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

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 cor-
       responds 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 termi-
       nate 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  proces-
       sors), 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 descrip-
	      tors 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  file  system
	      information.  This includes the root of the file system, the current working direc-
	      tory, 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 file system infor-
	      mation 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 con-
	      text, 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 mecha-
	      nisms 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 names-
	      paces.

	      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  CON-
	      FIG_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 implemen-
	      tation 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  physi-
	      cal  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 physi-
	      cal 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 names-
	      pace.

	      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  pro-
	      cesses  in that namespace are visible in the PID namespace of the process that cre-
	      ated 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 names-
	      paces will both be visible in the grandparent PID namespace.  Conversely, the  pro-
	      cesses  in  the "child" PID namespace do not see the processes in the parent names-
	      pace.  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 iden-
	      tifiers 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	 sethost-
	      name(2), respectively.  Changes made to the identifiers in a UTS namespace are vis-
	      ible 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  getp-
	      pid(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 call-
	      ing 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 sigac-
	      tion(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 han-
	      dlers 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 spec-
	      ified

       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 termi-
	      nates, 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 speci-
	      fied.

	      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  deliv-
	      ered  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 sig-
	      nal.  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 schedula-
	      ble 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 mem-
	      ory 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  map-
	      pings/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 descrip-
       tions 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  exe-
       cution.	On failure, -1 is returned in the caller's context, no child process will be cre-
       ated, 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 speci-
	      fied 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 man-
       ual 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  immedi-
       ately  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 follow-
       ing:

	   #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  exe-
       cutes  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
       This  page  is  part of release 3.53 of the Linux man-pages project.  A description of the
       project,    and	  information	 about	  reporting    bugs,	can    be    found     at
       http://www.kernel.org/doc/man-pages/.

Linux					    2013-04-16					 CLONE(2)
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