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
       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 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).

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       On the microblaze architecture, an additional argument is supplied:

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

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

   ia64
       On ia64, a different interface is used:

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

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

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

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

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

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

           #include <syscall.h>

           pid_t mypid;

           mypid = syscall(SYS_getpid);

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

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

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

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

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

   Program source
       #define _GNU_SOURCE
       #include <sys/wait.h>
       #include <sys/utsname.h>
       #include <sched.h>
       #include <string.h>
       #include <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 5.07 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                             2020-06-09                          CLONE(2)