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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 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 cor‐
       responds 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 termi‐
       nate 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  proces‐
       sors), 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 descrip‐
              tors 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 direc‐
              tory, 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 infor‐
              mation 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 con‐
              text, 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 mecha‐
              nisms 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 names‐
              paces.

              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  implemen‐
              tation 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 physi‐
              cal 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  physi‐
              cal 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,  ini‐
              tialized  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  per‐
              mitted 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  names‐
              paces 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_names‐
              paces(7)

              Before Linux 3.8, use of CLONE_NEWUSER required that the caller have three capabil‐
              ities:  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 iden‐
              tifiers 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  sethost‐
              name(2), respectively.  Changes made to the identifiers in a UTS namespace are vis‐
              ible 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 getp‐
              pid(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  call‐
              ing 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  sigac‐
              tion(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  han‐
              dlers  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 spec‐
              ified

       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 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 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 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 speci‐
              fied  (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 deliv‐
              ered 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  sig‐
              nal.   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  schedu‐
              lable  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  mem‐
              ory 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 map‐
              pings/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 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 exe‐
       cution.  On failure, -1 is returned in the caller's context, no child process will be cre‐
       ated, 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 speci‐
              fied 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 man‐
       ual 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 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 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 follow‐
       ing:

           #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 exe‐
       cutes 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 3.74 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                                       2014-09-21                                   CLONE(2)


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