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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 description of CLONE_PARENT 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 */
           };

       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 explic‐
       itly 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 indi‐
       vidually, 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()         clone(3)        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

   The child termination signal
       When the child process terminates, a signal may be sent to the par‐
       ent.  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 flags mask
       Both clone() and clone3() allow a flags bit mask that modifies their
       behavior and allows the caller to specify what is shared between the
       calling process and the child process.  This bit mask—the flags argu‐
       ment of clone() or the cl_args.flags field passed to clone3()—is
       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_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, per‐
              formed by either the calling process or the child process do
              not affect the other process.  Note, however, that the dupli‐
              cated 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_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 pro‐
              cesses share an I/O context, they will be allowed to inter‐
              leave 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 call‐
              ing 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 net‐
              work 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 call‐
              ing 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 com‐
              pletely 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 call‐
              ing 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 call‐
              er 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 call‐
              ing 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.

       CLONE_PARENT_SETTID (since Linux 2.5.49)
              Store the child thread ID at the location pointed to by par‐
              ent_tid (clone()) or cl_args.child_tid (clone3()) in the par‐
              ent's memory.  (In Linux 2.5.32-2.5.48 there was a flag
              CLONE_SETTID that did this.)  The store operation completes
              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 loca‐
              tion in the parent's memory.  The close-on-exec flag is set on
              this new file descriptor.  PID file descriptors can be used
              for the purposes described in pidfd_open(2).

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

              *  When using clone(), the PID file descriptor is placed at
                 the location pointed to by parent_tid.  Since the par‐
                 ent_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 corre‐
              sponding 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 archi‐
              tecture 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 call‐
              ing 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 (sys‐
              tem-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 par‐
              ent 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 termi‐
              nated, and the new program is executed in the thread group
              leader.

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

              Since Linux 2.5.35, the flags mask must also include
              CLONE_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 unhan‐
              dled 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 sig‐
              nal 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 excep‐
              tion (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 arbi‐
              trarily 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 map‐
              ping 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).

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.

   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 argu‐
       ments is reversed:

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

       On the microblaze architecture, an additional argument is supplied:

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

   blackfin, m68k, and sparc
       The argument-passing conventions on blackfin, m68k, and sparc are
       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.

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).

       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 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.

       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).

       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).

       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

       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.

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.

EXAMPLE         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 <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         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 5.04 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                            2019-11-19                         CLONE(2)

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