clone(2)



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

NAME
       clone - create a child process

SYNOPSIS
       #include <sched.h>

       int clone(int (*fn)(void *), void *child_stack, int flags, void *arg);

       _syscall2(int, clone, int, flags, void *, child_stack)

       _syscall5(int, clone, int, flags, void *, child_stack,
            int *, parent_tidptr, struct user_desc *, newtls,
            int *, child_tidptr)
               /* Using syscall(2) may be preferable; see intro(2) */

DESCRIPTION
       clone() creates a new process, in a manner similar to fork(2).  clone()
       is 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_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_FS
              If CLONE_FS is set, the caller and the child processes 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_FILES
              If CLONE_FILES is set, the calling process and  the  child  pro-
              cesses  share the same file descriptor table.  Any file descrip-
              tor created by the calling process or by the  child  process  is
              also  valid in the other process.  Similarly, if one of the pro-
              cesses 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_NEWNS (since Linux 2.4.19)
              Start the child in a new namespace.

              Every  process  lives in a 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(2) where the
              CLONE_NEWNS flag is not set, the child lives in the same  names-
              pace  as  the  parent.   The system calls mount(2) and umount(2)
              change the namespace of the calling process,  and  hence  affect
              all processes that live in the same namespace, but do not affect
              processes in a different namespace.

              After a clone(2) where the CLONE_NEWNS flag is set,  the  cloned
              child  is started in a new 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_SIGHAND
              If  CLONE_SIGHAND is set, the calling process and the child pro-
              cesses 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_PTRACE
              If CLONE_PTRACE is specified, and the calling process  is  being
              traced, then trace the child also (see ptrace(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_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.

       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  processes
              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).

       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_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_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_SETTLS (since Linux 2.5.32)
              The newtls parameter is  the  new  TLS  (Thread  Local  Storage)
              descriptor.  (See set_thread_area(2).)

       CLONE_PARENT_SETTID (since Linux 2.5.49)
              Store  child  thread  ID at location parent_tidptr in parent and
              child  memory.   (In  Linux  2.5.32-2.5.48  there  was  a   flag
              CLONE_SETTID that did this.)

       CLONE_CHILD_SETTID (since Linux 2.5.49)
              Store  child thread ID at location child_tidptr in child memory.

       CLONE_CHILD_CLEARTID (since Linux 2.5.49)
              Erase child thread ID at location child_tidptr 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.

   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.

       Since Linux 2.5.49 the system call has five parameters.   The  two  new
       parameters  are  parent_tidptr  which points to the location (in parent
       and child memory) where the child thread ID will  be  written  in  case
       CLONE_PARENT_SETTID was specified, and child_tidptr which points to the
       location (in child memory) where the child thread ID will be written in
       case CLONE_CHILD_SETTID was specified.

RETURN VALUE
       On  success,  the  thread  ID  of  the child process is returned in the
       caller's thread of execution.  On failure, a -1 will be 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 Returned  by  clone()  when  a  zero  value  is  specified   for
              child_stack.

       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_NEWNS was specified by a non-root process (process without
              CAP_SYS_ADMIN).

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

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

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).

       On x86, clone() should not be called  through  vsyscall,  but  directly
       through int $0x80.

CONFORMING TO
       The  clone()  and  sys_clone calls are Linux-specific and should not be
       used in programs intended to be portable.

BUGS
       Versions of the GNU C library that include the NPTL  threading  library
       contain  a wrapper function for getpid() that performs caching of PIDs.
       In programs linked against such libraries, calls to getpid() may return
       the   same  value,  even  when  the  threads  were  not  created  using
       CLONE_THREAD (and thus are not in the same thread group).  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)

Linux 2.6                         2005-05-17                          CLONE(2)

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