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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 fea-
   ture_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.)

       One 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	 func-
       tion 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 begin-
       ning of its execution.  The arg argument is passed to the fn function.

       When the fn(arg) function application returns, the child process termi-
       nates.	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  mem-
       ory,  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 con-
       stants, in order to specify what is shared between the calling  process
       and the child process:

       CLONE_CHILD_CLEARTID (since Linux 2.5.49)
	      Erase  the  child thread ID at the 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  the  child	 thread ID at the location ctid in the child's
	      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 cre-
	      ated 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 a process sharing a file  descriptor  table	 calls
	      execve(2), its file descriptor table is duplicated (unshared).

	      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	corre-
	      sponding	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 call-
	      ing 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 con-
	      tainers.

	      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 visi-
	      ble 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.

	      Only   a	 privileged   process	(CAP_SYS_ADMIN)	  can	employ
	      CLONE_NEWIPC.   This flag can't be specified in conjunction with
	      CLONE_SYSVSEM.

	      For further information on IPC namespaces, see namespaces(7).

       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  con-
	      tainers.

	      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 net-
	      work  device  can live in exactly one network namespace.	A vir-
	      tual network device ("veth") pair provides a pipe-like  abstrac-
	      tion  that  can be used to create tunnels between network names-
	      paces, 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).	For further information on network namespaces,
	      see namespaces(7).

	      Only   a	 privileged   process	(CAP_SYS_ADMIN)	  can	employ
	      CLONE_NEWNET.

       CLONE_NEWNS (since Linux 2.4.19)
	      If CLONE_NEWNS is set, the cloned child  is  started  in	a  new
	      mount namespace, initialized with a copy of the namespace of the
	      parent.  If CLONE_NEWNS is not set, the child lives in the  same
	      mount namespace as the parent.

	      For further information on mount namespaces, see namespaces(7).

	      Only   a	 privileged   process	(CAP_SYS_ADMIN)	  can	employ
	      CLONE_NEWNS.  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  con-
	      tainers.

	      For further information on PID namespaces, see namespaces(7) and
	      pid_namespaces(7)

	      Only a privileged process (CAP_SYS_ADMIN) can employ  CLONE_NEW-
	      PID.    This   flag  can't  be  specified	 in  conjunction  with
	      CLONE_THREAD or CLONE_PARENT.

       CLONE_NEWUSER
	      (This flag first became meaningful for clone() in Linux  2.6.23,
	      the  current clone() semantics were merged in Linux 3.5, and the
	      final pieces to make the user namespaces completely usable  were
	      merged in Linux 3.8.)

	      If  CLONE_NEWUSER	 is set, then create the process in a new user
	      namespace.  If this flag is not set, then (as with fork(2))  the
	      process  is  created  in	the same user namespace as the calling
	      process.

	      For further information on user  namespaces,  see	 namespaces(7)
	      and user_namespaces(7)

	      Before  Linux 3.8, use of CLONE_NEWUSER required that the caller
	      have three capabilities: CAP_SYS_ADMIN, CAP_SETUID, and CAP_SET-
	      GID.   Starting with Linux 3.8, no privileges are needed to cre-
	      ate a user namespace.

	      This flag can't be specified in conjunction with CLONE_THREAD or
	      CLONE_PARENT.   For  security  reasons,  CLONE_NEWUSER cannot be
	      specified in conjunction with CLONE_FS.

	      For further information  on  user	 namespaces,  see  user_names-
	      paces(7).

       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 cre-
	      ated  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 hostname 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  pro-
	      cesses in other UTS namespaces.

	      Only   a	 privileged   process	(CAP_SYS_ADMIN)	  can	employ
	      CLONE_NEWUTS.

	      For further information on UTS namespaces, see namespaces(7).

       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 the child thread ID at the location ptid in	 the  parent's
	      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.  Since then,	 the  kernel  silently
	      ignores it without error.

       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 behav-
	      ior 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.	Since then, the kernel
	      silently ignores it without error.

       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 adjustment (semadj)
	      values (see semop(2)).  In this case, the	 shared	 list  accumu-
	      lates  semadj  values across all processes sharing the list, and
	      semaphore adjustments are performed only when the	 last  process
	      that  is sharing the list terminates (or ceases sharing the list
	      using unshare(2)).  If this flag is not set, then the child  has
	      a separate semadj list that 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 dis-
	      cussion 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	 (sys-
	      tem-wide) unique thread IDs (TID).  A new thread's TID is avail-
	      able 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 termina-
	      tion) signal.

	      If  any  of the threads in a thread group performs an execve(2),
	      then all threads other than the thread group leader  are	termi-
	      nated,  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   (and   note  that,  since	 Linux
	      2.6.0-test6,  CLONE_SIGHAND  also	 requires   CLONE_VM   to   be
	      included).

	      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 unhan-
	      dled signal is delivered to a thread, then it will affect	 (ter-
	      minate, 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  sig-
	      nal 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 per-
	      formed  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).

   C library/kernel differences
       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 modi-
       fies 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 dif-
       ferent from the 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 call-
       er'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 appro-
       priately.

ERRORS
       EAGAIN Too many processes are already running; see fork(2).

       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 (since Linux 3.9)
	      Both CLONE_NEWUSER and CLONE_FS were specified in flags.

       EINVAL Both CLONE_NEWIPC and CLONE_SYSVSEM were specified in flags.

       EINVAL One (or both) of CLONE_NEWPID or CLONE_NEWUSER and one (or both)
	      of CLONE_THREAD or CLONE_PARENT 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 con-
	      figured with the CONFIG_SYSVIPC and CONFIG_IPC_NS options.

       EINVAL CLONE_NEWNET was specified in flags, but the kernel was not con-
	      figured with the CONFIG_NET_NS option.

       EINVAL CLONE_NEWPID was specified in flags, but the kernel was not con-
	      figured with the CONFIG_PID_NS option.

       EINVAL CLONE_NEWUTS was specified in flags, but the kernel was not con-
	      figured 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.

       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
       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  circum-
       stances.	  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 cre-
       ated 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
       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 dif-
       fers in the UTS namespaces of the parent and child.  For an example  of
       the use of this program, see setns(2).

   Program source
       #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),  capabili-
       ties(7), namespaces(7), pthreads(7)

COLOPHON
       This  page  is  part of release 4.04 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				  2015-07-23			      CLONE(2)