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NAME

       clone, __clone2 - create a child process

SYNOPSIS

       #define _GNU_SOURCE
       #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 */ );

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
       function  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
       terminates.   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
       memory, 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
       constants, 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
              calling  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)
              performed  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
              processes  share  the  same  file  descriptor  table.   Any file
              descriptor created 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 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
              corresponding  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
              namespace   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
              capability)  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
              processes share the same  table  of  signal  handlers.   If  the
              calling  process  or  child process calls sigaction(2) to change
              the behavior 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.

              From  Linux  2.6.25 this flag is deprecated.  You probably never
              wanted to use it, you certainly shouldn’t be using it, and  soon
              it will go away.

       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
              performed  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
              discussion  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
              (system-wide)  unique  thread  IDs (TID).  A new thread’s TID is
              available 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
              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
              termination) signal.

              If  any  of the threads in a thread group performs an execve(2),
              then  all  threads  other  than  the  thread  group  leader  are
              terminated,  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
              unhandled signal is delivered to a thread, then it  will  affect
              (terminate,  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
              signal  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
              separate 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
       arguments 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  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 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)

COLOPHON

       This page is part of release 3.01 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/.