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

       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 */ );

DESCRIPTION
       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-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 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
	      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
       caller'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.

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

CONFORMING TO
       The  clone()  and  sys_clone calls are 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 x86, clone() should not be called  through  vsyscall,	 but  directly
       through int $0x80.

       On IA-64, 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 child's stack
       area,  and  stack_size  specifies  the  size of the stack pointed to by
       child_stack_base.

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.
       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

	   #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			  2007-06-01			      CLONE(2)
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