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CLONE(2)		   Linux Programmers Manual		     CLONE(2)

       clone, __clone2 - create a child process

       #define _GNU_SOURCE

       int clone(int (*fn)(void *), void *child_stack,
		 int flags, void *arg, ...
		 /* pid_t *pid, struct user_desc *tls, pid_t *ctid */ );

       clone()	creates  a new process, in a manner similar to fork(2).  It is
       actually a library function layered on top of  the  underlying  clone()
       system  call,  hereinafter  referred to as sys_clone.  A description of
       sys_clone is given towards the end of this page.

       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.

       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 pro
       cess.  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-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_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

	      If  CLONE_PARENT	is not set, then (as with fork(2)) the childs
	      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.

	      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

	      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() 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() 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.

	      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

	      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.

	      From Linux 2.6.25 this flag is deprecated.  You  probably  never
	      wanted  to use it, you certainly shouldnt be using it, and soon
	      it will go away.

	      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.

	      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 threads 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 threads TID.  This thread is the leader
	      of the new thread group.

	      A  new thread created with CLONE_THREAD has the same parent pro
	      cess 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  terminates,  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

	      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

	      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

	      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

	      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 argument  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

       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  arguments.	 The  two  new
       arguments 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.

       On  success,  the  thread  ID  of  the child process is returned in the
       callers thread of execution.   On  failure,  -1	is  returned  in  the
       callers	context,  no child process will be created, and errno will be
       set appropriately.

       EAGAIN Too many processes are already running.

       EINVAL CLONE_SIGHAND was specified, but CLONE_VM was not.  (Since Linux

       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

       ENOMEM Cannot  allocate	sufficient memory to allocate a task structure
	      for the child, or to copy those parts of	the  callers  context
	      that need to be copied.

       EPERM  CLONE_NEWNS was specified by a non-root process (process without

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

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

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

       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

       On i386, clone() should not be called through  vsyscall,  but  directly
       through int $0x80.

       On ia64, a different system call is used:

       int __clone2(int (*fn)(void *),
		    void *child_stack_base, size_t stack_size,
		    int flags, void *arg, ...
		 /* pid_t *pid, struct user_desc *tls, pid_t *ctid */ );

       The  __clone2() system call operates in the same way as clone(), except
       that child_stack_base points to the lowest address of the childs stack
       area,  and  stack_size  specifies  the  size of the stack pointed to by

       Versions of the GNU C library that include the NPTL  threading  library
       contain a wrapper function for getpid(2) that performs caching of PIDs.
       In programs linked against  such  libraries,  calls  to	getpid(2)  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


	   pid_t mypid;

	   mypid = syscall(SYS_getpid);

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

       This  page  is  part of release 3.05 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				  2008-04-13			      CLONE(2)

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