oracular (2) clone.2.gz

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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/types.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)