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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.)

       One 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  the  child  thread  ID  at  the 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 the child thread ID at the location ctid in the child's 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  a  process
              sharing  a  file  descriptor  table  calls  execve(2), its file descriptor table is
              duplicated (unshared).

              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.

              Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWIPC.  This flag can't
              be specified in conjunction with CLONE_SYSVSEM.

              For further information on IPC namespaces, see namespaces(7).

       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).  For further information on network
              namespaces, see namespaces(7).

              Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWNET.

       CLONE_NEWNS (since Linux 2.4.19)
              If CLONE_NEWNS is set, the cloned child  is  started  in  a  new  mount  namespace,
              initialized with a copy of the namespace of the parent.  If CLONE_NEWNS is not set,
              the child lives in the same mount namespace as the parent.

              For further information on mount namespaces, see namespaces(7).

              Only a privileged process  (CAP_SYS_ADMIN)  can  employ  CLONE_NEWNS.   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.

              For further information on PID namespaces, see namespaces(7) and pid_namespaces(7)

              Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWPID.  This flag can't
              be specified in conjunction with CLONE_THREAD or CLONE_PARENT.

       CLONE_NEWUSER
              (This flag first became meaningful for clone() in Linux 2.6.23, the current clone()
              semantics were merged in  Linux  3.5,  and  the  final  pieces  to  make  the  user
              namespaces completely usable were merged in Linux 3.8.)

              If  CLONE_NEWUSER is set, then create the process in a new user namespace.  If this
              flag is not set, then (as with fork(2)) the process is created  in  the  same  user
              namespace as the calling process.

              For    further    information   on   user   namespaces,   see   namespaces(7)   and
              user_namespaces(7)

              Before Linux 3.8,  use  of  CLONE_NEWUSER  required  that  the  caller  have  three
              capabilities:  CAP_SYS_ADMIN, CAP_SETUID, and CAP_SETGID.  Starting with Linux 3.8,
              no privileges are needed to create a user namespace.

              This flag can't be specified in conjunction with CLONE_THREAD or CLONE_PARENT.  For
              security reasons, CLONE_NEWUSER cannot be specified in conjunction with CLONE_FS.

              For further information on user namespaces, see user_namespaces(7).

       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.

              Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWUTS.

              For further information on UTS namespaces, see namespaces(7).

       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  the  child thread ID at the location ptid in the parent's 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.  Since then, the kernel silently ignores it
              without error.

       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.  Since then, the kernel silently ignores it without error.

       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 adjustment (semadj) values (see semop(2)).  In this case, the
              shared list accumulates semadj values across all processes sharing  the  list,  and
              semaphore  adjustments are performed only when the last process that is sharing the
              list terminates (or ceases sharing the list using unshare(2)).  If this flag is not
              set, then the child has a separate semadj list that 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).

   C library/kernel 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 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 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  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; 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 (since Linux 3.9)
              Both CLONE_NEWUSER and CLONE_FS were specified in flags.

       EINVAL Both CLONE_NEWIPC and CLONE_SYSVSEM were specified in flags.

       EINVAL One (or both) of CLONE_NEWPID or CLONE_NEWUSER and one (or both) of CLONE_THREAD or
              CLONE_PARENT 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.

       EPERM  CLONE_NEWUSER  was  specified  in  flags,  but  either the effective user ID or the
              effective group ID of the caller does not have a mapping in  the  parent  namespace
              (see user_namespaces(7)).

       EPERM (since Linux 3.9)
              CLONE_NEWUSER  was  specified  in  flags  and the caller is in a chroot environment
              (i.e., the caller's root directory does not match the root directory of  the  mount
              namespace in which it resides).

       EUSERS (since Linux 3.11)
              CLONE_NEWUSER  was  specified  in  flags, and the call would cause the limit on the
              number of nested user namespaces to be exceeded.  See user_namespaces(7).

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\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), capabilities(7), namespaces(7), pthreads(7)

COLOPHON

       This  page  is  part of release 4.04 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/.