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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 */

       #define _GNU_SOURCE
       #include <sched.h>

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

       /* For the prototype of the raw system call, see NOTES */

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 virtual address
       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 flows  of  control
       in a program that run concurrently in a shared address space.

       When  the child process is created with clone(), it commences execution
       by calling the function pointed to by the argument fn.	(This  differs
       from  fork(2), where execution continues in the child from the point of
       the fork(2) call.)  The arg argument is passed as the argument  of  the
       function fn.

       When  the  fn(arg) function returns, the child process terminates.  The
       integer returned by fn is the exit status 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-ORed 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)
	      Clear (zero) 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.	The  store  operation completes before clone() returns
	      control to user space.

       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().   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.  Note,	 however,  that	 the  duplicated  file
	      descriptors  in  the  child refer to the same open file descrip‐
	      tions as the  corresponding  file	 descriptors  in  the  calling
	      process,	and thus share file offsets and file status flags (see
	      open(2)).

       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),  or	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_NEWCGROUP (since Linux 4.6)
	      Create  the  process in a new cgroup namespace.  If this flag is
	      not set, then (as with fork(2)) the process is  created  in  the
	      same  cgroup  namespaces	as  the calling process.  This flag is
	      intended for the implementation of containers.

	      For further information on cgroup namespaces, see	 cgroup_names‐
	      paces(7).

	      Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWC‐
	      GROUP.

       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 (veth(4)) device 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.

	      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.

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

       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.

	      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	 namespaces(7)
	      and user_namespaces(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.)  The store operation  completes  before  clone()
	      returns control to user space.

       CLONE_PID (Linux 2.0 to 2.5.15)
	      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.   From Linux 2.3.21
	      onward, this flag could be specified only	 by  the  system  boot
	      process  (PID 0).	 The flag disappeared completely from the ker‐
	      nel sources in Linux 2.5.16.  Since then,	 the  kernel  silently
	      ignores this bit if it is specified in flags.

       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 TLS (Thread Local Storage) descriptor is set to newtls.

	      The interpretation of newtls and the resulting effect is	archi‐
	      tecture  dependent.   On	x86, newtls is interpreted as a struct
	      user_desc * (see set_thread_area(2)).  On x86-64 it is  the  new
	      value  to	 be set for the %fs base register (see the ARCH_SET_FS
	      argument to arch_prctl(2)).  On architectures with  a  dedicated
	      TLS register, it is the new value of that register.

       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 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.  Starting with Linux 4.6, the
	      same bit was reused for the CLONE_NEWCGROUP flag.

       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).

NOTES
       Note that the glibc clone() wrapper function makes some changes in  the
       memory  pointed to by child_stack (changes required to set the stack up
       correctly for the child) before invoking the clone() system call.   So,
       in  cases  where clone() is used to recursively create children, do not
       use the buffer employed for the parent's stack  as  the	stack  of  the
       child.

   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.

       Another	difference  for	 the  raw  clone()  system  call  is  that the
       child_stack argument may be zero, in which case the child uses a dupli‐
       cate  of	 the parent's stack.  (Copy-on-write semantics ensure that the
       child gets separate copies of stack pages when either process  modifies
       the  stack.)   In this case, for correct operation, the CLONE_VM option
       should not be specified.	 (If the  child	 shares	 the  parent's	memory
       because of the use of the CLONE_VM flag, then no copy-on-write duplica‐
       tion occurs and chaos is likely to result.)

       The order of the arguments also differs in the  raw  system  call,  and
       there are variations in the arguments across architectures, as detailed
       in the following paragraphs.

       The raw system call interface on x86-64 and  some  other	 architectures
       (including sh, tile, and alpha) is roughly:

	   long clone(unsigned long flags, void *child_stack,
		      int *ptid, int *ctid,
		      unsigned long newtls);

       On  x86-32,  and	 several  other common architectures (including score,
       ARM, ARM 64, PA-RISC, arc, Power PC, xtensa, and MIPS),	the  order  of
       the last two arguments is reversed:

	   long clone(unsigned long flags, void *child_stack,
		     int *ptid, unsigned long newtls,
		     int *ctid);

       On  the	cris  and s390 architectures, the order of the first two argu‐
       ments is reversed:

	   long clone(void *child_stack, unsigned long flags,
		      int *ptid, int *ctid,
		      unsigned long newtls);

       On the microblaze architecture, an additional argument is supplied:

	   long clone(unsigned long flags, void *child_stack,
		      int stack_size,	      /* Size of stack */
		      int *ptid, int *ctid,
		      unsigned long newtls);

   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  the	 glibc	clone()	 wrapper  function  when fn or
	      child_stack is specified as NULL.

       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.

       EINVAL child_stack is not aligned  to  a	 suitable  boundary  for  this
	      architecture.   For  example,  on aarch64, child_stack must be a
	      multiple of 16.

       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.

       ENOSPC (since Linux 3.7)
	      CLONE_NEWPID was specified in flags, but the limit on the	 nest‐
	      ing  depth  of  PID  namespaces  would  have  been exceeded; see
	      pid_namespaces(7).

       ENOSPC (since Linux 4.9; beforehand EUSERS)
	      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).

	      From Linux 3.11 to Linux 4.8, the error diagnosed in  this  case
	      was EUSERS.

       ENOSPC (since Linux 4.9)
	      One  of the values in flags specified the creation of a new user
	      namespace, but doing so would have caused the limit  defined  by
	      the  corresponding  file	in /proc/sys/user to be exceeded.  For
	      further details, see namespaces(7).

       EPERM  CLONE_NEWCGROUP,	 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.
	      (This error occurs only on Linux 2.5.15 and earlier.)

       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).

       ERESTARTNOINTR (since Linux 2.6.17)
	      System call was interrupted by a signal and will	be  restarted.
	      (This can be seen only during a trace.)

       EUSERS (Linux 3.11 to Linux 4.8)
	      CLONE_NEWUSER  was specified in flags, and the limit on the num‐
	      ber of nested user namespaces would be exceeded.	See  the  dis‐
	      cussion of the ENOSPC error above.

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

NOTES
       The kcmp(2) system call can be used to test whether two processes share
       various	resources  such as a file descriptor table, System V semaphore
       undo operations, or a virtual address space.

       Handlers registered using pthread_atfork(3) are not executed  during  a
       call to clone().

       In  the	Linux  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 Linux 2.6.0 and later).

       For a while there was CLONE_DETACHED  (introduced  in  2.5.32):	parent
       wants no child-exit signal.  In Linux 2.6.2, the need to give this flag
       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
       GNU C library versions 2.3.4 up to and including 2.24 contained a wrap‐
       per  function  for  getpid(2)  that  performed  caching	of PIDs.  This
       caching relied on support in the glibc wrapper for clone(), but limita‐
       tions  in the implementation meant that the cache was not up to date in
       some circumstances.  In particular, if a signal was  delivered  to  the
       child immediately after the clone() call, then a call to getpid(2) in a
       handler for the signal could return the	PID  of	 the  calling  process
       ("the parent"), if the clone wrapper had not yet had a chance to update
       the PID cache in the child.  (This discussion ignores  the  case	 where
       the  child was created using CLONE_THREAD, when getpid(2) should return
       the same value in the child and in the  process	that  called  clone(),
       since  the  caller  and	the  child  are in the same thread group.  The
       stale-cache problem also does not occur if the flags argument  includes
       CLONE_VM.)   To	get  the truth, it was sometimes necessary to use code
       such as the following:

	   #include <syscall.h>

	   pid_t mypid;

	   mypid = syscall(SYS_getpid);

       Because of the stale-cache problem, as well as other problems noted  in
       getpid(2), the PID caching feature was removed in glibc 2.25.

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 4.14 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
       https://www.kernel.org/doc/man-pages/.

Linux				  2017-09-15			      CLONE(2)
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