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, 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);

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)
              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  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().  (The duplicated file descriptors in the child refer
              to  the  same open file descriptions (see open(2)) as the corre-
              sponding file descriptors in the calling  process.)   Subsequent
              operations  that  open or close file descriptors, or change file
              descriptor flags, performed by either the calling process or the
              child process do not affect the other process.

       CLONE_FS (since Linux 2.0)
              If  CLONE_FS  is set, the caller and the child process share the
              same filesystem information.  This  includes  the  root  of  the
              filesystem,  the  current working directory, and the umask.  Any
              call to chroot(2), chdir(2), or umask(2) performed by the  call-
              ing process or the child process also affects the other process.

              If CLONE_FS is not set, the child process works on a copy of the
              filesystem information of the calling process at the time of the
              clone()  call.  Calls to chroot(2), chdir(2), umask(2) performed
              later by one of the processes do not affect the other process.

       CLONE_IO (since Linux 2.6.25)
              If CLONE_IO is set, then the new process shares an  I/O  context
              with  the  calling  process.   If this flag is not set, then (as
              with fork(2)) the new process has its own I/O context.

              The I/O context is the I/O scope of  the  disk  scheduler  (i.e,
              what  the  I/O scheduler uses to model scheduling of a process's
              I/O).  If processes share the same I/O context, they are treated
              as  one  by  the  I/O  scheduler.  As a consequence, they get to
              share disk time.  For some  I/O  schedulers,  if  two  processes
              share  an  I/O context, they will be allowed to interleave their
              disk access.  If several threads are doing I/O on behalf of  the
              same  process  (aio_read(3),  for  instance), they should employ
              CLONE_IO to get better I/O performance.

              If the kernel is not configured with  the  CONFIG_BLOCK  option,
              this flag is a no-op.

       CLONE_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.

              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  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.)

       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 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.  The raw system call interface
       on x86 and many other architectures is roughly:

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

       Another difference for the raw system  call  is  that  the  child_stack
       argument may be zero, in which case copy-on-write semantics ensure that
       the child gets separate copies of stack pages when either process modi-
       fies  the  stack.   In  this  case, for correct operation, the CLONE_VM
       option should not be specified.

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

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

   ia64
       On ia64, a different interface is used:

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

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

       __clone2()   operates   in   the  same  way  as  clone(),  except  that
       child_stack_base points to the lowest  address  of  the  child's  stack
       area,  and  stack_size  specifies  the  size of the stack pointed to by
       child_stack_base.

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

RETURN VALUE
       On success, the thread ID of the child process is returned in the call-
       er's thread of execution.  On failure, -1 is returned in  the  caller's
       context, no child process will be created, and errno will be set appro-
       priately.

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

       EINVAL CLONE_SIGHAND was specified, but CLONE_VM was not.  (Since Linux
              2.6.0-test6.)

       EINVAL CLONE_THREAD  was  specified, but CLONE_SIGHAND was not.  (Since
              Linux 2.5.35.)

       EINVAL Both CLONE_FS and CLONE_NEWNS were specified in flags.

       EINVAL (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 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.

       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).

       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.)

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  circum-
       stances.   In particular, if a signal is delivered to the child immedi-
       ately after the clone() call, then a call to getpid(2) in a handler for
       the signal may return the PID of the calling process ("the parent"), if
       the clone wrapper has not yet had a chance to update the PID  cache  in
       the  child.  (This discussion ignores the case where the child was cre-
       ated using CLONE_THREAD, when getpid(2) should return the same value in
       the  child and in the process that called clone(), since the caller and
       the child are in the same thread group.  The stale-cache  problem  also
       does  not  occur  if the flags argument includes CLONE_VM.)  To get the
       truth, it may be necessary to use code such as the following:

           #include <syscall.h>

           pid_t mypid;

           mypid = syscall(SYS_getpid);

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

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

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

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

           /* Change hostname in UTS namespace of child */

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

           /* Retrieve and display hostname */

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

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

           sleep(200);

           return 0;           /* Child terminates now */
       }

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

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

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

           /* Allocate stack for child */

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

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

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

           /* Parent falls through to here */

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

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

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

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

           exit(EXIT_SUCCESS);
       }

SEE ALSO
       fork(2),  futex(2),  getpid(2), gettid(2), kcmp(2), set_thread_area(2),
       set_tid_address(2), setns(2), tkill(2), unshare(2), wait(2),  capabili-
       ties(7), namespaces(7), pthreads(7)

COLOPHON
       This  page  is  part of release 4.06 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                             2016-03-15                          CLONE(2)

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