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NAME

       clone, __clone2, clone3 - create a child process

SYNOPSIS

       /* Prototype for the glibc wrapper function */

       #define _GNU_SOURCE
       #include <sched.h>

       int clone(int (*fn)(void *), void *stack, int flags, void *arg, ...
                 /* pid_t *parent_tid, void *tls, pid_t *child_tid */ );

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

       long clone3(struct clone_args *cl_args, size_t size);

       Note: There is not yet a glibc wrapper for clone3(); see NOTES.

DESCRIPTION

       These system calls create a new ("child") process, in a manner similar to fork(2).

       By contrast with fork(2), these system calls provide more precise control over what pieces
       of execution context are shared between the calling process and the  child  process.   For
       example, using these system calls, the caller can control whether or not the two processes
       share the virtual address space, the table of file descriptors, and the  table  of  signal
       handlers.   These  system  calls also allow the new child process to be placed in separate
       namespaces(7).

       Note that in this manual page, "calling process" normally corresponds to "parent process".
       But see the descriptions of CLONE_PARENT and CLONE_THREAD below.

       This page describes the following interfaces:

       *  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.

       *  The newer clone3() system call.

       In  the  remainder  of  this  page,  the  terminology "the clone call" is used when noting
       details that apply to all of these interfaces,

   The clone() wrapper function
       When the child process  is  created  with  the  clone()  wrapper  function,  it  commences
       execution  by  calling  the  function  pointed  to by the argument fn.  (This differs from
       fork(2), where execution continues in the child from the point of the fork(2) call.)   The
       arg argument is passed as the argument of the function fn.

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

       The  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  downward  on all processors that run Linux (except the HP PA processors), so
       stack usually points to the topmost address of the memory  space  set  up  for  the  child
       stack.   Note  that  clone()  does  not  provide a means whereby the caller can inform the
       kernel of the size of the stack area.

       The remaining arguments to clone() are discussed below.

   clone3()
       The clone3() system call provides a superset of the functionality  of  the  older  clone()
       interface.  It also provides a number of API improvements, including: space for additional
       flags bits; cleaner separation in the use of various arguments; and the ability to specify
       the size of the child's stack area.

       As  with  fork(2), clone3() returns in both the parent and the child.  It returns 0 in the
       child process and returns the PID of the child in the parent.

       The cl_args argument of clone3() is a structure of the following form:

           struct clone_args {
               u64 flags;        /* Flags bit mask */
               u64 pidfd;        /* Where to store PID file descriptor
                                    (pid_t *) */
               u64 child_tid;    /* Where to store child TID,
                                    in child's memory (pid_t *) */
               u64 parent_tid;   /* Where to store child TID,
                                    in parent's memory (int *) */
               u64 exit_signal;  /* Signal to deliver to parent on
                                    child termination */
               u64 stack;        /* Pointer to lowest byte of stack */
               u64 stack_size;   /* Size of stack */
               u64 tls;          /* Location of new TLS */
               u64 set_tid;      /* Pointer to a pid_t array
                                    (since Linux 5.5) */
               u64 set_tid_size; /* Number of elements in set_tid
                                    (since Linux 5.5) */
               u64 cgroup;       /* File descriptor for target cgroup
                                    of child (since Linux 5.7) */
           };

       The size argument that is supplied to clone3() should be initialized to the size  of  this
       structure.   (The  existence  of  the  size  argument  permits  future  extensions  to the
       clone_args structure.)

       The stack for the child process is specified via cl_args.stack, which points to the lowest
       byte  of  the stack area, and cl_args.stack_size, which specifies the size of the stack in
       bytes.  In the case where the CLONE_VM flag (see below) is  specified,  a  stack  must  be
       explicitly  allocated and specified.  Otherwise, these two fields can be specified as NULL
       and 0, which causes the child to use the same stack area as the parent (in the child's own
       virtual address space).

       The remaining fields in the cl_args argument are discussed below.

   Equivalence between clone() and clone3() arguments
       Unlike  the older clone() interface, where arguments are passed individually, in the newer
       clone3() interface the arguments are packaged into the clone_args structure  shown  above.
       This structure allows for a superset of the information passed via the clone() arguments.

       The  following table shows the equivalence between the arguments of clone() and the fields
       in the clone_args argument supplied to clone3():

              clone()         clone3()        Notes
                              cl_args field
              flags & ~0xff   flags           For most flags; details below
              parent_tid      pidfd           See CLONE_PIDFD
              child_tid       child_tid       See CLONE_CHILD_SETTID
              parent_tid      parent_tid      See CLONE_PARENT_SETTID
              flags & 0xff    exit_signal
              stack           stack
              ---             stack_size
              tls             tls             See CLONE_SETTLS
              ---             set_tid         See below for details

              ---             set_tid_size
              ---             cgroup          See CLONE_INTO_CGROUP

   The child termination signal
       When the child process terminates, a signal may be sent to the  parent.   The  termination
       signal  is  specified  in  the  low  byte  of  flags  (clone())  or in cl_args.exit_signal
       (clone3()).  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 (i.e., zero) is specified, then the parent process is not  signaled
       when the child terminates.

   The set_tid array
       By  default, the kernel chooses the next sequential PID for the new process in each of the
       PID namespaces where it is present.  When creating a process with  clone3(),  the  set_tid
       array  (available  since Linux 5.5) can be used to select specific PIDs for the process in
       some or all of the PID namespaces where it is present.  If the PID of  the  newly  created
       process  should  be  set  only  for  the current PID namespace or in the newly created PID
       namespace (if flags contains CLONE_NEWPID) then the first element in the set_tid array has
       to be the desired PID and set_tid_size needs to be 1.

       If  the  PID  of  the  newly  created  process should have a certain value in multiple PID
       namespaces, then the set_tid array can have multiple entries.  The first entry defines the
       PID in the most deeply nested PID namespace and each of the following entries contains the
       PID in the corresponding ancestor PID namespace.  The number of PID namespaces in which  a
       PID  should  be  set  is defined by set_tid_size which cannot be larger than the number of
       currently nested PID namespaces.

       To create a process with the following PIDs in a PID namespace hierarchy:

              PID NS level   Requested PID   Notes
              0              31496           Outermost PID namespace
              1              42
              2              7               Innermost PID namespace

       Set the array to:

           set_tid[0] = 7;
           set_tid[1] = 42;
           set_tid[2] = 31496;
           set_tid_size = 3;

       If only the PIDs in the two innermost PID namespaces need to be specified, set  the  array
       to:

           set_tid[0] = 7;
           set_tid[1] = 42;
           set_tid_size = 2;

       The  PID  in  the PID namespaces outside the two innermost PID namespaces will be selected
       the same way as any other PID is selected.

       The set_tid feature requires CAP_SYS_ADMIN or (since Linux 5.9) CAP_CHECKPOINT_RESTORE  in
       all owning user namespaces of the target PID namespaces.

       Callers  may  only choose a PID greater than 1 in a given PID namespace if an init process
       (i.e., a process with PID 1) already exists in that namespace.  Otherwise  the  PID  entry
       for this PID namespace must be 1.

   The flags mask
       Both  clone()  and clone3() allow a flags bit mask that modifies their behavior and allows
       the caller to specify what is shared between the calling process and  the  child  process.
       This  bit  mask—the  flags  argument  of  clone()  or  the  cl_args.flags  field passed to
       clone3()—is referred to as the flags mask in the remainder of this page.

       The flags mask is specified as a bitwise-OR of zero or more of the constants listed below.
       Except  as  noted  below,  these  flags  are  available (and have the same effect) in both
       clone() and clone3().

       CLONE_CHILD_CLEARTID (since Linux 2.5.49)
              Clear (zero) the child thread ID at the location pointed to by child_tid  (clone())
              or  cl_args.child_tid  (clone3())  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 pointed to by child_tid (clone()) or
              cl_args.child_tid (clone3()) in the child's memory.  The store operation  completes
              before  the  clone  call returns control to user space in the child process.  (Note
              that the store operation may not have completed before the clone  call  returns  in
              the parent process, which will be relevant if the CLONE_VM flag is also employed.)

       CLONE_CLEAR_SIGHAND (since Linux 5.5)
              By  default, signal dispositions in the child thread are the same as in the parent.
              If this flag is specified, then all signals that are  handled  in  the  parent  are
              reset to their default dispositions (SIG_DFL) in the child.

              Specifying this flag together with CLONE_SIGHAND is nonsensical and disallowed.

       CLONE_DETACHED (historical)
              For  a  while  (during the Linux 2.5 development series) there was a CLONE_DETACHED
              flag, which caused the parent not to receive a signal when  the  child  terminated.
              Ultimately, the effect of this flag was subsumed under the CLONE_THREAD flag and by
              the time Linux 2.6.0 was released, this flag had  no  effect.   Starting  in  Linux
              2.6.2, the need to give this flag together with CLONE_THREAD disappeared.

              This  flag  is  still  defined,  but  it  is  usually ignored when calling clone().
              However, see the description of CLONE_PIDFD for some exceptions.

       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 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  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  the  clone call.
              Subsequent  operations  that  open  or  close  file  descriptors,  or  change  file
              descriptor  flags,  performed by either the calling process or the child process do
              not affect the other process.  Note, however, that the duplicated file  descriptors
              in  the  child  refer  to the same open file descriptions as the corresponding file
              descriptors in the calling process, and thus share file  offsets  and  file  status
              flags (see open(2)).

       CLONE_FS (since Linux 2.0)
              If  CLONE_FS  is  set,  the  caller and the child process share the same filesystem
              information.  This includes  the  root  of  the  filesystem,  the  current  working
              directory,  and  the umask.  Any call to chroot(2), chdir(2), or umask(2) performed
              by the 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  filesystem
              information  of  the  calling  process  at  the  time  of the clone call.  Calls to
              chroot(2), chdir(2), or umask(2) performed later by one of  the  processes  do  not
              affect the other process.

       CLONE_INTO_CGROUP (since Linux 5.7)
              By  default,  a child process is placed in the same version 2 cgroup as its parent.
              The CLONE_INTO_CGROUP flag allows the child process to be created  in  a  different
              version  2  cgroup.   (Note  that  CLONE_INTO_CGROUP  has effect only for version 2
              cgroups.)

              In order to place the child process in a different  cgroup,  the  caller  specifies
              CLONE_INTO_CGROUP  in  cl_args.flags  and passes a file descriptor that refers to a
              version 2 cgroup in  the  cl_args.cgroup  field.   (This  file  descriptor  can  be
              obtained  by  opening a cgroup v2 directory using either the O_RDONLY or the O_PATH
              flag.)  Note that all of  the  usual  restrictions  (described  in  cgroups(7))  on
              placing a process into a version 2 cgroup apply.

              Among the possible use cases for CLONE_INTO_CGROUP are the following:

              *  Spawning  a  process  into  a cgroup different from the parent's cgroup makes it
                 possible for a service manager to directly spawn  new  services  into  dedicated
                 cgroups.   This  eliminates  the  accounting  jitter that would be caused if the
                 child process was first created in the same cgroup as the parent and then  moved
                 into the target cgroup.  Furthermore, spawning the child process directly into a
                 target cgroup is significantly cheaper than moving the child  process  into  the
                 target cgroup after it has been created.

              *  The CLONE_INTO_CGROUP flag also allows the creation of frozen child processes by
                 spawning them into a frozen cgroup.  (See cgroups(7) for a  description  of  the
                 freezer controller.)

              *  For  threaded  applications  (or  even  thread implementations which make use of
                 cgroups to limit individual threads), it is possible to establish a fixed cgroup
                 layout before spawning each thread directly into its target cgroup.

       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.

              For further information on cgroup namespaces, see cgroup_namespaces(7).

              Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWCGROUP.

       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.

              For further information on IPC namespaces, see ipc_namespaces(7).

              Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWIPC.  This flag can't
              be specified in conjunction with CLONE_SYSVSEM.

       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.

              For further information on network namespaces, see network_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)   and
              mount_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.

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

              Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWPID.  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_SETGID.  Starting with Linux  3.8,
              no privileges are needed to create 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.

       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.

              For further information on UTS namespaces, see uts_namespaces(7).

              Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWUTS.

       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.

              The  CLONE_PARENT flag can't be used in clone calls by the global init process (PID
              1 in the initial PID namespace) and init processes in other PID  namespaces.   This
              restriction  prevents  the  creation  of  multi-rooted process trees as well as the
              creation of unreapable zombies in the initial PID namespace.

       CLONE_PARENT_SETTID (since Linux 2.5.49)
              Store the child thread ID at the location pointed to  by  parent_tid  (clone())  or
              cl_args.parent_tid  (clone3())  in  the  parent's  memory.  (In Linux 2.5.32-2.5.48
              there was a flag CLONE_SETTID that did this.)  The store operation completes before
              the clone call returns control to user space.

       CLONE_PID (Linux 2.0 to 2.5.15)
              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.   From  Linux  2.3.21  onward, this flag could be specified only by the system
              boot process (PID 0).  The flag disappeared completely from the kernel  sources  in
              Linux  2.5.16.   Subsequently,  the  kernel  silently  ignored  this  bit if it was
              specified in the flags mask.  Much later, the same bit was recycled for use as  the
              CLONE_PIDFD flag.

       CLONE_PIDFD (since Linux 5.2)
              If  this flag is specified, a PID file descriptor referring to the child process is
              allocated and placed at a specified location in the parent's memory.  The close-on-
              exec flag is set on this new file descriptor.  PID file descriptors can be used for
              the purposes described in pidfd_open(2).

              *  When using clone3(), the PID file descriptor is placed at the  location  pointed
                 to by cl_args.pidfd.

              *  When using clone(), the PID file descriptor is placed at the location pointed to
                 by parent_tid.  Since the parent_tid argument is used to  return  the  PID  file
                 descriptor,  CLONE_PIDFD  cannot  be  used with CLONE_PARENT_SETTID when calling
                 clone().

              It is currently not possible to use this flag  together  with  CLONE_THREAD.   This
              means  that  the  process  identified  by  the PID file descriptor will always be a
              thread group leader.

              If the obsolete CLONE_DETACHED flag is specified alongside CLONE_PIDFD when calling
              clone(),  an  error  is  returned.   An  error  also  results  if CLONE_DETACHED is
              specified when  calling  clone3().   This  error  behavior  ensures  that  the  bit
              corresponding  to  CLONE_DETACHED  can  be  reused  for further PID file descriptor
              features in the future.

       CLONE_PTRACE (since Linux 2.2)
              If CLONE_PTRACE is specified, and the calling process is being traced,  then  trace
              the child also (see ptrace(2)).

       CLONE_SETTLS (since Linux 2.5.32)
              The TLS (Thread Local Storage) descriptor is set to tls.

              The  interpretation  of tls and the resulting effect is architecture dependent.  On
              x86, tls is interpreted as  a  struct  user_desc *  (see  set_thread_area(2)).   On
              x86-64 it is the new value to be set for the %fs base register (see the ARCH_SET_FS
              argument to arch_prctl(2)).  On architectures with a dedicated TLS register, it  is
              the new value of that register.

              Use  of  this  flag requires detailed knowledge and generally it should not be used
              except in libraries implementing threading.

       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 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 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  of  the  clone  call.  Calls to
              sigaction(2) performed later by one of the processes have no effect  on  the  other
              process.

              Since  Linux  2.6.0,  the flags mask must also include CLONE_VM if CLONE_SIGHAND is
              specified

       CLONE_STOPPED (since Linux 2.6.0)
              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  accumulates  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)
              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, and a thread can obtain its own TID using gettid(2).

              When  a  clone  call  is  made  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 process
              that made the clone call (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 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, the flags mask must also include CLONE_SIGHAND if  CLONE_THREAD
              is  specified  (and  note  that,  since  Linux  2.6.0,  CLONE_SIGHAND also requires
              CLONE_VM to be included).

              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).

              A signal may be process-directed or thread-directed.  A process-directed signal  is
              targeted  at  a  thread  group  (i.e.,  a TGID), and is delivered to an arbitrarily
              selected thread from among those that are not blocking the signal.  A signal may be
              process-directed  because  it  was generated by the kernel for reasons other than a
              hardware exception, or because it was sent using kill(2) or sigqueue(3).  A thread-
              directed  signal  is  targeted at (i.e., delivered to) a specific thread.  A signal
              may be thread directed because it was sent using tgkill(2) or  pthread_sigqueue(3),
              or  because  the  thread  executed  a machine language instruction that triggered a
              hardware exception (e.g., invalid memory access triggering SIGSEGV or  a  floating-
              point exception triggering SIGFPE).

              A  call  to  sigpending(2)  returns  a  signal set that is the union of the pending
              process-directed signals and the signals that are pending for the calling thread.

              If a process-directed signal is delivered 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
              the signal.

       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 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 the clone call.  Memory writes or file
              mappings/unmappings performed by one of the processes do not affect the  other,  as
              with fork(2).

              If  the CLONE_VM flag is specified and the CLONE_VM flag is not specified, then any
              alternate signal stack that was established by sigaltstack(2)  is  cleared  in  the
              child process.

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; see fork(2).

       EBUSY (clone3() only)
              CLONE_INTO_CGROUP was specified in cl_args.flags, but the file descriptor specified
              in cl_args.cgroup refers to a version 2 cgroup in  which  a  domain  controller  is
              enabled.

       EEXIST (clone3() only)
              One  (or more) of the PIDs specified in set_tid already exists in the corresponding
              PID namespace.

       EINVAL Both CLONE_SIGHAND and CLONE_CLEAR_SIGHAND were specified in the flags mask.

       EINVAL CLONE_SIGHAND was specified in the flags mask, but CLONE_VM was not.  (Since  Linux
              2.6.0.)

       EINVAL CLONE_THREAD  was  specified  in the flags mask, but CLONE_SIGHAND was not.  (Since
              Linux 2.5.35.)

       EINVAL CLONE_THREAD was specified in the flags mask, but the  current  process  previously
              called unshare(2) with the CLONE_NEWPID flag or used setns(2) to reassociate itself
              with a PID namespace.

       EINVAL Both CLONE_FS and CLONE_NEWNS were specified in the flags mask.

       EINVAL (since Linux 3.9)
              Both CLONE_NEWUSER and CLONE_FS were specified in the flags mask.

       EINVAL Both CLONE_NEWIPC and CLONE_SYSVSEM were specified in the flags mask.

       EINVAL One (or both) of CLONE_NEWPID or CLONE_NEWUSER and one (or both) of CLONE_THREAD or
              CLONE_PARENT were specified in the flags mask.

       EINVAL (since Linux 2.6.32)
              CLONE_PARENT was specified, and the caller is an init process.

       EINVAL Returned  by  the  glibc  clone() wrapper function when fn or stack is specified as
              NULL.

       EINVAL CLONE_NEWIPC was specified in the flags mask, but the  kernel  was  not  configured
              with the CONFIG_SYSVIPC and CONFIG_IPC_NS options.

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

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

       EINVAL CLONE_NEWUSER  was  specified  in the flags mask, but the kernel was not configured
              with the CONFIG_USER_NS option.

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

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

       EINVAL (clone3() only)
              CLONE_DETACHED was specified in the flags mask.

       EINVAL (clone() only)
              CLONE_PIDFD was specified together with CLONE_DETACHED in the flags mask.

       EINVAL CLONE_PIDFD was specified together with CLONE_THREAD in the flags mask.

       EINVAL (clone() only)
              CLONE_PIDFD was specified together with CLONE_PARENT_SETTID in the flags mask.

       EINVAL (clone3() only)
              set_tid_size is greater than the number of nested PID namespaces.

       EINVAL (clone3() only)
              One of the PIDs specified in set_tid was an invalid.

       EINVAL (AArch64 only, Linux 4.6 and earlier)
              stack was not aligned to a 126-bit boundary.

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

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

       ENOSPC (since Linux 4.9; beforehand EUSERS)
              CLONE_NEWUSER was specified in the flags mask, and the call would cause  the  limit
              on the number of nested user namespaces to be exceeded.  See user_namespaces(7).

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

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

       EOPNOTSUPP (clone3() only)
              CLONE_INTO_CGROUP was specified in cl_args.flags, but the file descriptor specified
              in cl_args.cgroup refers to a version 2 cgroup that is in the domain invalid state.

       EPERM  CLONE_NEWCGROUP,  CLONE_NEWIPC,   CLONE_NEWNET,   CLONE_NEWNS,   CLONE_NEWPID,   or
              CLONE_NEWUTS   was   specified   by   an   unprivileged  process  (process  without
              CAP_SYS_ADMIN).

       EPERM  CLONE_PID was specified by a process other than process 0.  (This error occurs only
              on Linux 2.5.15 and earlier.)

       EPERM  CLONE_NEWUSER  was specified in the flags mask, 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  the  flags  mask  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).

       EPERM (clone3() only)
              set_tid_size  was  greater  than  zero,  and  the  caller  lacks  the CAP_SYS_ADMIN
              capability in one or more of the user namespaces that  own  the  corresponding  PID
              namespaces.

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

       EUSERS (Linux 3.11 to Linux 4.8)
              CLONE_NEWUSER was specified in the flags mask, and  the  limit  on  the  number  of
              nested  user  namespaces would be exceeded.  See the discussion of the ENOSPC error
              above.

VERSIONS

       The clone3() system call first appeared in Linux 5.3.

CONFORMING TO

       These system calls are Linux-specific and should not be used in programs  intended  to  be
       portable.

NOTES

       One  use  of  these  systems calls is to implement threads: multiple flows of control in a
       program that run concurrently in a shared address space.

       Glibc does not provide a wrapper for clone3(); call it using syscall(2).

       Note that the glibc clone() wrapper function makes some changes in the memory  pointed  to
       by  stack  (changes  required to set the stack up correctly for the child) before invoking
       the clone() system call.  So, in  cases  where  clone()  is  used  to  recursively  create
       children, do not use the buffer employed for the parent's stack as the stack of the child.

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

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

       In  the  Linux  2.4.x  series,  CLONE_THREAD generally does not make the parent of the new
       thread the same as the parent of the calling process.  However, for kernel versions  2.4.7
       to  2.4.18  the  CLONE_THREAD  flag  implied  the CLONE_PARENT flag (as in Linux 2.6.0 and
       later).

       On i386, clone() should not be called through vsyscall, but directly through int $0x80.

   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.

       In contrast to the glibc wrapper, the raw clone() system call  accepts  NULL  as  a  stack
       argument (and clone3() likewise allows cl_args.stack to be NULL).  In this case, the child
       uses a duplicate of the parent's stack.  (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.  (If  the  child
       shares  the parent's memory because of the use of the CLONE_VM flag, then no copy-on-write
       duplication occurs and chaos is likely to result.)

       The order of the arguments also differs in the raw system call, and there  are  variations
       in the arguments across architectures, as detailed in the following paragraphs.

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

           long clone(unsigned long flags, void *stack,
                      int *parent_tid, int *child_tid,
                      unsigned long tls);

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

           long clone(unsigned long flags, void *stack,
                     int *parent_tid, unsigned long tls,
                     int *child_tid);

       On the cris and s390 architectures, the order of the first two arguments is reversed:

           long clone(void *stack, unsigned long flags,
                      int *parent_tid, int *child_tid,
                      unsigned long tls);

       On the microblaze architecture, an additional argument is supplied:

           long clone(unsigned long flags, void *stack,
                      int stack_size,         /* Size of stack */
                      int *parent_tid, int *child_tid,
                      unsigned long tls);

   blackfin, m68k, and sparc
       The  argument-passing  conventions  on  blackfin,  m68k,  and sparc are different 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 *stack_base, size_t stack_size,
                        int flags, void *arg, ...
                     /* pid_t *parent_tid, struct user_desc *tls,
                        pid_t *child_tid */ );

       The prototype shown above is for the glibc wrapper function; for the system  call  itself,
       the  prototype  can  be  described as follows (it is identical to the clone() prototype on
       microblaze):

           long clone2(unsigned long flags, void *stack_base,
                       int stack_size,         /* Size of stack */
                       int *parent_tid, int *child_tid,
                       unsigned long tls);

       __clone2() operates in the same way as clone(),  except  that  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 stack_base.

   Linux 2.4 and earlier
       In Linux 2.4 and earlier, clone() does not take arguments parent_tid, tls, and child_tid.

BUGS

       GNU C library versions 2.3.4 up to and including 2.24 contained  a  wrapper  function  for
       getpid(2)  that  performed  caching  of PIDs.  This caching relied on support in the glibc
       wrapper for clone(), but limitations in the implementation meant that the cache was not up
       to  date  in  some  circumstances.   In particular, if a signal was delivered to the child
       immediately after the clone() call, then a call to getpid(2) in a handler for  the  signal
       could  return  the PID of the calling process ("the parent"), if the clone wrapper had not
       yet had a chance to update the PID cache in the child.  (This discussion ignores the  case
       where  the  child  was  created  using CLONE_THREAD, when getpid(2) should return the same
       value in the child and in the process that called clone(), since the caller and the  child
       are  in  the  same thread group.  The stale-cache problem also does not occur if the flags
       argument includes CLONE_VM.)  To get the truth, it was sometimes  necessary  to  use  code
       such as the following:

           #include <syscall.h>

           pid_t mypid;

           mypid = syscall(SYS_getpid);

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

EXAMPLES

       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  differs  in  the UTS namespaces of the parent and child.  For an
       example of the use of this program, see setns(2).

       Within the sample program, we allocate the memory that is to be used for the child's stack
       using mmap(2) rather than malloc(3) for the following reasons:

       *  mmap(2) allocates a block of memory that starts on a page boundary and is a multiple of
          the page size.  This is useful if we want to  establish  a  guard  page  (a  page  with
          protection PROT_NONE) at the end of the stack using mprotect(2).

       *  We  can  specify  the MAP_STACK flag to request a mapping that is suitable for a stack.
          For the moment, this flag is a no-op on Linux, but it exists and  has  effect  on  some
          other systems, so we should include it for portability.

   Program source
       #define _GNU_SOURCE
       #include <sys/wait.h>
       #include <sys/utsname.h>
       #include <sched.h>
       #include <string.h>
       #include <stdint.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>
       #include <sys/mman.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 memory to be used for the stack of the child */

           stack = mmap(NULL, STACK_SIZE, PROT_READ | PROT_WRITE,
                        MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
           if (stack == MAP_FAILED)
               errExit("mmap");

           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 %jd\n", (intmax_t) 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),    mmap(2),   pidfd_open(2),
       set_thread_area(2),   set_tid_address(2),   setns(2),   tkill(2),   unshare(2),   wait(2),
       capabilities(7), namespaces(7), pthreads(7)

COLOPHON

       This  page  is  part of release 5.10 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/.