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NAME

       clone, __clone2 - create a child process

SYNOPSIS

       /* Prototype for the glibc wrapper function */

       #include <sched.h>

       int clone(int (*fn)(void *), void *child_stack,
                 int flags, void *arg, ...
                 /* pid_t *ptid, struct user_desc *tls, pid_t *ctid */ );

       /* Prototype for the raw system call */

       long clone(unsigned long flags, void *child_stack,
                 void *ptid, void *ctid,
                 struct pt_regs *regs);

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

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

DESCRIPTION

       clone() creates a new process, in a manner similar to fork(2).

       This page describes both the glibc clone() wrapper function and the underlying system call
       on which it is based.  The main text describes the wrapper function; the  differences  for
       the raw system call are described toward the end of this page.

       Unlike  fork(2),  clone() allows the child process to share parts of its execution context
       with the calling process, such as the memory space, the table of file descriptors, and the
       table  of  signal  handlers.   (Note  that on this manual page, "calling process" normally
       corresponds to "parent process".  But see the description of CLONE_PARENT below.)

       The main use of clone() is to implement threads: multiple threads of control in a  program
       that run concurrently in a shared memory space.

       When  the  child process is created with clone(), it executes the function fn(arg).  (This
       differs from fork(2), where execution continues in the child from the point of the fork(2)
       call.)   The fn argument is a pointer to a function that is called by the child process at
       the beginning of its execution.  The arg argument is passed to the fn function.

       When the fn(arg) function application returns, the child process terminates.  The  integer
       returned  by  fn  is  the  exit  code  for  the child process.  The child process may also
       terminate explicitly by calling exit(2) or after receiving a fatal signal.

       The child_stack argument specifies the location of the stack used by  the  child  process.
       Since  the  child  and  calling process may share memory, it is not possible for the child
       process to execute in the same stack as the calling process.   The  calling  process  must
       therefore  set  up  memory  space  for the child stack and pass a pointer to this space to
       clone().  Stacks grow downward on  all  processors  that  run  Linux  (except  the  HP  PA
       processors),  so child_stack usually points to the topmost address of the memory space set
       up for the child stack.

       The low byte of flags contains the number of the termination signal  sent  to  the  parent
       when the child dies.  If this signal is specified as anything other than SIGCHLD, then the
       parent process must specify the __WALL or __WCLONE options when waiting for the child with
       wait(2).   If  no  signal  is  specified, then the parent process is not signaled when the
       child terminates.

       flags may also be bitwise-or'ed with zero or more of the following constants, in order  to
       specify what is shared between the calling process and the child process:

       CLONE_CHILD_CLEARTID (since Linux 2.5.49)
              Erase child thread ID at location ctid in child memory when the child exits, and do
              a wakeup on the futex at that address.  The address involved may be changed by  the
              set_tid_address(2) system call.  This is used by threading libraries.

       CLONE_CHILD_SETTID (since Linux 2.5.49)
              Store child thread ID at location ctid in child memory.

       CLONE_FILES (since Linux 2.0)
              If  CLONE_FILES  is  set,  the calling process and the child process share the same
              file descriptor table.  Any file descriptor created by the calling  process  or  by
              the  child  process  is  also valid in the other process.  Similarly, if one of the
              processes closes a file descriptor, or changes  its  associated  flags  (using  the
              fcntl(2) F_SETFD operation), the other process is also affected.

              If  CLONE_FILES  is  not  set,  the  child  process  inherits  a  copy  of all file
              descriptors opened in the calling process at the time of clone().  (The  duplicated
              file  descriptors  in  the  child  refer  to  the  same open file descriptions (see
              open(2)) as the corresponding file descriptors in the calling process.)  Subsequent
              operations  that  open  or close file descriptors, or change file descriptor flags,
              performed by either the calling process or the child  process  do  not  affect  the
              other process.

       CLONE_FS (since Linux 2.0)
              If  CLONE_FS  is  set,  the  caller and the child process share the same filesystem
              information.  This includes  the  root  of  the  filesystem,  the  current  working
              directory,  and  the umask.  Any call to chroot(2), chdir(2), or umask(2) performed
              by the calling process or the child process also affects the other process.

              If CLONE_FS is not set, the child  process  works  on  a  copy  of  the  filesystem
              information  of  the  calling  process  at  the time of the clone() call.  Calls to
              chroot(2), chdir(2), umask(2) performed later by one of the processes do not affect
              the other process.

       CLONE_IO (since Linux 2.6.25)
              If  CLONE_IO  is  set,  then the new process shares an I/O context with the calling
              process.  If this flag is not set, then (as with fork(2)) the new process  has  its
              own I/O context.

              The I/O context is the I/O scope of the disk scheduler (i.e, what the I/O scheduler
              uses to model scheduling of a process's I/O).  If  processes  share  the  same  I/O
              context,  they are treated as one by the I/O scheduler.  As a consequence, they get
              to share disk time.  For some  I/O  schedulers,  if  two  processes  share  an  I/O
              context,  they will be allowed to interleave their disk access.  If several threads
              are doing I/O on behalf of the  same  process  (aio_read(3),  for  instance),  they
              should employ CLONE_IO to get better I/O performance.

              If the kernel is not configured with the CONFIG_BLOCK option, this flag is a no-op.

       CLONE_NEWIPC (since Linux 2.6.19)
              If  CLONE_NEWIPC  is  set, then create the process in a new IPC namespace.  If this
              flag is not set, then (as with fork(2)), the process is created  in  the  same  IPC
              namespace  as the calling process.  This flag is intended for the implementation of
              containers.

              An IPC namespace provides an isolated view of System V IPC objects  (see  svipc(7))
              and  (since  Linux  2.6.30)  POSIX message queues (see mq_overview(7)).  The common
              characteristic of these IPC mechanisms  is  that  IPC  objects  are  identified  by
              mechanisms other than filesystem pathnames.

              Objects  created  in  an  IPC namespace are visible to all other processes that are
              members of  that  namespace,  but  are  not  visible  to  processes  in  other  IPC
              namespaces.

              When an IPC namespace is destroyed (i.e., when the last process that is a member of
              the namespace terminates), all IPC  objects  in  the  namespace  are  automatically
              destroyed.

              Use  of  this  flag  requires:  a  kernel  configured  with  the CONFIG_SYSVIPC and
              CONFIG_IPC_NS options and that the process  be  privileged  (CAP_SYS_ADMIN).   This
              flag can't be specified in conjunction with CLONE_SYSVSEM.

       CLONE_NEWNET (since Linux 2.6.24)
              (The  implementation  of  this  flag  was  completed  only  by about kernel version
              2.6.29.)

              If CLONE_NEWNET is set, then create the process in a  new  network  namespace.   If
              this  flag  is  not set, then (as with fork(2)), the process is created in the same
              network  namespace  as  the  calling  process.   This  flag  is  intended  for  the
              implementation of containers.

              A  network  namespace  provides  an  isolated view of the networking stack (network
              device interfaces, IPv4 and IPv6  protocol  stacks,  IP  routing  tables,  firewall
              rules,  the  /proc/net  and  /sys/class/net  directory  trees,  sockets,  etc.).  A
              physical network device can live in  exactly  one  network  namespace.   A  virtual
              network  device  ("veth") pair provides a pipe-like abstraction that can be used to
              create tunnels between network namespaces, and can be used to create a bridge to  a
              physical network device in another namespace.

              When  a  network  namespace  is freed (i.e., when the last process in the namespace
              terminates), its physical network devices are moved back  to  the  initial  network
              namespace (not to the parent of the process).

              Use  of  this  flag requires: a kernel configured with the CONFIG_NET_NS option and
              that the process be privileged (CAP_SYS_ADMIN).

       CLONE_NEWNS (since Linux 2.4.19)
              Start the child in a new mount namespace.

              Every process lives in a mount namespace.  The namespace of a process is  the  data
              (the set of mounts) describing the file hierarchy as seen by that process.  After a
              fork(2) or clone() where the CLONE_NEWNS flag is not set, the child  lives  in  the
              same mount namespace as the parent.  The system calls mount(2) and umount(2) change
              the mount namespace of the calling process, and hence  affect  all  processes  that
              live  in  the  same  namespace,  but  do  not affect processes in a different mount
              namespace.

              After a clone() where the CLONE_NEWNS flag is set, the cloned child is started in a
              new mount namespace, initialized with a copy of the namespace of the parent.

              Only a privileged process (one having the CAP_SYS_ADMIN capability) may specify the
              CLONE_NEWNS flag.  It is not permitted to specify both CLONE_NEWNS and CLONE_FS  in
              the same clone() call.

       CLONE_NEWPID (since Linux 2.6.24)
              If  CLONE_NEWPID  is  set, then create the process in a new PID namespace.  If this
              flag is not set, then (as with fork(2)), the process is created  in  the  same  PID
              namespace  as the calling process.  This flag is intended for the implementation of
              containers.

              A PID namespace provides an isolated environment for PIDs: PIDs in a new  namespace
              start  at  1, somewhat like a standalone system, and calls to fork(2), vfork(2), or
              clone() will produce processes with PIDs that are unique within the namespace.

              The first process created in a new namespace (i.e., the process created  using  the
              CLONE_NEWPID  flag)  has  the  PID  1, and is the "init" process for the namespace.
              Children that are orphaned within the namespace will be reparented to this  process
              rather  than init(8).  Unlike the traditional init process, the "init" process of a
              PID namespace can terminate, and if it does, all of the processes in the  namespace
              are terminated.

              PID  namespaces  form  a  hierarchy.   When  a  new  PID  namespace is created, the
              processes in that namespace are visible in the PID namespace of  the  process  that
              created  the  new namespace; analogously, if the parent PID namespace is itself the
              child of another PID  namespace,  then  processes  in  the  child  and  parent  PID
              namespaces  will both be visible in the grandparent PID namespace.  Conversely, the
              processes in the "child" PID namespace do not  see  the  processes  in  the  parent
              namespace.   The existence of a namespace hierarchy means that each process may now
              have multiple PIDs: one for each namespace in which it is visible;  each  of  these
              PIDs  is  unique  within  the corresponding namespace.  (A call to getpid(2) always
              returns the PID associated with the namespace in which the process lives.)

              After creating the new namespace, it is useful for the child  to  change  its  root
              directory and mount a new procfs instance at /proc so that tools such as ps(1) work
              correctly.  (If CLONE_NEWNS is also included in flags, then it isn't  necessary  to
              change  the  root  directory:  a  new  procfs instance can be mounted directly over
              /proc.)

              Use of this flag requires: a kernel configured with the  CONFIG_PID_NS  option  and
              that  the  process  be privileged (CAP_SYS_ADMIN).  This flag can't be specified in
              conjunction with CLONE_THREAD.

       CLONE_NEWUTS (since Linux 2.6.19)
              If CLONE_NEWUTS is set, then create the process  in  a  new  UTS  namespace,  whose
              identifiers  are  initialized by duplicating the identifiers from the UTS namespace
              of the calling process.  If this flag is not  set,  then  (as  with  fork(2)),  the
              process  is created in the same UTS namespace as the calling process.  This flag is
              intended for the implementation of containers.

              A UTS namespace is the set of identifiers returned by uname(2);  among  these,  the
              domain   name  and  the  host  name  can  be  modified  by  setdomainname(2)  and
              sethostname(2), respectively.  Changes made to the identifiers in a  UTS  namespace
              are  visible  to  all other processes in the same namespace, but are not visible to
              processes in other UTS namespaces.

              Use of this flag requires: a kernel configured with the  CONFIG_UTS_NS  option  and
              that the process be privileged (CAP_SYS_ADMIN).

       CLONE_PARENT (since Linux 2.3.12)
              If  CLONE_PARENT  is  set,  then  the  parent  of  the  new  child  (as returned by
              getppid(2)) will be the same as that of the calling process.

              If CLONE_PARENT is not set, then (as  with  fork(2))  the  child's  parent  is  the
              calling process.

              Note  that  it  is the parent process, as returned by getppid(2), which is signaled
              when the child terminates, so that if CLONE_PARENT is set, then the parent  of  the
              calling process, rather than the calling process itself, will be signaled.

       CLONE_PARENT_SETTID (since Linux 2.5.49)
              Store  child  thread  ID  at  location  ptid in parent and child memory.  (In Linux
              2.5.32-2.5.48 there was a flag CLONE_SETTID that did this.)

       CLONE_PID (obsolete)
              If CLONE_PID is set, the child process is created with the same process ID  as  the
              calling  process.   This  is good for hacking the system, but otherwise of not much
              use.  Since 2.3.21 this flag can be specified only by the system boot process  (PID
              0).  It disappeared in Linux 2.5.16.

       CLONE_PTRACE (since Linux 2.2)
              If  CLONE_PTRACE  is specified, and the calling process is being traced, then trace
              the child also (see ptrace(2)).

       CLONE_SETTLS (since Linux 2.5.32)
              The newtls argument is  the  new  TLS  (Thread  Local  Storage)  descriptor.   (See
              set_thread_area(2).)

       CLONE_SIGHAND (since Linux 2.0)
              If  CLONE_SIGHAND  is set, the calling process and the child process share the same
              table  of  signal  handlers.   If  the  calling  process  or  child  process  calls
              sigaction(2)  to  change  the  behavior  associated  with a signal, the behavior is
              changed in the other process as well.   However,  the  calling  process  and  child
              processes still have distinct signal masks and sets of pending signals.  So, one of
              them may block or unblock some signals using sigprocmask(2) without  affecting  the
              other process.

              If  CLONE_SIGHAND  is  not  set,  the  child  process inherits a copy of the signal
              handlers of  the  calling  process  at  the  time  clone()  is  called.   Calls  to
              sigaction(2)  performed  later  by one of the processes have no effect on the other
              process.

              Since Linux 2.6.0-test6, flags must  also  include  CLONE_VM  if  CLONE_SIGHAND  is
              specified

       CLONE_STOPPED (since Linux 2.6.0-test2)
              If CLONE_STOPPED is set, then the child is initially stopped (as though it was sent
              a SIGSTOP signal), and must be resumed by sending it a SIGCONT signal.

              This flag was deprecated from Linux 2.6.25 onward, and was  removed  altogether  in
              Linux 2.6.38.

       CLONE_SYSVSEM (since Linux 2.5.10)
              If CLONE_SYSVSEM is set, then the child and the calling process share a single list
              of System V semaphore undo values (see semop(2)).  If this flag is  not  set,  then
              the child has a separate undo list, which is initially empty.

       CLONE_THREAD (since Linux 2.4.0-test8)
              If CLONE_THREAD is set, the child is placed in the same thread group as the calling
              process.  To make the remainder of the discussion of  CLONE_THREAD  more  readable,
              the term "thread" is used to refer to the processes within a thread group.

              Thread groups were a feature added in Linux 2.4 to support the POSIX threads notion
              of a set of threads that share a single PID.  Internally, this shared  PID  is  the
              so-called  thread  group  identifier (TGID) for the thread group.  Since Linux 2.4,
              calls to getpid(2) return the TGID of the caller.

              The threads within a group can  be  distinguished  by  their  (system-wide)  unique
              thread  IDs (TID).  A new thread's TID is available as the function result returned
              to the caller of clone(), and a thread can obtain its own TID using gettid(2).

              When a call is made to clone() without specifying CLONE_THREAD, then the  resulting
              thread  is placed in a new thread group whose TGID is the same as the thread's TID.
              This thread is the leader of the new thread group.

              A new thread created with CLONE_THREAD has the same parent process as the caller of
              clone()  (i.e.,  like  CLONE_PARENT),  so  that calls to getppid(2) return the same
              value for all of the threads  in  a  thread  group.   When  a  CLONE_THREAD  thread
              terminates,  the  thread  that  created  it using clone() is not sent a SIGCHLD (or
              other termination) signal; nor can the status of such a thread  be  obtained  using
              wait(2).  (The thread is said to be detached.)

              After  all  of  the  threads  in a thread group terminate the parent process of the
              thread group is sent a SIGCHLD (or other termination) signal.

              If any of the threads in a thread group performs an  execve(2),  then  all  threads
              other  than the thread group leader are terminated, and the new program is executed
              in the thread group leader.

              If one of the threads in a thread group creates a child  using  fork(2),  then  any
              thread in the group can wait(2) for that child.

              Since  Linux  2.5.35,  flags  must  also  include  CLONE_SIGHAND if CLONE_THREAD is
              specified.

              Signals may be sent to a thread group as a whole (i.e., a TGID) using  kill(2),  or
              to a specific thread (i.e., TID) using tgkill(2).

              Signal  dispositions  and  actions  are  process-wide:  if  an  unhandled signal is
              delivered to a thread, then it will affect (terminate, stop, continue,  be  ignored
              in) all members of the thread group.

              Each  thread  has its own signal mask, as set by sigprocmask(2), but signals can be
              pending either: for the whole process (i.e.,  deliverable  to  any  member  of  the
              thread  group), when sent with kill(2); or for an individual thread, when sent with
              tgkill(2).  A call to sigpending(2) returns a signal set that is the union  of  the
              signals  pending  for  the  whole  process and the signals that are pending for the
              calling thread.

              If kill(2) is used to send a signal to a thread group, and  the  thread  group  has
              installed  a  handler  for  the signal, then the handler will be invoked in exactly
              one, arbitrarily selected member of the thread  group  that  has  not  blocked  the
              signal.  If multiple threads in a group are waiting to accept the same signal using
              sigwaitinfo(2), the kernel will arbitrarily select one of these threads to  receive
              a signal sent using kill(2).

       CLONE_UNTRACED (since Linux 2.5.46)
              If CLONE_UNTRACED is specified, then a tracing process cannot force CLONE_PTRACE on
              this child process.

       CLONE_VFORK (since Linux 2.2)
              If CLONE_VFORK is set, the execution of the calling process is suspended until  the
              child releases its virtual memory resources via a call to execve(2) or _exit(2) (as
              with vfork(2)).

              If CLONE_VFORK is not  set  then  both  the  calling  process  and  the  child  are
              schedulable  after  the  call,  and  an  application  should  not rely on execution
              occurring in any particular order.

       CLONE_VM (since Linux 2.0)
              If CLONE_VM is set, the calling process and the  child  process  run  in  the  same
              memory  space.  In particular, memory writes performed by the calling process or by
              the child process are also visible in the  other  process.   Moreover,  any  memory
              mapping  or  unmapping  performed with mmap(2) or munmap(2) by the child or calling
              process also affects the other process.

              If CLONE_VM is not set, the child process runs in a separate  copy  of  the  memory
              space  of  the  calling  process  at  the  time  of clone().  Memory writes or file
              mappings/unmappings performed by one of the processes do not affect the  other,  as
              with fork(2).

   The raw system call interface
       The  raw  clone() system call corresponds more closely to fork(2) in that execution in the
       child continues from the point of the call.  As such, the fn  and  arg  arguments  of  the
       clone()  wrapper  function are omitted.  Furthermore, the argument order changes.  The raw
       system call interface on x86 and many other architectures is roughly:

           long clone(unsigned long flags, void *child_stack,
                      void *ptid, void *ctid,
                      struct pt_regs *regs);

       Another difference for the raw system call is that the child_stack argument may  be  zero,
       in  which case copy-on-write semantics ensure that the child gets separate copies of stack
       pages when either process modifies the stack.  In this case, for  correct  operation,  the
       CLONE_VM option should not be specified.

       For  some  architectures, the order of the arguments for the system call differs from that
       shown above.  On the score, microblaze, ARM, ARM 64, PA-RISC, arc, Power PC,  xtensa,  and
       MIPS  architectures, the order of the fourth and fifth arguments is reversed.  On the cris
       and s390 architectures, the order of the first and second arguments is reversed.

   blackfin, m68k, and sparc
       The  argument-passing  conventions  on  blackfin,  m68k,  and  sparc  are  different  from
       descriptions above.  For details, see the kernel (and glibc) source.

   ia64
       On ia64, a different interface is used:

       int __clone2(int (*fn)(void *),
                    void *child_stack_base, size_t stack_size,
                    int flags, void *arg, ...
                 /* pid_t *ptid, struct user_desc *tls, pid_t *ctid */ );

       The prototype shown above is for the glibc wrapper function; the raw system call interface
       has no fn or arg argument, and changes the order of the arguments so  that  flags  is  the
       first argument, and tls is the last argument.

       __clone2() operates in the same way as clone(), except that child_stack_base points to the
       lowest address of the child's stack area, and stack_size specifies the size of  the  stack
       pointed to by child_stack_base.

   Linux 2.4 and earlier
       In Linux 2.4 and earlier, clone() does not take arguments ptid, tls, and ctid.

RETURN VALUE

       On  success,  the  thread  ID  of  the child process is returned in the caller's thread of
       execution.  On failure, -1 is returned in the caller's context, no child process  will  be
       created, and errno will be set appropriately.

ERRORS

       EAGAIN Too many processes are already running.

       EINVAL CLONE_SIGHAND was specified, but CLONE_VM was not.  (Since Linux 2.6.0-test6.)

       EINVAL CLONE_THREAD was specified, but CLONE_SIGHAND was not.  (Since Linux 2.5.35.)

       EINVAL Both CLONE_FS and CLONE_NEWNS were specified in flags.

       EINVAL Both CLONE_NEWIPC and CLONE_SYSVSEM were specified in flags.

       EINVAL Both CLONE_NEWPID and CLONE_THREAD were specified in flags.

       EINVAL Returned by clone() when a zero value is specified for child_stack.

       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_NEWUTS was specified in flags, but the kernel was  not  configured  with  the
              CONFIG_UTS option.

       ENOMEM Cannot allocate sufficient memory to allocate a task structure for the child, or to
              copy those parts of the caller's context that need to be copied.

       EPERM  CLONE_NEWIPC,  CLONE_NEWNET,  CLONE_NEWNS,  CLONE_NEWPID,   or   CLONE_NEWUTS   was
              specified by an unprivileged process (process without CAP_SYS_ADMIN).

       EPERM  CLONE_PID was specified by a process other than process 0.

VERSIONS

       There  is  no  entry  for  clone() in libc5.  glibc2 provides clone() as described in this
       manual page.

CONFORMING TO

       clone() is Linux-specific and should not be used in programs intended to be portable.

NOTES

       In the kernel 2.4.x series, CLONE_THREAD generally does not make the  parent  of  the  new
       thread  the same as the parent of the calling process.  However, for kernel versions 2.4.7
       to 2.4.18 the CLONE_THREAD flag implied the CLONE_PARENT flag (as in kernel 2.6).

       For a while there was CLONE_DETACHED (introduced in 2.5.32): parent  wants  no  child-exit
       signal.  In 2.6.2 the need to give this together with CLONE_THREAD disappeared.  This flag
       is still defined, but has no effect.

       On i386, clone() should not be called through vsyscall, but directly through int $0x80.

BUGS

       Versions of the GNU C library that include the NPTL threading library  contain  a  wrapper
       function  for  getpid(2) that performs caching of PIDs.  This caching relies on support in
       the glibc wrapper for clone(), but as currently implemented, the cache may not  be  up  to
       date  in  some  circumstances.   In  particular,  if  a  signal  is delivered to the child
       immediately after the clone() call, then a call to getpid(2) in a handler for  the  signal
       may return the PID of the calling process ("the parent"), if the clone wrapper has not yet
       had a chance to update the PID cache in the child.   (This  discussion  ignores  the  case
       where  the  child  was  created  using CLONE_THREAD, when getpid(2) should return the same
       value in the child and in the process that called clone(), since the caller and the  child
       are  in  the  same thread group.  The stale-cache problem also does not occur if the flags
       argument includes CLONE_VM.)  To get the truth, it may be necessary to use  code  such  as
       the following:

           #include <syscall.h>

           pid_t mypid;

           mypid = syscall(SYS_getpid);

EXAMPLE

   Create a child that executes in a separate UTS namespace
       The  following  program  demonstrates  the  use  of clone() to create a child process that
       executes in a separate  UTS  namespace.   The  child  changes  the  hostname  in  its  UTS
       namespace.   Both parent and child then display the system hostname, making it possible to
       see that the hostname differs in the UTS namespaces of  the  parent  and  child.   For  an
       example of the use of this program, see setns(2).

       #define _GNU_SOURCE
       #include <sys/wait.h>
       #include <sys/utsname.h>
       #include <sched.h>
       #include <string.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>

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

       static int              /* Start function for cloned child */
       childFunc(void *arg)
       {
           struct utsname uts;

           /* Change hostname in UTS namespace of child */

           if (sethostname(arg, strlen(arg)) == -1)
               errExit("sethostname");

           /* Retrieve and display hostname */

           if (uname(&uts) == -1)
               errExit("uname");
           printf("uts.nodename in child:  %s\n", uts.nodename);

           /* Keep the namespace open for a while, by sleeping.
              This allows some experimentation--for example, another
              process might join the namespace. */

           sleep(200);

           return 0;           /* Child terminates now */
       }

       #define STACK_SIZE (1024 * 1024)    /* Stack size for cloned child */

       int
       main(int argc, char *argv[])
       {
           char *stack;                    /* Start of stack buffer */
           char *stackTop;                 /* End of stack buffer */
           pid_t pid;
           struct utsname uts;

           if (argc < 2) {
               fprintf(stderr, "Usage: %s <child-hostname>\n", argv[0]);
               exit(EXIT_SUCCESS);
           }

           /* Allocate stack for child */

           stack = malloc(STACK_SIZE);
           if (stack == NULL)
               errExit("malloc");
           stackTop = stack + STACK_SIZE;  /* Assume stack grows downward */

           /* Create child that has its own UTS namespace;
              child commences execution in childFunc() */

           pid = clone(childFunc, stackTop, CLONE_NEWUTS | SIGCHLD, argv[1]);
           if (pid == -1)
               errExit("clone");
           printf("clone() returned %ld\n", (long) pid);

           /* Parent falls through to here */

           sleep(1);           /* Give child time to change its hostname */

           /* Display hostname in parent's UTS namespace. This will be
              different from hostname in child's UTS namespace. */

           if (uname(&uts) == -1)
               errExit("uname");
           printf("uts.nodename in parent: %s\n", uts.nodename);

           if (waitpid(pid, NULL, 0) == -1)    /* Wait for child */
               errExit("waitpid");
           printf("child has terminated\n");

           exit(EXIT_SUCCESS);
       }

SEE ALSO

       fork(2),  futex(2), getpid(2), gettid(2), kcmp(2), set_thread_area(2), set_tid_address(2),
       setns(2), tkill(2), unshare(2), wait(2), capabilities(7), pthreads(7)

COLOPHON

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       project,     and    information    about    reporting    bugs,    can    be    found    at
       http://www.kernel.org/doc/man-pages/.