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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 func-
       tion 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 termi-
       nates.   The integer returned by fn is the exit code for the child pro-
       cess.  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  downwards  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  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 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 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
              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  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
              file  system  information  of the calling process at the time of
              the clone() call.  Calls to chroot(2), chdir(2),  umask(2)  per-
              formed  later  by  one  of the processes do not affect the other
              process.

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

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

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

       CLONE_NEWIPC (since Linux 2.6.19)
              If  CLONE_NEWIPC  is  set,  then create the process in a new IPC
              namespace.  If this flag is not set, then (as with fork(2)), the
              process is created in the same IPC namespace as the calling pro-
              cess.  This flag is intended for the implementation of  contain-
              ers.

              An IPC namespace consists of the set of identifiers for System V
              IPC objects.  (These objects are created using  msgctl(2),  sem-
              ctl(2), and shmctl(2)).  Objects created in an IPC namespace are
              visible to all other processes that are members of  that  names-
              pace,  but are not visible to processes in other IPC namespaces.

              When an IPC namespace is destroyed (i.e, when the  last  process
              that  is  a member of the namespace terminates), all IPC objects
              in the namespace are automatically destroyed.

              Use of this flag requires: a kernel  configured  with  the  CON-
              FIG_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 proba-
              bly 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 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).

              Use  of  this  flag  requires: a kernel configured with the CON-
              FIG_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 capabil-
              ity)  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 pro-
              cess.  This flag is intended for the implementation of  contain-
              ers.

              A  PID namespace provides an isolated environment for PIDs: PIDs
              in a new namespace start at 1, somewhat like a  standalone  sys-
              tem,  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 terminated.

              PID namespaces form a hierarchy.  When a new  PID  namespace  is
              created,  the processes in that namespace are visible in the PID
              namespace of the process that created the new namespace;  analo-
              gously,  if  the  parent  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  names-
              pace  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
              visible; each of these PIDs is unique within  the  corresponding
              namespace.   (A call to getpid(2) always returns the PID associ-
              ated 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 CON-
              FIG_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 initialized by duplicating the
              identifiers from the UTS namespace of the calling  process.   If
              this  flag  is  not  set, then (as with fork(2)), the process is
              created in the same UTS namespace as the calling process.   This
              flag is intended for the implementation of containers.

              A  UTS namespace is the set of identifiers returned by uname(2);
              among these, the domain name and the host name can  be  modified
              by  setdomainname(2) and  sethostname(2), respectively.  Changes
              made to the identifiers in a UTS namespace are  visible  to  all
              other  processes  in  the same namespace, but are not visible to
              processes in other UTS namespaces.

              Use of this flag requires: a kernel  configured  with  the  CON-
              FIG_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  terminates,  so  that  if
              CLONE_PARENT  is  set,  then  the parent of the calling process,
              rather than the calling process itself, will be signaled.

       CLONE_PARENT_SETTID (since Linux 2.5.49)
              Store child thread ID at location ptid in parent and child  mem-
              ory.  (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  pro-
              cess  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.

              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 sepa-
              rate 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 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 pro-
              cess 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 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.

              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
              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 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).

   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 argu-
       ments 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 ctid.

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

ERRORS
       EAGAIN Too many processes are already running.

       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 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.

       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.

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 lowest 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
       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() 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() 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);

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 3.25 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux                             2009-07-18                          CLONE(2)
 

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