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



NAME
       clone, __clone2 - create a child process

SYNOPSIS
       #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 */ );

DESCRIPTION
       clone()  creates  a  new process, in a manner similar to fork(2).  It is actually a library function layered on
       top of the underlying clone() system call, hereinafter referred to as sys_clone.  A description of sys_clone is
       given towards the end of this page.

       Unlike  fork(2),  these  calls allow the child process to share parts of its execution context with the calling
       process, such as the memory space, the table of file descriptors, and the table of signal handlers.  (Note that
       on  this  manual  page, "calling process" normally corresponds to "parent process".  But see the description of
       CLONE_PARENT below.)

       The main use of clone() is to implement threads: multiple threads of control in a program that run concurrently
       in a shared memory space.

       When  the  child  process is created with clone(), it executes the function application fn(arg).  (This differs
       from fork(2), where execution continues in the child from the point of the fork(2) call.)  The fn argument is a
       pointer  to a function that is called by the child process at the beginning of its execution.  The arg argument
       is passed to the fn function.

       When the fn(arg) function application returns, the child process terminates.  The integer returned by fn is the
       exit  code  for the child process.  The child process may also terminate explicitly by calling exit(2) or after
       receiving a fatal signal.

       The child_stack argument specifies the location of the stack used by the child process.  Since  the  child  and
       calling  process may share memory, it is not possible for the child process to execute in the same stack as the
       calling process.  The calling process must therefore set up memory space for the child stack and pass a pointer
       to  this  space  to  clone().  Stacks grow downwards 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
              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 descriptors opened in the
              calling process at the time of clone().  (The duplicated file descriptors in the child refer to the same
              open  file  descriptions  (see  open(2))  as the corresponding file descriptors in the calling process.)
              Subsequent operations that open or close file descriptors, or change file descriptor flags, performed by
              either the calling process or the child process do not affect the other process.

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

              If CLONE_FS is not set, the child process works on a copy of the file system 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 pro-
              cesses 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 consists of the set of identifiers for System V IPC objects.  (These objects  are  cre-
              ated  using  msgctl(2),  semctl(2),  and shmctl(2)).  Objects created in an IPC namespace are visible to
              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 termi-
              nates), all IPC objects in the namespace are automatically destroyed.

              Use of this flag requires: a kernel configured with the CONFIG_SYSVIPC  and  CONFIG_IPC_NS  options  and
              that  the  process  be  privileged  (CAP_SYS_ADMIN).   This  flag can't be specified in conjunction with
              CLONE_SYSVSEM.

       CLONE_NEWNET (since Linux 2.6.24)
              (The implementation of this flag is not yet complete, but probably will  be  mostly  complete  by  about
              Linux 2.6.28.)

              If  CLONE_NEWNET  is  set, then create the process in a new network namespace.  If this flag is not set,
              then (as with fork(2)), the process is created in the same network namespace  as  the  calling  process.
              This flag is intended for the implementation of containers.

              A  network  namespace provides an isolated view of the networking stack (network device interfaces, IPv4
              and IPv6 protocol stacks, IP routing tables, firewall rules, the /proc/net and /sys/class/net  directory
              trees,  sockets, etc.).  A physical network device can live in exactly one network namespace.  A virtual
              network device ("veth") pair provides a pipe-like abstraction that can be used to create tunnels between
              network  namespaces,  and  can be used to create a bridge to a physical network device in another names-
              pace.

              When a network namespace is freed (i.e., when the last process in the namespace terminates), its  physi-
              cal  network devices are moved back to the initial network namespace (not to the parent of the process).

              Use of this flag requires: a kernel configured with the CONFIG_NET_NS option and  that  the  process  be
              privileged (CAP_SYS_ADMIN).

       CLONE_NEWNS (since Linux 2.4.19)
              Start the child in a new mount namespace.

              Every  process  lives  in a mount namespace.  The namespace of a process is the data (the set of mounts)
              describing the file hierarchy as seen by that process.  After a fork(2) or clone() where the CLONE_NEWNS
              flag  is  not set, the child lives in the same mount namespace as the parent.  The system calls mount(2)
              and umount(2) change the mount namespace of the calling process, and hence  affect  all  processes  that
              live in the same namespace, but do not affect processes in a different mount namespace.

              After a clone() where the CLONE_NEWNS flag is set, the cloned child is started in a new mount namespace,
              initialized with a copy of the namespace of the parent.

              Only a privileged process (one having the CAP_SYS_ADMIN capability) may specify  the  CLONE_NEWNS  flag.
              It is not permitted to specify both CLONE_NEWNS and CLONE_FS in the same clone() call.

       CLONE_NEWPID (since Linux 2.6.24)
              If  CLONE_NEWPID  is set, then create the process in a new PID namespace.  If this flag is not set, then
              (as with fork(2)), the process is created in the same PID namespace as the calling process.   This  flag
              is intended for the implementation of containers.

              A  PID namespace provides an isolated environment for PIDs: PIDs in a new namespace start at 1, somewhat
              like a standalone system, and calls to fork(2), vfork(2), or clone(2) will produce processes  with  PIDs
              that are unique within the namespace.

              The first process created in a new namespace (i.e., the process created using the CLONE_NEWPID flag) has
              the PID 1, and is the "init" process for the namespace.  Children that are orphaned within the namespace
              will be reparented to this process rather than init(8).  Unlike the traditional init process, the "init"
              process of a PID namespace can terminate, and if it does, all of the processes in the namespace are ter-
              minated.

              PID  namespaces  form a hierarchy.  When a PID new namespace is created, the processes in that namespace
              are visible in the PID namespace of the process that created the new namespace; analogously, if the par-
              ent  PID  namespace is itself the child of another PID namespace, then processes in the child and parent
              PID namespaces will both be visible in the grandparent PID namespace.  Conversely, the processes in  the
              "child"  PID  namespace  do not see the processes in the parent namespace.  The existence of a namespace
              hierarchy means that each process may now have multiple PIDs: one for each namespace in which it is vis-
              ible;  each  of  these  PIDs  is unique within the corresponding namespace.  (A call to getpid(2) always
              returns the PID associated with the namespace in which the process lives.)

              After creating the new namespace, it is useful for the child to change its root directory  and  mount  a
              new  procfs  instance  at  /proc  so  that  tools such as ps(1) work correctly.  (If CLONE_NEWNS is also
              included in flags, then it isn't necessary to change the root directory: a new procfs  instance  can  be
              mounted directly over /proc.)

              Use  of  this  flag  requires: a kernel configured with the CONFIG_PID_NS option and that the process be
              privileged (CAP_SYS_ADMIN).  This flag can't be specified in conjunction with CLONE_THREAD.

       CLONE_NEWUTS (since Linux 2.6.19)
              If CLONE_NEWUTS is set, then create the process in a new UTS namespace, whose identifiers  are  initial-
              ized  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
              host name can be modified by setdomainname(2) and  sethostname(2), respectively.  Changes made to  these
              identifiers in one UTS namespace are visible to other processes in the same namespace, but are not visi-
              ble to processes in other UTS namespaces.

              Use of this flag requires: a kernel configured with the CONFIG_UTS_NS option and  that  the  process  be
              privileged (CAP_SYS_ADMIN).

       CLONE_PARENT (since Linux 2.3.12)
              If CLONE_PARENT is set, then the parent of the new child (as returned by getppid(2)) will be the same as
              that of the calling process.

              If CLONE_PARENT is not set, then (as with fork(2)) the child's parent is the calling process.

              Note that it is the parent process, as returned by getppid(2), which is signaled when the  child  termi-
              nates,  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
              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
              If CLONE_SIGHAND is set, the calling process and the child process share the same table of  signal  han-
              dlers.   If  the  calling  process or child process calls sigaction(2) to change the behavior associated
              with a signal, the behavior is changed in the other process as well.  However, the calling  process  and
              child processes still have distinct signal masks and sets of pending signals.  So, one of them may block
              or unblock some signals using sigprocmask(2) without affecting the other process.

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

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

       CLONE_STOPPED (since Linux 2.6.0-test2)
              If CLONE_STOPPED is set, then the child is initially stopped (as though it was sent a  SIGSTOP  signal),
              and must be resumed by sending it a SIGCONT signal.

              From Linux 2.6.25 this flag is deprecated.  You probably never wanted to use it, you certainly shouldn't
              be using it, and soon it will go away.

       CLONE_SYSVSEM (since Linux 2.5.10)
              If CLONE_SYSVSEM is set, then the child and the  calling  process  share  a  single  list  of  System  V
              semaphore undo values (see semop(2)).  If this flag is not set, then the child has a separate undo list,
              which is initially empty.

       CLONE_THREAD (since Linux 2.4.0-test8)
              If CLONE_THREAD is set, the child is placed in the same thread group as the calling  process.   To  make
              the remainder of the discussion of CLONE_THREAD more readable, the term "thread" is used to refer to the
              processes within a thread group.

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

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

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

              A new thread created with CLONE_THREAD has the same parent process as the caller of clone() (i.e.,  like
              CLONE_PARENT),  so  that  calls  to  getppid(2) return the same value for all of the threads in a thread
              group.  When a CLONE_THREAD thread terminates, the thread that created it using clone() is  not  sent  a
              SIGCHLD  (or  other  termination) signal; nor can the status of such a thread be obtained using wait(2).
              (The thread is said to be detached.)

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

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

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

              Since Linux 2.5.35, flags must also include CLONE_SIGHAND if CLONE_THREAD is specified.

              Signals  may  be sent to a thread group as a whole (i.e., a TGID) using kill(2), or to a specific thread
              (i.e., TID) using tgkill(2).

              Signal dispositions and actions are process-wide: if an unhandled signal is delivered to a thread,  then
              it will affect (terminate, stop, continue, be ignored in) all members of the thread group.

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

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

       CLONE_UNTRACED (since Linux 2.5.46)
              If  CLONE_UNTRACED is specified, then a tracing process cannot force CLONE_PTRACE on this child process.

       CLONE_VFORK
              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
              If CLONE_VM is set, the calling process and the child process run in the same memory space.  In particu-
              lar,  memory  writes  performed  by  the calling process or by the child process are also visible in the
              other process.  Moreover, any memory mapping or unmapping performed with mmap(2)  or  munmap(2)  by  the
              child or calling process also affects the other process.

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

   sys_clone
       The sys_clone system call corresponds more closely to fork(2) in that execution in the child continues from the
       point of the call.  Thus, sys_clone only requires the flags and child_stack  arguments,  which  have  the  same
       meaning as for clone().  (Note that the order of these arguments differs from clone().)

       Another  difference  for  sys_clone  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.

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

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

ERRORS
       EAGAIN Too many processes are already running.

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

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

       EINVAL Both CLONE_FS and CLONE_NEWNS were specified in flags.

       EINVAL Both CLONE_NEWIPC and CLONE_SYSVSEM were specified in flags.

       EINVAL Both CLONE_NEWPID and CLONE_THREAD were specified in flags.

       EINVAL Returned by clone() when a zero value is specified for child_stack.

       EINVAL CLONE_NEWIPC was specified in flags, but the kernel was not configured with the CONFIG_SYSVIPC and  CON-
              FIG_IPC_NS options.

       EINVAL CLONE_NEWNET was specified in flags, but the kernel was not configured with the CONFIG_NET_NS option.

       EINVAL CLONE_NEWPID was specified in flags, but the kernel was not configured with the CONFIG_PID_NS option.

       EINVAL CLONE_NEWUTS was specified in flags, but the kernel was not configured with the CONFIG_UTS option.

       ENOMEM Cannot  allocate sufficient memory to allocate a task structure for the child, or to copy those parts of
              the caller's context that need to be copied.

       EPERM  CLONE_NEWIPC, CLONE_NEWNET, CLONE_NEWNS, CLONE_NEWPID, or CLONE_NEWUTS was specified by a non-root  pro-
              cess (process without CAP_SYS_ADMIN).

       EPERM  CLONE_PID was specified by a process other than process 0.

VERSIONS
       There is no entry for clone() in libc5.  glibc2 provides clone() as described in this manual page.

CONFORMING TO
       The  clone() and sys_clone calls are 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.

       On ia64, a different system call 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 __clone2() system call operates in the same way as clone(), except that child_stack_base points to the low-
       est address of the child's stack  area,  and  stack_size  specifies  the  size  of  the  stack  pointed  to  by
       child_stack_base.

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  cur-
       rently  implemented,  the  cache  may  not  be up to date in some circumstances.  In particular, if a signal is
       delivered to the child immediately after the clone() call, then a call to getpid() 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() 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);

SEE ALSO
       fork(2), futex(2), getpid(2), gettid(2), set_thread_area(2), set_tid_address(2), tkill(2), unshare(2), wait(2),
       capabilities(7), pthreads(7)

COLOPHON
       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2009-07-18                          CLONE(2)