clone(2)



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

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 fea-
   ture_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  func-
       tion 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 begin-
       ning of its execution.  The arg argument is passed to the fn function.

       When the fn(arg) function application returns, the child process termi-
       nates.   The  integer  returned  by  fn  is the exit code for the child
       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  mem-
       ory,  it  is  not possible for the child process to execute in the same
       stack as the calling process.  The calling process must  therefore  set
       up memory space for the child stack and pass a pointer to this space to
       clone().  Stacks grow 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 con-
       stants, 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 cre-
              ated 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 corre-
              sponding file descriptors in the calling  process.)   Subsequent
              operations  that  open or close file descriptors, or change file
              descriptor flags, performed by either the calling process or the
              child process do not affect the other process.

       CLONE_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  call-
              ing process or the child process also affects the other process.

              If CLONE_FS is not set, the child process works on a copy of the
              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  con-
              tainers.

              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  visi-
              ble 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 CON-
              FIG_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  con-
              tainers.

              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 net-
              work  device  can live in exactly one network namespace.  A vir-
              tual network device ("veth") pair provides a pipe-like  abstrac-
              tion  that  can be used to create tunnels between network names-
              paces, 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  CON-
              FIG_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 capabil-
              ity) may specify the CLONE_NEWNS flag.  It is not  permitted  to
              specify both CLONE_NEWNS and CLONE_FS in the same clone() call.

       CLONE_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  con-
              tainers.

              A  PID namespace provides an isolated environment for PIDs: PIDs
              in a new namespace start at 1, somewhat like a  standalone  sys-
              tem,  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;  analo-
              gously,  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  names-
              pace  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 associ-
              ated 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 CON-
              FIG_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 pro-
              cesses in other UTS namespaces.

              Use of this flag requires: a kernel  configured  with  the  CON-
              FIG_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  mem-
              ory.  (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 behav-
              ior  associated  with  a  signal, the behavior is changed in the
              other process as well.  However, the calling process  and  child
              processes  still  have distinct signal masks and sets of pending
              signals.  So, one of them may  block  or  unblock  some  signals
              using sigprocmask(2) without affecting the other process.

              If  CLONE_SIGHAND  is not set, the child process inherits a copy
              of the signal handlers  of  the  calling  process  at  the  time
              clone() is called.  Calls to sigaction(2) performed later by one
              of the processes have no effect on the other process.

              Since Linux 2.6.0-test6, flags must  also  include  CLONE_VM  if
              CLONE_SIGHAND is specified

       CLONE_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  sepa-
              rate 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 dis-
              cussion of CLONE_THREAD more readable, the term "thread" is used
              to refer to the processes within a thread group.

              Thread groups were a feature added in Linux 2.4 to  support  the
              POSIX  threads  notion  of  a set of threads that share a single
              PID.  Internally, this shared PID is the so-called thread  group
              identifier  (TGID) for the thread group.  Since Linux 2.4, calls
              to getpid(2) return the TGID of the caller.

              The threads within a group can be distinguished by  their  (sys-
              tem-wide) unique thread IDs (TID).  A new thread's TID is avail-
              able as the function result returned to the caller  of  clone(),
              and a thread can obtain its own TID using gettid(2).

              When  a call is made to clone() without specifying CLONE_THREAD,
              then the resulting thread is placed in a new thread group  whose
              TGID is the same as the thread's TID.  This thread is the leader
              of the new thread group.

              A new thread created  with  CLONE_THREAD  has  the  same  parent
              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 termi-
              nates, the thread that created it using clone() is  not  sent  a
              SIGCHLD  (or  other  termination)  signal; nor can the status of
              such a thread be obtained using wait(2).  (The thread is said to
              be detached.)

              After  all of the threads in a thread group terminate the parent
              process of the thread group is sent a SIGCHLD (or other termina-
              tion) signal.

              If  any  of the threads in a thread group performs an execve(2),
              then all threads other than the thread group leader  are  termi-
              nated,  and  the  new  program  is  executed in the thread group
              leader.

              If one of the threads in a thread group creates  a  child  using
              fork(2),  then  any  thread  in  the  group can wait(2) for that
              child.

              Since Linux 2.5.35, flags must  also  include  CLONE_SIGHAND  if
              CLONE_THREAD   is   specified   (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 unhan-
              dled signal is delivered to a thread, then it will affect  (ter-
              minate, stop, continue, be ignored in) all members of the thread
              group.

              Each thread has its own signal mask, as set  by  sigprocmask(2),
              but  signals can be pending either: for the whole process (i.e.,
              deliverable to any member of the thread group), when  sent  with
              kill(2);  or for an individual thread, when sent with tgkill(2).
              A call to sigpending(2) returns a signal set that is  the  union
              of  the  signals  pending  for the whole process and the signals
              that are pending for the calling thread.

              If kill(2) is used to send a signal to a thread group,  and  the
              thread  group  has  installed a handler for the signal, then the
              handler will be invoked in  exactly  one,  arbitrarily  selected
              member  of the thread group that has not blocked the signal.  If
              multiple threads in a group are waiting to accept the same  sig-
              nal using sigwaitinfo(2), the kernel will arbitrarily select one
              of these threads to receive a signal sent using kill(2).

       CLONE_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 per-
              formed  by  the calling process or by the child process are also
              visible in the other process.  Moreover, any memory  mapping  or
              unmapping  performed  with  mmap(2) or munmap(2) by the child or
              calling process also affects the other process.

              If CLONE_VM is not set, the child process  runs  in  a  separate
              copy  of  the memory space of the calling process at the time of
              clone().  Memory writes or file mappings/unmappings performed by
              one of the processes do not affect the other, as with fork(2).

   C library/kernel ABI differences
       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 modi-
       fies 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 dif-
       ferent from the 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 call-
       er'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 appro-
       priately.

ERRORS
       EAGAIN Too many processes are already running; see fork(2).

       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 con-
              figured with the CONFIG_SYSVIPC and CONFIG_IPC_NS options.

       EINVAL CLONE_NEWNET was specified in flags, but the kernel was not con-
              figured with the CONFIG_NET_NS option.

       EINVAL CLONE_NEWPID was specified in flags, but the kernel was not con-
              figured with the CONFIG_PID_NS option.

       EINVAL CLONE_NEWUTS was specified in flags, but the kernel was not con-
              figured 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.

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 circum-
       stances.  In particular, if a signal is delivered to the child  immedi-
       ately 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  cre-
       ated 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  dif-
       fers  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\n", 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>\n", 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\n", (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\n", uts.nodename);

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

           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), capabili-
       ties(7), pthreads(7)

COLOPHON
       This page is part of release 3.71 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at
       http://www.kernel.org/doc/man-pages/.

Linux                             2014-08-19                          CLONE(2)

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