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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_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.

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

       CLONE_FILES
              If CLONE_FILES is set, the calling process and the child process
              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_FS
              If  CLONE_FS  is set, the caller and the child process 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_IO (since Linux 2.6.25)
              If CLONE_IO is set, then the new process shares an  I/O  context
              with  the  calling  process.   If this flag is not set, then (as
              with fork(2)) the new process has its own I/O context.

              The I/O context is the I/O scope of  the  disk  scheduler  (i.e,
              what  the  I/O scheduler uses to model scheduling of a process’s
              I/O).  If processes share the same I/O context, they are treated
              as  one  by  the  I/O  scheduler.  As a consequence, they get to
              share disk time.  For some  I/O  schedulers,  if  two  processes
              share  an  I/O context, they will be allowed to interleave their
              disk access.  If several threads are doing I/O on behalf of  the
              same  process  (aio_read(3),  for  instance), they should employ
              CLONE_IO to get better I/O performance.

              If the kernel is not configured with  the  CONFIG_BLOCK  option,
              this flag is a no-op.

       CLONE_NEWIPC (since Linux 2.4.19)
              If  CLONE_NEWIPC  is  set,  then create the process in a new IPC
              namespace.  If this flag is not set, then (as with fork(2)), the
              process  is  created  in  the  same IPC namespace as the calling
              process.  This  flag  is  intended  for  the  implementation  of
              containers.

              An IPC namespace consists of the set of identifiers for System V
              IPC  objects.   (These  objects  are  created  using  msgctl(2),
              semctl(2),  and shmctl(2)).  Objects created in an IPC namespace
              are  visible  to  other  processes  that  are  members  of  that
              namespace,  but  are  not  visible  to  processes  in  other IPC
              namespaces.

              When an IPC namespace is destroyed (i.e, when the  last  process
              that  is  a member of the namespace terminates), all IPC objects
              in the namespace are automatically destroyed.

              Use  of  this  flag  requires:  a  kernel  configured  with  the
              CONFIG_SYSVIPC and CONFIG_IPC_NS options and that the process be
              privileged (CAP_SYS_ADMIN).  This flag  can’t  be  specified  in
              conjunction with CLONE_SYSVSEM.

       CLONE_NEWNET (since Linux 2.6.24)
              (The  implementation  of  this  flag  is  not  yet complete, but
              probably will be mostly complete by about Linux 2.6.28.)

              If CLONE_NEWNET is set, then create the process in a new network
              namespace.  If this flag is not set, then (as with fork(2)), the
              process is created in the same network namespace as the  calling
              process.   This  flag  is  intended  for  the  implementation of
              containers.

              A network namespace provides an isolated view of the  networking
              stack (network device interfaces, IPv4 and IPv6 protocol stacks,
              IP  routing  tables,   firewall   rules,   the   /proc/net   and
              /sys/class/net  directory  trees,  sockets,  etc.).   A physical
              network device can live in exactly  one  network  namespace.   A
              virtual  network  device  ("veth")  pair  provides  a  pipe-like
              abstraction that can be used to create tunnels  between  network
              namespaces,  and  can  be  used to create a bridge to a physical
              network device in another namespace.

              When a network namespace is freed (i.e., when the  last  process
              in  the  namespace terminates), its physical network devices are
              moved back to the initial network namespace (not to  the  parent
              of the process).

              Use  of  this  flag  requires:  a  kernel  configured  with  the
              CONFIG_NET_NS  option  and  that  the  process   be   privileged
              (CAP_SYS_ADMIN).

       CLONE_NEWNS (since Linux 2.4.19)
              Start the child in a new mount namespace.

              Every  process  lives  in a mount 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  mount  namespace as the parent.  The system calls mount(2)
              and umount(2) change the mount namespace of the calling process,
              and  hence affect all processes that live in the same namespace,
              but do not affect processes in a different mount namespace.

              After a clone() where the CLONE_NEWNS flag is  set,  the  cloned
              child  is  started  in a new mount 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_NEWPID (since Linux 2.6.24)
              If  CLONE_NEWPID  is  set,  then create the process in a new PID
              namespace.  If this flag is not set, then (as with fork(2)), the
              process  is  created  in  the  same PID namespace as the calling
              process.  This  flag  is  intended  for  the  implementation  of
              containers.

              A  PID namespace provides an isolated environment for PIDs: PIDs
              in a new namespace  start  at  1,  somewhat  like  a  standalone
              system, and calls to fork(2), vfork(2), or clone(2) will produce
              processes with PIDs that are unique within the namespace.

              The first process created in a new namespace (i.e., the  process
              created  using  the CLONE_NEWPID flag) has the PID 1, and is the
              "init" process for the namespace.  Children  that  are  orphaned
              within  the  namespace will be reparented to this process rather
              than init(8).  Unlike the traditional init process,  the  "init"
              process of a PID namespace can terminate, and if it does, all of
              the processes in the namespace are terminated.

              PID namespaces form a hierarchy.  When a PID  new  namespace  is
              created,  the processes in that namespace are visible in the PID
              namespace  of  the  process  that  created  the  new  namespace;
              analogously,  if the parent PID namespace is itself the child of
              another PID namespace, then processes in the  child  and  parent
              PID  namespaces  will  both  be  visible  in the grandparent PID
              namespace.   Conversely,  the  processes  in  the  "child"   PID
              namespace do not see the processes in the parent namespace.  The
              existence of a namespace hierarchy means that each  process  may
              now  have  multiple  PIDs: one for each namespace in which it is
              visible; each of these PIDs is unique within  the  corresponding
              namespace.    (A  call  to  getpid(2)  always  returns  the  PID
              associated with the namespace in which the process lives.)

              After creating the new namespace, it is useful for the child  to
              change  its  root  directory  and mount a new procfs instance at
              /proc  so  that  tools  such  as  ps(1)  work  correctly.    (If
              CLONE_NEWNS  is  also included in flags, then it isn’t necessary
              to change the root directory:  a  new  procfs  instance  can  be
              mounted directly over /proc.)

              Use  of  this  flag  requires:  a  kernel  configured  with  the
              CONFIG_PID_NS  option  and  that  the  process   be   privileged
              (CAP_SYS_ADMIN).   This  flag  can’t be specified in conjunction
              with CLONE_THREAD.

       CLONE_NEWUTS (since Linux 2.6.19)
              If CLONE_NEWUTS is set, then create the process  in  a  new  UTS
              namespace,  whose identifiers are initialized by duplicating the
              identifiers from the UTS namespace of the calling  process.   If
              this  flag  is  not  set, then (as with fork(2)), the process is
              created in the same UTS namespace as the calling process.   This
              flag is intended for the implementation of containers.

              A  UTS namespace is the set of identifiers returned by uname(2);
              among these, the domain name and the host name can  be  modified
              by  setdomainname(2) and  sethostname(2), respectively.  Changes
              made to these identifiers in one UTS namespace  are  visible  to
              other  processes  in  the same namespace, but are not visible to
              processes in other UTS namespaces.

              Use  of  this  flag  requires:  a  kernel  configured  with  the
              CONFIG_UTS_NS   option   and  that  the  process  be  privileged
              (CAP_SYS_ADMIN).

       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_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_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_PTRACE
              If CLONE_PTRACE is specified, and the calling process  is  being
              traced, then trace the child also (see ptrace(2)).

       CLONE_SETTLS (since Linux 2.5.32)
              The  newtls  argument  is  the  new  TLS  (Thread Local Storage)
              descriptor.  (See set_thread_area(2).)

       CLONE_SIGHAND
              If CLONE_SIGHAND is set,  the  calling  process  and  the  child
              process 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_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_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_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_UNTRACED (since Linux 2.5.46)
              If CLONE_UNTRACED is specified, then a  tracing  process  cannot
              force CLONE_PTRACE on this child process.

       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 process
              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).

   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 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.

RETURN VALUE

       On success, the thread ID of the  child  process  is  returned  in  the
       caller’s  thread  of  execution.   On  failure,  -1  is 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 Both CLONE_NEWIPC and CLONE_SYSVSEM were specified in flags.

       EINVAL Both CLONE_NEWPID and CLONE_THREAD were specified in flags.

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

       EINVAL CLONE_NEWIPC was specified in flags,  but  the  kernel  was  not
              configured with the CONFIG_SYSVIPC and CONFIG_IPC_NS options.

       EINVAL CLONE_NEWNET  was  specified  in  flags,  but the kernel was not
              configured with the CONFIG_NET_NS option.

       EINVAL CLONE_NEWPID was specified in flags,  but  the  kernel  was  not
              configured with the CONFIG_PID_NS option.

       EINVAL CLONE_NEWUTS  was  specified  in  flags,  but the kernel was not
              configured with the CONFIG_UTS option.

       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_NEWIPC,  CLONE_NEWNET,   CLONE_NEWNS,   CLONE_NEWPID,   or
              CLONE_NEWUTS  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.
       This caching relies on support in the glibc wrapper for clone(), but as
       currently  implemented,  the  cache  may  not  be  up  to  date in some
       circumstances.  In particular, if a signal is delivered  to  the  child
       immediately  after  the  clone()  call,  then  a  call to getpid() in a
       handler for the signal may return the PID of the calling process  ("the
       parent"),  if  the clone wrapper has not yet had a chance to update the
       PID cache in the child.  (This discussion ignores the  case  where  the
       child  was  created using CLONE_THREAD, when getpid() should return the
       same value in the child and in the process that called  clone(),  since
       the caller and the child are in the same thread group.  The stale-cache
       problem also does not occur if the flags argument  includes  CLONE_VM.)
       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

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