NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON

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

NAME         top

       unshare - disassociate parts of the process execution context

SYNOPSIS         top

       #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         top

       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 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 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_NEWIPC (since Linux 2.6.19)
              This flag has the same effect as the clone(2) CLONE_NEWIPC
              flag.  Unshare the System 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 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.

       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_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         top

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

ERRORS         top

       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         top

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

CONFORMING TO         top

       The unshare() system call is Linux-specific.

NOTES         top

       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         top

       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 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 <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         top

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

       Documentation/unshare.txt in the Linux kernel source tree

COLOPHON         top

       This page is part of release 3.75 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)