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NAME

       clone, __clone2, clone3 - create a child process

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       /* Prototype for the glibc wrapper function */

       #define _GNU_SOURCE
       #include <sched.h>

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

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

       #include <linux/sched.h>    /* Definition of struct clone_args */
       #include <sched.h>          /* Definition of CLONE_* constants */
       #include <sys/syscall.h>    /* Definition of SYS_* constants */
       #include <unistd.h>

       long syscall(SYS_clone3, struct clone_args *cl_args, size_t size);

       Note: glibc provides no wrapper for clone3(), necessitating the use of syscall(2).

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
                                    (int *) */
               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 (pid_t *) */
               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  is  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 is 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 (and not
              set to SIG_IGN) 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 Linux 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.

       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, is 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 Linux 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 is 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_VFORK 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  is
       created, and errno is set to indicate the error.

ERRORS

       EACCES (clone3() only)
              CLONE_INTO_CGROUP  was  specified in cl_args.flags, but the restrictions (described
              in cgroups(7)) on placing the child process into the version 2 cgroup  referred  to
              by cl_args.cgroup are not met.

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

       EINVAL CLONE_NEWUSER and CLONE_THREAD 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 (clone3() only)
              CLONE_THREAD or CLONE_PARENT was specified in the flags  mask,  but  a  signal  was
              specified in exit_signal.

       EINVAL (AArch64 only, Linux 4.6 and earlier)
              stack was not aligned to a 128-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  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.

       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.

STANDARDS

       Linux.

HISTORY

       clone3()
              Linux 5.3.

   Linux 2.4 and earlier
       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, from Linux 2.4.7 to Linux
       2.4.18 the CLONE_THREAD flag implied the CLONE_PARENT flag (as in Linux 2.6.0 and later).

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

NOTES

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

       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.

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 <err.h>
       #include <sched.h>
       #include <signal.h>
       #include <stdint.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <string.h>
       #include <sys/mman.h>
       #include <sys/utsname.h>
       #include <sys/wait.h>
       #include <unistd.h>

       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)
               err(EXIT_FAILURE, "sethostname");

           /* Retrieve and display hostname. */

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

           if (waitpid(pid, NULL, 0) == -1)    /* Wait for child */
               err(EXIT_FAILURE, "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)