clone(2) — Linux manual page

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

NAME         top

       clone, __clone2, clone3 - create a child process

SYNOPSIS         top

       /* 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         top

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

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

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

       This page describes the following interfaces:

       *  The glibc clone() wrapper function and the underlying system
          call on which it is based.  The main text describes the
          wrapper function; the differences for the raw system call are
          described toward the end of this page.

       *  The newer clone3() system call.

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

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

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

       The stack argument specifies the location of the stack used by
       the child process.  Since the child and calling process may share
       memory, it is not possible for the child process to execute in
       the same stack as the calling process.  The calling process must
       therefore set up memory space for the child stack and pass a
       pointer to this space to clone().  Stacks grow downward on all
       processors that run Linux (except the HP PA processors), so stack
       usually points to the topmost address of the memory space set up
       for the child stack.  Note that clone() does not provide a means
       whereby the caller can inform the kernel of the size of the stack
       area.

       The remaining arguments to clone() are discussed below.

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

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

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

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

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

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

       The remaining fields in the cl_args argument are discussed below.

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

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

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

   The child termination signal
       When the child process terminates, a signal may be sent to the
       parent.  The termination signal is specified in the low byte of
       flags (clone()) or in cl_args.exit_signal (clone3()).  If this
       signal is specified as anything other than SIGCHLD, then the
       parent process must specify the __WALL or __WCLONE options when
       waiting for the child with wait(2).  If no signal (i.e., zero) is
       specified, then the parent process is not signaled when the child
       terminates.

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

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

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

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

       Set the array to:

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

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

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

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

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

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

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

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

       CLONE_CHILD_CLEARTID (since Linux 2.5.49)
              Clear (zero) the child thread ID at the location pointed
              to by child_tid (clone()) or cl_args.child_tid (clone3())
              in child memory when the child exits, and do a wakeup on
              the futex at that address.  The address involved may be
              changed by the set_tid_address(2) system call.  This is
              used by threading libraries.

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

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

              Specifying this flag together with CLONE_SIGHAND is
              nonsensical and disallowed.

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

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

       CLONE_FILES (since Linux 2.0)
              If CLONE_FILES is set, the calling process and the child
              process share the same file descriptor table.  Any file
              descriptor created by the calling process or by the child
              process is also valid in the other process.  Similarly, if
              one of the processes closes a file descriptor, or changes
              its associated flags (using the fcntl(2) F_SETFD
              operation), the other process is also affected.  If a
              process sharing a file descriptor table calls execve(2),
              its file descriptor table is duplicated (unshared).

              If CLONE_FILES is not set, the child process inherits a
              copy of all file descriptors opened in the calling process
              at the time of the clone call.  Subsequent operations that
              open or close file descriptors, or change file descriptor
              flags, performed by either the calling process or the
              child process do not affect the other process.  Note,
              however, that the duplicated file descriptors in the child
              refer to the same open file descriptions as the
              corresponding file descriptors in the calling process, and
              thus share file offsets and file status flags (see
              open(2)).

       CLONE_FS (since Linux 2.0)
              If CLONE_FS is set, the caller and the child process share
              the same filesystem information.  This includes the root
              of the filesystem, the current working directory, and the
              umask.  Any call to chroot(2), chdir(2), or umask(2)
              performed by the calling process or the child process also
              affects the other process.

              If CLONE_FS is not set, the child process works on a copy
              of the filesystem information of the calling process at
              the time of the clone call.  Calls to chroot(2), chdir(2),
              or umask(2) performed later by one of the processes do not
              affect the other process.

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

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

              Among the possible use cases for CLONE_INTO_CGROUP are the
              following:

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

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

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

       CLONE_IO (since Linux 2.6.25)
              If CLONE_IO is set, then the new process shares an I/O
              context with the calling process.  If this flag is not
              set, then (as with fork(2)) the new process has its own
              I/O context.

              The I/O context is the I/O scope of the disk scheduler
              (i.e., what the I/O scheduler uses to model scheduling of
              a process's I/O).  If processes share the same I/O
              context, they are treated as one by the I/O scheduler.  As
              a consequence, they get to share disk time.  For some I/O
              schedulers, if two processes share an I/O context, they
              will be allowed to interleave their disk access.  If
              several threads are doing I/O on behalf of the same
              process (aio_read(3), for instance), they should employ
              CLONE_IO to get better I/O performance.

              If the kernel is not configured with the CONFIG_BLOCK
              option, this flag is a no-op.

       CLONE_NEWCGROUP (since Linux 4.6)
              Create the process in a new cgroup namespace.  If this
              flag is not set, then (as with fork(2)) the process is
              created in the same cgroup namespaces as the calling
              process.

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

              Only a privileged process (CAP_SYS_ADMIN) can employ
              CLONE_NEWCGROUP.

       CLONE_NEWIPC (since Linux 2.6.19)
              If CLONE_NEWIPC is set, then create the process in a new
              IPC namespace.  If this flag is not set, then (as with
              fork(2)), the process is created in the same IPC namespace
              as the calling process.

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

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

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

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

              For further information on network namespaces, see
              network_namespaces(7).

              Only a privileged process (CAP_SYS_ADMIN) can employ
              CLONE_NEWNET.

       CLONE_NEWNS (since Linux 2.4.19)
              If CLONE_NEWNS is set, the cloned child is started in a
              new mount namespace, initialized with a copy of the
              namespace of the parent.  If CLONE_NEWNS is not set, the
              child lives in the same mount namespace as the parent.

              For further information on mount namespaces, see
              namespaces(7) and mount_namespaces(7).

              Only a privileged process (CAP_SYS_ADMIN) can employ
              CLONE_NEWNS.  It is not permitted to specify both
              CLONE_NEWNS and CLONE_FS in the same clone call.

       CLONE_NEWPID (since Linux 2.6.24)
              If CLONE_NEWPID is set, then create the process in a new
              PID namespace.  If this flag is not set, then (as with
              fork(2)) the process is created in the same PID namespace
              as the calling process.

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

              Only a privileged process (CAP_SYS_ADMIN) can employ
              CLONE_NEWPID.  This flag can't be specified in conjunction
              with CLONE_THREAD or CLONE_PARENT.

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

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

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

              Before Linux 3.8, use of CLONE_NEWUSER required that the
              caller have three capabilities: CAP_SYS_ADMIN, CAP_SETUID,
              and CAP_SETGID.  Starting with Linux 3.8, no privileges
              are needed to create a user namespace.

              This flag can't be specified in conjunction with
              CLONE_THREAD or CLONE_PARENT.  For security reasons,
              CLONE_NEWUSER cannot be specified in conjunction with
              CLONE_FS.

       CLONE_NEWUTS (since Linux 2.6.19)
              If CLONE_NEWUTS is set, then create the process in a new
              UTS namespace, whose identifiers are initialized by
              duplicating the identifiers from the UTS namespace of the
              calling process.  If this flag is not set, then (as with
              fork(2)) the process is created in the same UTS namespace
              as the calling process.

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

              Only a privileged process (CAP_SYS_ADMIN) can employ
              CLONE_NEWUTS.

       CLONE_PARENT (since Linux 2.3.12)
              If CLONE_PARENT is set, then the parent of the new child
              (as returned by getppid(2)) will be the same as that of
              the calling process.

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

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

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

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

       CLONE_PID (Linux 2.0 to 2.5.15)
              If CLONE_PID is set, the child process is created with the
              same process ID as the calling process.  This is good for
              hacking the system, but otherwise of not much use.  From
              Linux 2.3.21 onward, this flag could be specified only by
              the system boot process (PID 0).  The flag disappeared
              completely from the kernel sources in Linux 2.5.16.
              Subsequently, the kernel silently ignored this bit if it
              was specified in the flags mask.  Much later, the same bit
              was recycled for use as the CLONE_PIDFD flag.

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

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

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

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

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

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

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

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

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

       CLONE_SIGHAND (since Linux 2.0)
              If CLONE_SIGHAND is set, the calling process and the child
              process share the same table of signal handlers.  If the
              calling process or child process calls sigaction(2) to
              change the behavior associated with a signal, the behavior
              is changed in the other process as well.  However, the
              calling process and child processes still have distinct
              signal masks and sets of pending signals.  So, one of them
              may block or unblock signals using sigprocmask(2) without
              affecting the other process.

              If CLONE_SIGHAND is not set, the child process inherits a
              copy of the signal handlers of the calling process at the
              time of the clone call.  Calls to sigaction(2) performed
              later by one of the processes have no effect on the other
              process.

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

       CLONE_STOPPED (since Linux 2.6.0)
              If CLONE_STOPPED is set, then the child is initially
              stopped (as though it was sent a SIGSTOP signal), and must
              be resumed by sending it a SIGCONT signal.

              This flag was deprecated from Linux 2.6.25 onward, and was
              removed altogether in Linux 2.6.38.  Since then, the
              kernel silently ignores it without error.  Starting with
              Linux 4.6, the same bit was reused for the CLONE_NEWCGROUP
              flag.

       CLONE_SYSVSEM (since Linux 2.5.10)
              If CLONE_SYSVSEM is set, then the child and the calling
              process share a single list of System V semaphore
              adjustment (semadj) values (see semop(2)).  In this case,
              the shared list accumulates semadj values across all
              processes sharing the list, and semaphore adjustments are
              performed only when the last process that is sharing the
              list terminates (or ceases sharing the list using
              unshare(2)).  If this flag is not set, then the child has
              a separate semadj list that is initially empty.

       CLONE_THREAD (since Linux 2.4.0)
              If CLONE_THREAD is set, the child is placed in the same
              thread group as the calling process.  To make the
              remainder of the discussion of CLONE_THREAD more readable,
              the term "thread" is used to refer to the processes within
              a thread group.

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

              The threads within a group can be distinguished by their
              (system-wide) unique thread IDs (TID).  A new thread's TID
              is available as the function result returned to the
              caller, and a thread can obtain its own TID using
              gettid(2).

              When a clone call is made without specifying CLONE_THREAD,
              then the resulting thread is placed in a new thread group
              whose TGID is the same as the thread's TID.  This thread
              is the leader of the new thread group.

              A new thread created with CLONE_THREAD has the same parent
              process as the process that made the clone call (i.e.,
              like CLONE_PARENT), so that calls to getppid(2) return the
              same value for all of the threads in a thread group.  When
              a CLONE_THREAD thread terminates, the thread that created
              it is not sent a SIGCHLD (or other termination) signal;
              nor can the status of such a thread be obtained using
              wait(2).  (The thread is said to be detached.)

              After all of the threads in a thread group terminate the
              parent process of the thread group is sent a SIGCHLD (or
              other termination) signal.

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

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

              Since Linux 2.5.35, the flags mask must also include
              CLONE_SIGHAND if CLONE_THREAD is specified (and note that,
              since Linux 2.6.0, CLONE_SIGHAND also requires CLONE_VM to
              be included).

              Signal dispositions and actions are process-wide: if an
              unhandled signal is delivered to a thread, then it will
              affect (terminate, stop, continue, be ignored in) all
              members of the thread group.

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

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

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

              If a process-directed signal is delivered to a thread
              group, and the thread group has installed a handler for
              the signal, then the handler will be invoked in exactly
              one, arbitrarily selected member of the thread group that
              has not blocked the signal.  If multiple threads in a
              group are waiting to accept the same signal using
              sigwaitinfo(2), the kernel will arbitrarily select one of
              these threads to receive the signal.

       CLONE_UNTRACED (since Linux 2.5.46)
              If CLONE_UNTRACED is specified, then a tracing process
              cannot force CLONE_PTRACE on this child process.

       CLONE_VFORK (since Linux 2.2)
              If CLONE_VFORK is set, the execution of the calling
              process is suspended until the child releases its virtual
              memory resources via a call to execve(2) or _exit(2) (as
              with vfork(2)).

              If CLONE_VFORK is not set, then both the calling process
              and the child are schedulable after the call, and an
              application should not rely on execution occurring in any
              particular order.

       CLONE_VM (since Linux 2.0)
              If CLONE_VM is set, the calling process and the child
              process run in the same memory space.  In particular,
              memory writes performed by the calling process or by the
              child process are also visible in the other process.
              Moreover, any memory mapping or unmapping performed with
              mmap(2) or munmap(2) by the child or calling process also
              affects the other process.

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

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

RETURN VALUE         top

       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         top

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       EPERM  CLONE_NEWUSER was specified in the flags mask, but either
              the effective user ID or the effective group ID of the
              caller does not have a mapping in the parent namespace
              (see user_namespaces(7)).

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

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

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

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

VERSIONS         top

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

CONFORMING TO         top

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

NOTES         top

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

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

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

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

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

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

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

   C library/kernel differences
       The raw clone() system call corresponds more closely to fork(2)
       in that execution in the child continues from the point of the
       call.  As such, the fn and arg arguments of the clone() wrapper
       function are omitted.

       In contrast to the glibc wrapper, the raw clone() system call
       accepts NULL as a stack argument (and clone3() likewise allows
       cl_args.stack to be NULL).  In this case, the child uses a
       duplicate of the parent's stack.  (Copy-on-write semantics ensure
       that the child gets separate copies of stack pages when either
       process modifies the stack.)  In this case, for correct
       operation, the CLONE_VM option should not be specified.  (If the
       child shares the parent's memory because of the use of the
       CLONE_VM flag, then no copy-on-write duplication occurs and chaos
       is likely to result.)

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

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

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

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

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

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

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

       On the microblaze architecture, an additional argument is
       supplied:

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

   blackfin, m68k, and sparc
       The argument-passing conventions on blackfin, m68k, and sparc are
       different from the descriptions above.  For details, see the
       kernel (and glibc) source.

   ia64
       On ia64, a different interface is used:

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

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

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

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

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

BUGS         top

       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         top

       The following program demonstrates the use of clone() to create a
       child process that executes in a separate UTS namespace.  The
       child changes the hostname in its UTS namespace.  Both parent and
       child then display the system hostname, making it possible to see
       that the hostname differs in the UTS namespaces of the parent and
       child.  For an example of the use of this program, see setns(2).

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

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

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

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

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

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

           /* Change hostname in UTS namespace of child */

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

           /* Retrieve and display hostname */

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

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

           sleep(200);

           return 0;           /* Child terminates now */
       }

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

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

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

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

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

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

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

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

           /* Parent falls through to here */

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

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

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

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

           exit(EXIT_SUCCESS);
       }

SEE ALSO         top

       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         top

       This page is part of release 5.10 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                          2020-11-01                       CLONE(2)

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