clone

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

NAME
       clone, __clone2, clone3 - create a child process

SYNOPSIS
       /* Prototype for the glibc wrapper function */

       #define _GNU_SOURCE
       #include <sched.h>

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

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

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

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

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

       By contrast with fork(2), these system calls provide more precise  con-
       trol over what pieces of execution context are shared between the call-
       ing 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 mem-
       ory  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 improve-
       ments, including: space for additional flags bits;  cleaner  separation
       in the use of various arguments; and the ability to specify the size of
       the child's stack area.

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

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

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

       The size argument that is supplied to clone3() should be initialized to
       the  size  of this structure.  (The existence of the size argument per-
       mits 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 al-
       located 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 individ-
       ually,  in the newer clone3() interface the arguments are packaged into
       the clone_args structure shown above.  This structure allows for a  su-
       perset 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

   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 creat-
       ing 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 en-
       tries.  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  hierar-
       chy:

              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 speci-
       fied, set the array to:

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

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

       The  set_tid  feature  requires  CAP_SYS_ADMIN in all owning user name-
       spaces 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  argu-
       ment  of  clone() or the cl_args.flags field passed to clone3()--is re-
       ferred 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 con-
       stants  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  li-
              braries.

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

       CLONE_CLEAR_SIGHAND (since Linux 5.5)
              By default, signal dispositions in the child thread are the same
              as in the parent.  If this flag is specified, then  all  signals
              that are handled in the parent are reset to their default dispo-
              sitions (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.  Start-
              ing 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 call-
              ing 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  cre-
              ated  by  the  calling  process  or by the child process is also
              valid in the other process.  Similarly, if one of the  processes
              closes a file descriptor, or changes its associated flags (using
              the fcntl(2) F_SETFD operation), the other process is  also  af-
              fected.   If a process sharing a file descriptor table calls ex-
              ecve(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 de-
              scriptors 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 call-
              ing process or the child process also affects the other process.

              If CLONE_FS is not set, the child process works on a copy of the
              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_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_name-
              spaces(7).

              Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWC-
              GROUP.

       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_name-
              spaces(7).

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

       CLONE_NEWNET (since Linux 2.6.24)
              (The implementation of this flag was  completed  only  by  about
              kernel version 2.6.29.)

              If CLONE_NEWNET is set, then create the process in a new network
              namespace.  If this flag is not set, then (as with fork(2))  the
              process  is created in the same network namespace as the calling
              process.

              For further information on network namespaces, see network_name-
              spaces(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_NEW-
              PID.    This   flag  can't  be  specified  in  conjunction  with
              CLONE_THREAD or CLONE_PARENT.

       CLONE_NEWUSER
              (This flag first became meaningful for clone() in Linux  2.6.23,
              the  current clone() semantics were merged in Linux 3.5, and the
              final pieces to make the user namespaces completely usable  were
              merged in Linux 3.8.)

              If  CLONE_NEWUSER  is set, then create the process in a new user
              namespace.  If this flag is not set, then (as with fork(2))  the
              process  is  created  in  the same user namespace as the calling
              process.

              For further information on user  namespaces,  see  namespaces(7)
              and user_namespaces(7).

              Before  Linux 3.8, use of CLONE_NEWUSER required that the caller
              have three capabilities: CAP_SYS_ADMIN, CAP_SETUID, and CAP_SET-
              GID.   Starting with Linux 3.8, no privileges are needed to cre-
              ate 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 cre-
              ated in the same UTS namespace as the calling process.

              For  further  information  on  UTS  namespaces,  see   uts_name-
              spaces(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 re-
              turned  by  getppid(2))  will be the same as that of the calling
              process.

              If CLONE_PARENT is not set, then (as with fork(2))  the  child's
              parent is the calling process.

              Note  that  it is the parent process, as returned by getppid(2),
              which  is  signaled  when  the  child  terminates,  so  that  if
              CLONE_PARENT  is  set,  then  the parent of the calling process,
              rather than the calling process itself, will be signaled.

              The CLONE_PARENT flag can't be used in clone calls by the global
              init  process (PID 1 in the initial PID namespace) and init pro-
              cesses 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  par-
              ent_tid  (clone())  or  cl_args.child_tid (clone3()) in the par-
              ent's  memory.   (In  Linux  2.5.32-2.5.48  there  was  a   flag
              CLONE_SETTID  that did this.)  The store operation completes be-
              fore the clone call returns control to user space.

       CLONE_PID (Linux 2.0 to 2.5.15)
              If CLONE_PID is set, the child process is created with the  same
              process ID as the calling process.  This is good for hacking the
              system, but otherwise of not much use.  From  Linux  2.3.21  on-
              ward,  this  flag  could  be  specified  only by the system boot
              process (PID 0).  The flag disappeared completely from the  ker-
              nel  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 loca-
              tion 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  ar-
                 gument 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 er-
              ror also results if CLONE_DETACHED  is  specified  when  calling
              clone3().   This error behavior ensures that the bit correspond-
              ing to CLONE_DETACHED can be reused for  further  PID  file  de-
              scriptor 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  architec-
              ture  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 behav-
              ior  associated  with  a  signal, the behavior is changed in the
              other process as well.  However, the calling process  and  child
              processes  still  have distinct signal masks and sets of pending
              signals.  So, one of them may block  or  unblock  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 re-
              moved 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  accumu-
              lates  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 dis-
              cussion of CLONE_THREAD more readable, the term "thread" is used
              to refer to the processes within a thread group.

              Thread groups were a feature added in Linux 2.4 to  support  the
              POSIX  threads  notion  of  a set of threads that share a single
              PID.  Internally, this shared PID is the so-called thread  group
              identifier  (TGID) for the thread group.  Since Linux 2.4, calls
              to getpid(2) return the TGID of the caller.

              The threads within a group can be distinguished by  their  (sys-
              tem-wide) unique thread IDs (TID).  A new thread's TID is avail-
              able as the function result returned to the caller, 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 termina-
              tion) signal.

              If  any  of the threads in a thread group performs an execve(2),
              then all threads other than the thread group leader  are  termi-
              nated,  and  the  new  program  is  executed in the thread group
              leader.

              If one of the threads in a thread group creates  a  child  using
              fork(2),  then  any  thread  in  the  group can wait(2) for that
              child.

              Since Linux 2.5.35, the flags mask must also include  CLONE_SIG-
              HAND  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  unhan-
              dled  signal is delivered to a thread, then it will affect (ter-
              minate, stop, continue, be ignored in) all members of the thread
              group.

              Each thread has its own signal mask, as set by sigprocmask(2).

              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 rea-
              sons other than a hardware exception, or because it was sent us-
              ing  kill(2)  or  sigqueue(3).  A thread-directed signal is tar-
              geted 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 ex-
              ception triggering SIGFPE).

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

              If a process-directed signal is delivered to a thread group, and
              the thread group has installed a handler for  the  signal,  then
              the handler will be invoked in exactly one, arbitrarily selected
              member of the thread group that has not blocked the signal.   If
              multiple  threads in a group are waiting to accept the same sig-
              nal 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 per-
              formed by the calling process or by the child process  are  also
              visible  in  the other process.  Moreover, any memory mapping or
              unmapping performed with mmap(2) or munmap(2) by  the  child  or
              calling process also affects the other process.

              If  CLONE_VM  is  not  set, the child process runs in a separate
              copy of the memory space of the calling process at the  time  of
              the  clone call.  Memory writes or file mappings/unmappings per-
              formed by one of the processes do not affect the other, as  with
              fork(2).

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

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

       Note that the glibc clone() wrapper function makes some changes in  the
       memory  pointed  to by stack (changes required to set the stack up cor-
       rectly 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.

   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 speci-
       fied.  (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 argu-
       ments 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  dif-
       ferent  from  the descriptions above.  For details, see the kernel (and
       glibc) source.

   ia64
       On ia64, a different interface is used:

           int __clone2(int (*fn)(void *),
                        void *stack_base, size_t stack_size,
                        int flags, void *arg, ...
                     /* pid_t *parent_tid, struct user_desc *tls,
                        pid_t *child_tid */ );

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

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

       __clone2() operates in the same way as clone(), except that  stack_base
       points  to the lowest address of the child's stack area, and stack_size
       specifies the size of the stack pointed to by stack_base.

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

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

ERRORS
       EAGAIN Too many processes are already running; see fork(2).

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

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

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

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

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

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

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

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

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

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

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

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

       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 architec-
              ture.  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 name-
              spaces.

       EINVAL (clone3() only)
              If one of the PIDs specified in set_tid was an invalid PID.

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

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

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

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

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

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

       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_name-
              spaces(7)).

       EPERM (since Linux 3.9)
              CLONE_NEWUSER was specified in the flags mask and the caller  is
              in  a chroot environment (i.e., the caller's root directory does
              not match the root directory of the mount namespace in which  it
              resides).

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

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

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

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

CONFORMING TO
       These system calls are Linux-specific and should not be  used  in  pro-
       grams intended to be portable.

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

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

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

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

BUGS
       GNU C library versions 2.3.4 up to and including 2.24 contained a wrap-
       per  function  for  getpid(2)  that  performed  caching  of PIDs.  This
       caching relied on support in the glibc wrapper for clone(), but limita-
       tions  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.

EXAMPLE
       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  dif-
       fers  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 fol-
       lowing 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 suit-
          able 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 in-
          clude it for portability.

   Program source
       #define _GNU_SOURCE
       #include <sys/wait.h>
       #include <sys/utsname.h>
       #include <sched.h>
       #include <string.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>
       #include <sys/mman.h>

       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                               } while (0)

       static int              /* Start function for cloned child */
       childFunc(void *arg)
       {
           struct utsname uts;

           /* Change hostname in UTS namespace of child */

           if (sethostname(arg, strlen(arg)) == -1)
               errExit("sethostname");

           /* Retrieve and display hostname */

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

           /* Keep the namespace open for a while, by sleeping.
              This allows some experimentation--for example, another
              process might join the namespace. */

           sleep(200);

           return 0;           /* Child terminates now */
       }

       #define STACK_SIZE (1024 * 1024)    /* Stack size for cloned child */

       int
       main(int argc, char *argv[])
       {
           char *stack;                    /* Start of stack buffer */
           char *stackTop;                 /* End of stack buffer */
           pid_t pid;
           struct utsname uts;

           if (argc < 2) {
               fprintf(stderr, "Usage: %s <child-hostname>\n", argv[0]);
               exit(EXIT_SUCCESS);
           }

           /* Allocate memory to be used for the stack of the child */

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

           stackTop = stack + STACK_SIZE;  /* Assume stack grows downward */

           /* Create child that has its own UTS namespace;
              child commences execution in childFunc() */

           pid = clone(childFunc, stackTop, CLONE_NEWUTS | SIGCHLD, argv[1]);
           if (pid == -1)
               errExit("clone");
           printf("clone() returned %ld\n", (long) pid);

           /* Parent falls through to here */

           sleep(1);           /* Give child time to change its hostname */

           /* Display hostname in parent's UTS namespace. This will be
              different from hostname in child's UTS namespace. */

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

           if (waitpid(pid, NULL, 0) == -1)    /* Wait for child */
               errExit("waitpid");
           printf("child has terminated\n");

           exit(EXIT_SUCCESS);
       }

SEE ALSO
       fork(2),   futex(2),   getpid(2),    gettid(2),    kcmp(2),    mmap(2),
       pidfd_open(2),    set_thread_area(2),   set_tid_address(2),   setns(2),
       tkill(2),   unshare(2),   wait(2),   capabilities(7),    namespaces(7),
       pthreads(7)

COLOPHON
       This  page  is  part of release 5.05 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

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