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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.)

       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	 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  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 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 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 file system information.  This includes  the	 root  of  the
	      file  system, 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
	      file system information of the calling process at	 the  time  of
	      the  clone()  call.  Calls to chroot(2), chdir(2), umask(2) per‐
	      formed 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.

	      Use of this flag requires: a kernel  configured  with  the  CON‐
	      FIG_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 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).

	      Use  of  this  flag  requires: a kernel configured with the CON‐
	      FIG_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 capabil‐
	      ity)  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 con‐
	      tainers.

	      A PID namespace provides an isolated environment for PIDs:  PIDs
	      in  a  new namespace start at 1, somewhat like a standalone sys‐
	      tem, 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; analo‐
	      gously, 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 names‐
	      pace 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  associ‐
	      ated 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  CON‐
	      FIG_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 CON‐
	      FIG_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 mem‐
	      ory.  (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 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.

       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 sepa‐
	      rate 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 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.

	      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).

   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 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  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.

       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 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.

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
   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  dif‐
       fers  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), capabili‐
       ties(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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