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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 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.

	      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 3.54 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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