL CLONE_NEWIPC** was specified in _flags_ , but the kernel was not
                  configured with the **CONFIG_SYSVIPC** and **CONFIG_IPC_NS**
                  options.
    
           **EINVAL CLONE_NEWNET** was specified in _flags_ , but the kernel was not
                  configured with the **CONFIG_NET_NS** option.
    
           **EINVAL CLONE_NEWPID** was specified in _flags_ , but the kernel was not
                  configured with the **CONFIG_PID_NS** option.
    
           **EINVAL CLONE_NEWUSER** was specified in _flags_ , but the kernel was
                  not configured with the **CONFIG_USER_NS** option.
    
           **EINVAL CLONE_NEWUTS** was specified in _flags_ , but the kernel was not
                  configured with the **CONFIG_UTS_NS** option.
    
           **EINVAL CLONE_NEWPID** was specified in _flags_ , but the process has
                  previously called **unshare**() with the **CLONE_NEWPID** flag.
    
           **ENOMEM** Cannot allocate sufficient memory to copy parts of caller's
                  context that need to be unshared.
    
           **ENOSPC**(since Linux 3.7)
                  **CLONE_NEWPID** was specified in flags, but the limit on the
                  nesting depth of PID namespaces would have been exceeded;
                  see pid_namespaces(7).
    
           **ENOSPC**(since Linux 4.9; beforehand **EUSERS**)
                  **CLONE_NEWUSER** was specified in _flags_ , and the call would
                  cause the limit on the number of nested user namespaces to
                  be exceeded.  See user_namespaces(7).
    
                  From Linux 3.11 to Linux 4.8, the error diagnosed in this
                  case was **EUSERS**.
    
           **ENOSPC**(since Linux 4.9)
                  One of the values in _flags_ specified the creation of a new
                  user namespace, but doing so would have caused the limit
                  defined by the corresponding file in _/proc/sys/user_ to be
                  exceeded.  For further details, see namespaces(7).
    
           **EPERM** The calling process did not have the required privileges
                  for this operation.
    
           **EPERM  CLONE_NEWUSER** was specified in _flags_ , but either the
                  effective user ID or the effective group ID of the caller
                  does not have a mapping in the parent namespace (see
                  user_namespaces(7)).
    
           **EPERM**(since Linux 3.9)
                  **CLONE_NEWUSER** was specified in _flags_ and the caller is in a
                  chroot environment (i.e., the caller's root directory does
                  not match the root directory of the mount namespace in
                  which it resides).
    
           **EUSERS**(from Linux 3.11 to Linux 4.8)
                  **CLONE_NEWUSER** was specified in _flags_ , and the limit on the
                  number of nested user namespaces would be exceeded.  See
                  the discussion of the **ENOSPC** error above.
    


## STANDARDS



           Linux.
    


## 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 3.8, **unshare**() does not implement
           flags that reverse the effects of **CLONE_SIGHAND** , **CLONE_THREAD** , or
           **CLONE_VM**.  Such functionality may be added in the future, if
           required.
    
           Creating all kinds of namespace, except user namespaces, requires
           the **CAP_SYS_ADMIN** capability.  However, since creating a user
           namespace automatically confers a full set of capabilities,
           creating both a user namespace and any other type of namespace in
           the same **unshare**() call does not require the **CAP_SYS_ADMIN**
           capability in the original namespace.
    


## EXAMPLES



           The program below provides a simple implementation of the
           unshare(1) command, which unshares one or more namespaces and
           executes the command supplied in its command-line arguments.
           Here's an example of the use of this program, running a shell in a
           new mount namespace, and verifying that the original shell and the
           new shell are in separate mount namespaces:
    
               $ **readlink /proc/$$/ns/mnt** ;
               mnt:[4026531840]
               $ **sudo ./unshare -m /bin/bash** ;
               # **readlink /proc/$$/ns/mnt** ;
               mnt:[4026532325]
    
           The differing output of the two readlink(1) commands shows that
           the two shells are in different mount namespaces.
    
       **Program source**
    
           /* unshare.c
    
              A simple implementation of the unshare(1) command: unshare
              namespaces and execute a command.
           */
           #define _GNU_SOURCE
           #include <err.h>
           #include <sched.h>
           #include <stdio.h>
           #include <stdlib.h>
           #include <unistd.h>
    
           static void
           usage(char *pname)
           {
               fprintf(stderr, "Usage: %s [options] program [arg...]\n", pname);
               fprintf(stderr, "Options can be:\n");
               fprintf(stderr, "    -C   unshare cgroup namespace\n");
               fprintf(stderr, "    -i   unshare IPC namespace\n");
               fprintf(stderr, "    -m   unshare mount namespace\n");
               fprintf(stderr, "    -n   unshare network namespace\n");
               fprintf(stderr, "    -p   unshare PID namespace\n");
               fprintf(stderr, "    -t   unshare time namespace\n");
               fprintf(stderr, "    -u   unshare UTS namespace\n");
               fprintf(stderr, "    -U   unshare user namespace\n");
               exit(EXIT_FAILURE);
           }
    
           int
           main(int argc, char *argv[])
           {
               int flags, opt;
    
               flags = 0;
    
               while ((opt = getopt(argc, argv, "CimnptuU")) != -1) {
                   switch (opt) {
                   case 'C': flags |= CLONE_NEWCGROUP;     break;
                   case 'i': flags |= CLONE_NEWIPC;        break;
                   case 'm': flags |= CLONE_NEWNS;         break;
                   case 'n': flags |= CLONE_NEWNET;        break;
                   case 'p': flags |= CLONE_NEWPID;        break;
                   case 't': flags |= CLONE_NEWTIME;       break;
                   case 'u': flags |= CLONE_NEWUTS;        break;
                   case 'U': flags |= CLONE_NEWUSER;       break;
                   default:  usage(argv[0]);
                   }
               }
    
               if (optind >= argc)
                   usage(argv[0]);
    
               if (unshare(flags) == -1)
                   err(EXIT_FAILURE, "unshare");
    
               execvp(argv[optind], &argv[optind]);
               err(EXIT_FAILURE, "execvp");
           }
    


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