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

NAME
       clone, clone2 - create a child process

SYNOPSIS
       #include <sched.h>

       int clone(int (*fn)(void *arg), void *child_stack, int flags, void *arg);

       _syscall2(int, clone, int, flags, void *, child_stack)

       _syscall5(int, clone, int, flags, void *, child_stack,
	    int *, parent_tidptr, struct user_desc *, newtls,
	    int *, child_tidptr)
	       /* Using syscall(2) may be preferable; see intro(2) */

       int __clone2(int (*fn)(void *arg), void *child_stack,
	      size_t stack_size, int flags, void *arg);

       _syscall2(int, clone2, int, flags, void *, child_stack,
	      int, child_stack_size, int *, parent_tidptr,
	      struct user_desc *, newtls, int *, child_tidptr)

DESCRIPTION
       clone()	system	call can be use on all architectures except IA-64.  On
       IA-64 there is available command __clone2().   Both  commands  has  the
       similar	behavior  - they creates a new process, in a manner similar to
       fork(2).	 clone() is a library function layered on top of the  underly‐
       ing   clone()  system  call,  hereinafter  referred  to	as  sys_clone.
       __clone2() is exported from sys_clone2 system call.

       Unlike fork(2), these calls allow 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.

       The  behavior  of __clone2() is the same as the behavior of clone() The
       difference is __clone2() has one extra  argument	 stack_size  which  is
       used to determine the size of child stack. Other variables has the same
       meaning.

       When the child process is created with clone(), it executes  the	 func‐
       tion  application fn(arg).  (This differs from fork(2), where execution
       continues in the child from the point of the  fork(2)  call.)   The  fn
       argument is a pointer to a function that is called by the child process
       at the beginning of its execution.  The arg argument is passed  to  the
       fn function.

       When the fn(arg) function application returns, the child process 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	downwards  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_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_FS
	      If CLONE_FS is set, the caller and the child processes 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_FILES
	      If CLONE_FILES is set, the calling process and  the  child  pro‐
	      cesses  share the same file descriptor table.  Any file descrip‐
	      tor created by the calling process or by the  child  process  is
	      also  valid in the other process.	 Similarly, if one of the pro‐
	      cesses 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_NEWNS (since Linux 2.4.19)
	      Start the child in a new namespace.

	      Every  process  lives in a 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(2) where the
	      CLONE_NEWNS flag is not set, the child lives in the same	names‐
	      pace  as	the  parent.   The system calls mount(2) and umount(2)
	      change the namespace of the calling process,  and	 hence	affect
	      all processes that live in the same namespace, but do not affect
	      processes in a different namespace.

	      After a clone(2) where the CLONE_NEWNS flag is set,  the	cloned
	      child  is started in a new 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_SIGHAND
	      If CLONE_SIGHAND is set, the calling process and the child  pro‐
	      cesses  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_PTRACE
	      If  CLONE_PTRACE	is specified, and the calling process is being
	      traced, then trace the child also (see ptrace(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_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.

       CLONE_VFORK
	      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
	      If  CLONE_VM is set, the calling process and the child processes
	      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).

       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_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_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_SETTLS (since Linux 2.5.32)
	      The  newtls  parameter  is  the  new  TLS (Thread Local Storage)
	      descriptor.  (See set_thread_area(2).)

       CLONE_PARENT_SETTID (since Linux 2.5.49)
	      Store child thread ID at location parent_tidptr  in  parent  and
	      child   memory.	(In  Linux  2.5.32-2.5.48  there  was  a  flag
	      CLONE_SETTID that did this.)

       CLONE_CHILD_SETTID (since Linux 2.5.49)
	      Store child thread ID at location child_tidptr in child memory.

       CLONE_CHILD_CLEARTID (since Linux 2.5.49)
	      Erase child thread ID at location child_tidptr 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.

   sys_clone
       The sys_clone system call corresponds more closely to fork(2)  in  that
       execution  in  the  child  continues from the point of the call.	 Thus,
       sys_clone only requires the flags and child_stack arguments, which have
       the  same  meaning as for clone().  (Note that the order of these argu‐
       ments differs from clone().)

       Another difference for sys_clone is that the child_stack	 argument  may
       be  zero,  in  which case copy-on-write semantics ensure that the child
       gets separate copies of stack pages when either	process	 modifies  the
       stack.  In this case, for correct operation, the CLONE_VM option should
       not be specified.

       Since Linux 2.5.49 the system call has five parameters.	 The  two  new
       parameters  are	parent_tidptr  which points to the location (in parent
       and child memory) where the child thread ID will	 be  written  in  case
       CLONE_PARENT_SETTID was specified, and child_tidptr which points to the
       location (in child memory) where the child thread ID will be written in
       case CLONE_CHILD_SETTID was specified.

RETURN VALUE
       On success, the thread ID of the child process is returned in the call‐
       er's thread of execution.  On failure, a -1 will	 be  returned  in  the
       caller's	 context,  no child process will be created, and errno will be
       set appropriately.

ERRORS
       EAGAIN Too many processes are already running.

       EINVAL CLONE_SIGHAND was specified, but CLONE_VM was not. (Since	 Linux
	      2.6.0-test6.)

       EINVAL CLONE_THREAD  was	 specified,  but CLONE_SIGHAND was not. (Since
	      Linux 2.5.35.)

       EINVAL Both CLONE_FS and CLONE_NEWNS were specified in flags.

       EINVAL Returned	by  clone()  when  a  zero  value  is  specified   for
	      child_stack.

       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_NEWNS was specified by a non-root process (process without
	      CAP_SYS_ADMIN).

       EPERM  CLONE_PID was specified by a process other than process 0.

AVAILABILITY
       There is no entry for clone() in libc5.	 glibc2	 provides  clone()  as
       described in this manual page.

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).

       On x86, clone() should not be called  through  vsyscall,	 but  directly
       through int $0x80.

CONFORMING TO
       The  clone()  and  sys_clone calls are Linux-specific and should not be
       used in programs intended to be portable.

BUGS
       Versions of the GNU C library that include the NPTL  threading  library
       contain	a wrapper function for getpid() that performs caching of PIDs.
       In programs linked against such libraries, calls to getpid() may return
       the   same  value,  even	 when  the  threads  were  not	created	 using
       CLONE_THREAD (and thus are not in the same thread group).  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);

SEE ALSO
       fork(2),	   futex(2),	getpid(2),    gettid(2),   set_thread_area(2),
       set_tid_address(2),  tkill(2),  unshare(2),  wait(2),  capabilities(7),
       pthreads(7)

Linux 2.6			  2005-05-17			      CLONE(2)
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