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clone(2) - clone, clone - create a child process - man 2 clone

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



NAME
       clone - create a child process

SYNOPSIS
       #include <sched.h>

       int clone(int (*fn)(void *), 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)

DESCRIPTION
       clone  creates  a  new  process, just like fork(2).  clone is a library
       function layered on top of the  underlying  clone  system  call,  here-
       inafter  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(1,n) descriptors, and the table of signal(2,7) 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 con-
       trol in(1,8) a program that run concurrently in(1,8) a shared memory space.

       When  the child process is created with clone, it executes the function
       application fn(arg).  (This differs from fork(2), where execution  con-
       tinues  in(1,8) the child from the point of the fork(2) call.)  The fn argu-
       ment 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(3,n,1 builtins) code for the child
       process.  The child process may also terminate  explicitly  by  calling
       exit(3,n,1 builtins)(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(1,8) the same
       stack as the calling process.  The calling process must  therefore  set(7,n,1 builtins)
       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(7,n,1 builtins) up for the child stack.

       The low byte of flags contains the number of the  signal(2,7)  sent  to  the
       parent  when  the  child dies.  If this signal(2,7) 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(2,7)
       is specified, then the parent process is not signaled  when  the  child
       terminates.

       flags  may  also  be bitwise-or'ed with one or several of the following
       constants, in(1,8) 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(7,n,1 builtins),  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(7,n,1 builtins), 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(3,n)
              CLONE_PARENT is set(7,n,1 builtins), then the parent  of  the  calling  process,
              rather than the calling process itself, will be signaled.


       CLONE_FS
              If CLONE_FS is set(7,n,1 builtins), the caller and the child processes share the
              same file(1,n) system information.  This includes  the  root  of  the
              file(1,n)  system, the current working directory, and the umask.  Any
              call to chroot(1,2)(2), chdir(2), or umask(2) performed by the  call-
              ing  process or the child process also takes effect in(1,8) the other
              process.

              If CLONE_FS is not set(7,n,1 builtins), the child process works on a copy of the
              file(1,n)  system  information  of the calling process at the time(1,2,n) of
              the clone call.  Calls to  chroot(1,2)(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(7,n,1 builtins), the calling process and  the  child  pro-
              cesses  share  the same file(1,n) descriptor table.  File descriptors
              always refer to the same files in(1,8) the calling process and in(1,8) the
              child  process.   Any  file(1,n)  descriptor  created  by the calling
              process or by the child process  is  also  valid  in(1,8)  the  other
              process.   Similarly,  if(3,n)  one  of  the  processes closes a file(1,n)
              descriptor, or changes its associated flags, the  other  process
              is also affected.

              If  CLONE_FILES is not set(7,n,1 builtins), the child process inherits a copy of
              all file(1,n) descriptors opened in(1,8) the calling process at  the  time(1,2,n)
              of  clone.   Operations  on  file(1,n) descriptors performed later 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(1,8) a new namespace.

              Every  process  lives in(1,8) a namespace. The namespace of a process
              is the data (the set(7,n,1 builtins) of mounts) describing the file(1,n) hierarchy as
              seen  by  that  process.  After  a fork(2) or clone(2) where the
              CLONE_NEWNS flag is not set(7,n,1 builtins), the child lives in(1,8) the same  names-
              pace  as  the  parent.   The system calls mount(2,8)(2) and umount(2)
              change the namespace of the calling process,  and  hence  affect
              all processes that live in(1,8) the same namespace, but do not affect
              processes in(1,8) a different namespace.

              After a clone(2) where the CLONE_NEWNS flag is set(7,n,1 builtins),  the  cloned
              child  is started in(1,8) 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(1,8) the same clone call.


       CLONE_SIGHAND
              If CLONE_SIGHAND is set(7,n,1 builtins), the calling process and the child  pro-
              cesses  share the same table of signal(2,7) handlers.  If the calling
              process or child process calls sigaction(2) to change the behav-
              ior  associated  with  a  signal(2,7), the behavior is changed in(1,8) the
              other process as well.  However, the calling process  and  child
              processes  still  have distinct signal(2,7) 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(7,n,1 builtins), the child process inherits a copy
              of the signal(2,7) handlers of the calling process at the time(1,2,n)  clone
              is  called.  Calls to sigaction(2) performed later by one of the
              processes have no effect on the other process.


       CLONE_PTRACE
              If CLONE_PTRACE is specified, and the calling process  is  being
              traced, then trace(3x,n,3x _nc_tracebits) the child also (see ptrace(2)).


       CLONE_VFORK
              If  CLONE_VFORK  is set(7,n,1 builtins), the execution of the calling process is
              suspended until the child releases its virtual(5,8) memory  resources
              via a call to execve(2) or _exit(2) (as with vfork(2)).

              If  CLONE_VFORK is not set(7,n,1 builtins) then both the calling process and the
              child are schedulable after the call, and an application  should
              not rely on execution occurring in(1,8) any particular order.


       CLONE_VM
              If  CLONE_VM is set(7,n,1 builtins), the calling process and the child processes
              run in(1,8) the same memory space.  In particular, memory writes per-
              formed  by  the calling process or by the child process are also
              visible in(1,8) 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(7,n,1 builtins), the child process  runs  in(1,8)  a  separate
              copy  of  the memory space of the calling process at the time(1,2,n) of
              clone.  Memory writes or file(1,n) 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(7,n,1 builtins), 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(1,8) Linux 2.5.16.


       CLONE_THREAD (since Linux 2.4.0-test8)
              If  CLONE_THREAD  is set(7,n,1 builtins), the child is placed in(1,8) the same thread
              group as the calling process.

              If CLONE_THREAD is not set(7,n,1 builtins), then the child is placed in(1,8) its  own
              (new) thread group, whose ID is the same as the process ID.

              (Thread  groups  are  feature  added in(1,8) Linux 2.4 to support the
              POSIX threads notion of a set(7,n,1 builtins) of threads sharing a  single  PID.
              In  Linux  since 2.4, calls to getpid(2) return the thread group
              ID of the caller.)


       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(1,8) 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(1,8) child memory.


       CLONE_CHILD_CLEARTID (since Linux 2.5.49)
              Erase child thread ID at location child_tidptr in(1,8)  child  memory
              when  the  child  exits,  and  do  a wakeup on the futex(2,4) 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(1,8)  that
       execution  in(1,8)  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 arguments
       differs from clone.)

       Another difference for sys_clone is that the child_stack  argument  may
       be  zero,  in(1,8)  which case copy-on-write semantics ensure that the child
       gets(3,n) 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(1,8) parent
       and child memory) where the parent thread ID will be  written  in(1,8)  case
       CLONE_PARENT_SETTID was specified, and child_tidptr which points to the
       location (in(1,8) child memory) where the child thread ID will be written in(1,8)
       case CLONE_CHILD_SETTID was specified.


RETURN VALUE
       On  success,  the  thread  ID  of  the child process is returned in(1,8) the
       caller's thread of execution.  On failure, a -1 will be returned in(1,8) the
       caller's  context,  no child process will be created, and errno will be
       set(7,n,1 builtins) 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 Precisely one of CLONE_DETACHED and CLONE_THREAD was  specified.
              (Since Linux 2.6.0-test6.)

       EINVAL Both CLONE_FS and CLONE_NEWNS were specified in(1,8) 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(1,8)  libc5.   glibc2 provides clone as
       described in(1,8) this manual page.


NOTES
       For kernel versions 2.4.7-2.4.18  the  CLONE_THREAD  flag  implied  the
       CLONE_PARENT flag.


CONFORMING TO
       The clone and sys_clone calls are Linux-specific and should not be used
       in(1,8) programs intended to be portable.


SEE ALSO
       fork(2), getpid(2), gettid(2), wait(2), capabilities(7)



Linux 2.6                         2004-09-10                          CLONE(2)

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