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NAME

       clone, __clone2 - create a child process

SYNOPSIS

       /* Prototype for the glibc wrapper function */

       #include <sched.h>

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

       /* Prototype for the raw system call */

       long clone(unsigned long flags, void *child_stack,
                 void *ptid, void *ctid,
                 struct pt_regs *regs);

   Feature   Test   Macro   Requirements   for  glibc  wrapper  function  (see
   feature_test_macros(7)):

       clone():
           Since glibc 2.14:
               _GNU_SOURCE
           Before glibc 2.14:
               _BSD_SOURCE || _SVID_SOURCE
                   /* _GNU_SOURCE also suffices */

DESCRIPTION

       clone() creates a new process, in a manner similar to fork(2).

       This page describes both the glibc clone()  wrapper  function  and  the
       underlying  system  call on which it is based.  The main text describes
       the wrapper function; the differences  for  the  raw  system  call  are
       described toward the end of this page.

       Unlike  fork(2), clone() allows 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   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 downward 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 ctid 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 ctid in child memory.

       CLONE_FILES (since Linux 2.0)
              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 (since Linux 2.0)
              If CLONE_FS is set, the caller and the child process  share  the
              same  filesystem  information.   This  includes  the root of the
              filesystem, 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
              filesystem 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 2.6.19)
              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  provides  an  isolated  view of System V IPC
              objects (see svipc(7)) and (since Linux  2.6.30)  POSIX  message
              queues (see mq_overview(7)).  The common characteristic of these
              IPC mechanisms is that IPC objects are identified by  mechanisms
              other than filesystem pathnames.

              Objects  created  in  an  IPC namespace are visible to all 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 was  completed  only  by  about
              kernel version 2.6.29.)

              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() 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 new PID 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 hostname can be modified by
              setdomainname(2) and sethostname(2), respectively.  Changes made
              to  the  identifiers in a UTS namespace are visible to all 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  ptid 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 (since Linux 2.2)
              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 (since Linux 2.0)
              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.

              This  flag  was  deprecated  from  Linux  2.6.25 onward, and was
              removed altogether in Linux 2.6.38.

       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   (and   note  that,  since  Linux
              2.6.0-test6,  CLONE_SIGHAND  also  requires   CLONE_VM   to   be
              included).

              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 (since Linux 2.2)
              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 (since Linux 2.0)
              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).

   The raw system call interface
       The raw clone() system call corresponds more closely to fork(2) in that
       execution in the child continues from the point of the call.  As  such,
       the  fn  and arg arguments of the clone() wrapper function are omitted.
       Furthermore, the argument order changes.  The raw system call interface
       on x86 and many other architectures is roughly:

           long clone(unsigned long flags, void *child_stack,
                      void *ptid, void *ctid,
                      struct pt_regs *regs);

       Another  difference  for  the  raw  system call 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.

       For  some architectures, the order of the arguments for the system call
       differs from that shown above.  On the score, microblaze, ARM, ARM  64,
       PA-RISC,  arc,  Power  PC, xtensa, and MIPS architectures, the order of
       the fourth and fifth arguments is  reversed.   On  the  cris  and  s390
       architectures, the order of the first and second arguments is reversed.

   blackfin, m68k, and sparc
       The  argument-passing  conventions  on  blackfin,  m68k,  and sparc are
       different from descriptions above.  For details, see  the  kernel  (and
       glibc) source.

   ia64
       On ia64, a different interface is used:

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

       The  prototype  shown  above is for the glibc wrapper function; the raw
       system call interface has no fn or arg argument, and changes the  order
       of  the  arguments  so that flags is the first argument, and tls is the
       last argument.

       __clone2()  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.

   Linux 2.4 and earlier
       In  Linux  2.4  and earlier, clone() does not take arguments ptid, tls,
       and ctid.

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 an unprivileged 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

       clone()  is  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.

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(2)  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(2) 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);

EXAMPLE

       The following program demonstrates the use of clone() to create a child
       process that executes in a separate UTS namespace.  The  child  changes
       the  hostname in its UTS namespace.  Both parent and child then display
       the system hostname, making  it  possible  to  see  that  the  hostname
       differs  in the UTS namespaces of the parent and child.  For an example
       of the use of this program, see setns(2).

   Program source
       #define _GNU_SOURCE
       #include <sys/wait.h>
       #include <sys/utsname.h>
       #include <sched.h>
       #include <string.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>

       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                               } while (0)

       static int              /* Start function for cloned child */
       childFunc(void *arg)
       {
           struct utsname uts;

           /* Change hostname in UTS namespace of child */

           if (sethostname(arg, strlen(arg)) == -1)
               errExit("sethostname");

           /* Retrieve and display hostname */

           if (uname(&uts) == -1)
               errExit("uname");
           printf("uts.nodename in child:  %s
", uts.nodename);

           /* Keep the namespace open for a while, by sleeping.
              This allows some experimentation--for example, another
              process might join the namespace. */

           sleep(200);

           return 0;           /* Child terminates now */
       }

       #define STACK_SIZE (1024 * 1024)    /* Stack size for cloned child */

       int
       main(int argc, char *argv[])
       {
           char *stack;                    /* Start of stack buffer */
           char *stackTop;                 /* End of stack buffer */
           pid_t pid;
           struct utsname uts;

           if (argc < 2) {
               fprintf(stderr, "Usage: %s <child-hostname>
", argv[0]);
               exit(EXIT_SUCCESS);
           }

           /* Allocate stack for child */

           stack = malloc(STACK_SIZE);
           if (stack == NULL)
               errExit("malloc");
           stackTop = stack + STACK_SIZE;  /* Assume stack grows downward */

           /* Create child that has its own UTS namespace;
              child commences execution in childFunc() */

           pid = clone(childFunc, stackTop, CLONE_NEWUTS | SIGCHLD, argv[1]);
           if (pid == -1)
               errExit("clone");
           printf("clone() returned %ld
", (long) pid);

           /* Parent falls through to here */

           sleep(1);           /* Give child time to change its hostname */

           /* Display hostname in parent's UTS namespace. This will be
              different from hostname in child's UTS namespace. */

           if (uname(&uts) == -1)
               errExit("uname");
           printf("uts.nodename in parent: %s
", uts.nodename);

           if (waitpid(pid, NULL, 0) == -1)    /* Wait for child */
               errExit("waitpid");
           printf("child has terminated
");

           exit(EXIT_SUCCESS);
       }

SEE ALSO

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

COLOPHON

       This page is part of release 3.65 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/.



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