clone(2)



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

NAME
       clone, __clone2 - create a child process

SYNOPSIS
       /* Prototype for the glibc wrapper function */

       #define _GNU_SOURCE
       #include <sched.h>

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

       /* For the prototype of the raw system call, see NOTES */

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 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)
              Clear (zero) 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.   The  store  operation completes before clone() returns
              control to user space.

       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 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().   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.  Note,  however,  that  the  duplicated  file
              descriptors  in  the  child refer to the same open file descrip-
              tions as the  corresponding  file  descriptors  in  the  calling
              process,  and thus share file offsets and file status flags (see
              open(2)).

       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_NEWCGROUP (since Linux 4.6)
              Create the process in a new cgroup namespace.  If this  flag  is
              not  set,  then  (as with fork(2)) the process is created in the
              same cgroup namespaces as the calling  process.   This  flag  is
              intended for the implementation of containers.

              For  further information on cgroup namespaces, see cgroup_names-
              paces(7).

              Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWC-
              GROUP.

       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.

              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 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).  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.

              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.

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

       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.

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

              Only a privileged process (CAP_SYS_ADMIN) can employ  CLONE_NEW-
              PID.    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_SET-
              GID.   Starting with Linux 3.8, no privileges are needed to cre-
              ate 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_names-
              paces(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 cre-
              ated  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.

              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.)  The store operation  completes  before  clone()
              returns control to user space.

       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 TLS (Thread Local Storage) descriptor is set to newtls.

              The  interpretation of newtls and the resulting effect is archi-
              tecture dependent.  On x86, newtls is interpreted  as  a  struct
              user_desc  *  (See set_thread_area(2)).  On x86_64 it is the new
              value to be set for the %fs base register (See  the  ARCH_SET_FS
              argument  to  arch_prctl(2)).  On architectures with a dedicated
              TLS register, it is the new value of that register.

       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.   Since  then,  the  kernel
              silently ignores it without error.  Starting with Linux 4.6, the
              same bit was reused for the CLONE_NEWCGROUP flag.

       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  accumu-
              lates  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 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 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.  In addition, there are varia-
       tions across architectures.

       The  raw  system  call interface on x86-64 and some other architectures
       (including sh, tile, and alpha) is roughly:

           long clone(unsigned long flags, void *child_stack,
                      int *ptid, int *ctid,
                      unsigned long newtls);

       On x86-32, and several other  common  architectures  (including  score,
       ARM,  ARM  64,  PA-RISC, arc, Power PC, xtensa, and MIPS), the order of
       the last two arguments is reversed:

           long clone(unsigned long flags, void *child_stack,
                     int *ptid, unsigned long newtls,
                     int *ctid);

       On the cris and s390 architectures, the order of the  first  two  argu-
       ments is reversed:

           long clone(void *child_stack, unsigned long flags,
                      int *ptid, int *ctid,
                      unsigned long newtls);

       On the microblaze architecture, an additional argument is supplied:

           long clone(unsigned long flags, void *child_stack,
                      int stack_size,         /* Size of stack */
                      int *ptid, int *ctid,
                      unsigned long newtls);

       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.

   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 (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 the  glibc  clone()  wrapper  function  when  fn  or
              child_stack is specified as NULL.

       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.

       EINVAL child_stack  is  not  aligned  to  a  suitable boundary for this
              architecture.  For example, on aarch64, child_stack  must  be  a
              multiple of 16.

       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.

       ENOSPC (since Linux 3.7)
              CLONE_NEWPID  was specified in flags, but the limit on the nest-
              ing depth of  PID  namespaces  would  have  been  exceeded;  see
              pid_namespaces(7).

       ENOSPC (since Linux 4.9; beforehand EUSERS)
              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).

              From  Linux  3.11 to Linux 4.8, the error diagnosed in this case
              was EUSERS.

       ENOSPC (since Linux 4.9)
              One of the values in flags specified the creation of a new  user
              namespace,  but  doing so would have caused the limit defined by
              the corresponding file in /proc/sys/user to  be  exceeded.   For
              further details, see namespaces(7).

       EPERM  CLONE_NEWCGROUP,    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).

       ERESTARTNOINTR (since Linux 2.6.17)
              System call was interrupted by a signal and will  be  restarted.
              (This can be seen only during a trace.)

       EUSERS (Linux 3.11 to Linux 4.8)
              CLONE_NEWUSER  was specified in flags, and the limit on the num-
              ber of nested user namespaces would be exceeded.  See  the  dis-
              cussion of the ENOSPC error above.

CONFORMING TO
       clone()  is  Linux-specific and should not be used in programs intended
       to be portable.

NOTES
       The kcmp(2) system call can be used to test whether two processes share
       various  resources  such as a file descriptor table, System V semaphore
       undo operations, or a virtual address space.

       Handlers registered using pthread_atfork(3) are not executed  during  a
       call to clone().

       In  the  Linux  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 Linux 2.6.0 and later).

       For a while there was CLONE_DETACHED  (introduced  in  2.5.32):  parent
       wants no child-exit signal.  In Linux 2.6.2, the need to give this flag
       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
       GNU C library versions 2.3.4 up to and including 2.24 contained a wrap-
       per  function  for  getpid(2)  that  performed  caching  of PIDs.  This
       caching relied on support in the glibc wrapper for clone(), but limita-
       tions  in the implementation meant that the cache was not up to date in
       some circumstances.  In particular, if a signal was  delivered  to  the
       child immediately after the clone() call, then a call to getpid(2) in a
       handler for the signal could return the  PID  of  the  calling  process
       ("the parent"), if the clone wrapper had 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 was sometimes necessary to use code
       such as the following:

           #include <syscall.h>

           pid_t mypid;

           mypid = syscall(SYS_getpid);

       Because of the stale-cache problem, as well as other problems noted  in
       getpid(2), the PID caching feature was removed in glibc 2.25.

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), namespaces(7), pthreads(7)

COLOPHON
       This page is part of release 4.13 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
       https://www.kernel.org/doc/man-pages/.

Linux                             2017-09-15                          CLONE(2)

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