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UNSHARE(2)                          Linux Programmer's Manual                          UNSHARE(2)



NAME
       unshare - disassociate parts of the process execution context

SYNOPSIS
       #include <sched.h>

       int unshare(int flags);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       unshare():
           Since glibc 2.14:
               _GNU_SOURCE
           Before glibc 2.14:
               _BSD_SOURCE || _SVID_SOURCE
                   /* _GNU_SOURCE also suffices */

DESCRIPTION
       unshare()  allows  a  process to disassociate parts of its execution context that are cur‐
       rently 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  execution  context
       should be unshared.  This argument is specified by ORing together zero or more of the fol‐
       lowing 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)), cur‐
              rent directory (chdir(2)), or umask (umask(2)) attributes with any other process.

       CLONE_NEWIPC (since Linux 2.6.19)
              This flag has the same effect as the clone(2) CLONE_NEWIPC flag.  Unshare the  Sys‐
              tem 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
              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  net‐
              work  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.

       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  names‐
              pace.   CLONE_NEWPID automatically implies CLONE_THREAD as well.  Use of CLONE_NEW‐
              PID requires the CAP_SYS_ADMIN capability.  For further information, see pid_names‐
              paces(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 capabili‐
              ties 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 call‐
       ing 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.

       ENOMEM Cannot allocate sufficient memory to copy parts of caller's context that need to be
              unshared.

       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 (since Linux 3.11)
              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).

VERSIONS
       The unshare() system call was added to Linux in kernel 2.6.16.

CONFORMING TO
       The unshare() system call is Linux-specific.

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.

EXAMPLE
       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/$$/ns/mnt
           mnt:[4026531840]
           $ sudo ./unshare -m /bin/bash
           [sudo] password for cecilia:
           # readlink /proc/$$/ns/mnt
           mnt:[4026532325]

       The differing output of the two readlink(1) commands shows that the two shells are in dif‐
       ferent mount namespaces.

   Program source

       /* unshare.c

          A simple implementation of the unshare(1) command: unshare
          namespaces and execute a command.
       */
       #define _GNU_SOURCE
       #include <sched.h>
       #include <unistd.h>
       #include <stdlib.h>
       #include <stdio.h>

       /* A simple error-handling function: print an error message based
          on the value in 'errno' and terminate the calling process */

       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                               } while (0)

       static void
       usage(char *pname)
       {
           fprintf(stderr, "Usage: %s [options] program [arg...]\n", pname);
           fprintf(stderr, "Options can be:\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, "    -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, "imnpuU")) != -1) {
               switch (opt) {
               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 '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)
               errExit("unshare");

           execvp(argv[optind], &argv[optind]);
           errExit("execvp");
       }

SEE ALSO
       unshare(1), clone(2), fork(2), kcmp(2), setns(2), vfork(2), namespaces(7)

       Documentation/unshare.txt in the Linux kernel source tree

COLOPHON
       This page is part of release 3.74 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at http://www.kernel.org/doc/man-pages/.



Linux                                       2014-09-21                                 UNSHARE(2)


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