clone

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



NAME
       clone, __clone2 - create a child process

SYNOPSIS
       /* Prototype for the glibc wrapper function */

       #define _GNU_SOURCE
       #include <sched.h>

       int clone(int (*fn)(void *), void *child_stack,
                 int flags, void *arg, ...
                 /* pid_t *ptid, void *newtls, pid_t *ctid */ );

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

DESCRIPTION
       clone() creates a new process, in a manner similar to fork(2).

       This page describes both 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.

       Unlike fork(2), clone() allows the child process to share parts of its
       execution context with the calling process, such as the virtual address
       space, the table of file descriptors, and the table of signal handlers.
       (Note that on this manual page, "calling process" normally corresponds
       to "parent process".  But see the description of CLONE_PARENT below.)

       One use of clone() is to implement threads: multiple flows of control
       in a program that run concurrently in a shared address space.

       When the child process is created with clone(), 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 child_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 child_stack usually points to the topmost
       address of the memory space set up for the child stack.

       The low byte of flags contains the number of the termination signal
       sent to the parent when the child dies.  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 is specified, then the parent process is not signaled when
       the child terminates.

       flags may also be bitwise-ORed with zero or more of the following
       constants, in order to specify what is shared between the calling
       process and the child process:

       CLONE_CHILD_CLEARTID (since Linux 2.5.49)
              Clear (zero) the child thread ID at the location ctid 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 ctid in the child's
              memory.  The store operation completes before clone() returns
              control to user space in the child process.  (Note that the
              store operation may not have completed before clone() returns in
              the parent process, which will be relevant if the CLONE_VM flag
              is also employed.)

       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 clone().  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_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.  This flag is
              intended for the implementation of containers.

              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.  This flag is intended for the implementation of
              containers.

              An IPC namespace provides an isolated view of System V IPC
              objects (see sysvipc(7)) and (since Linux 2.6.30) POSIX message
              queues (see mq_overview(7)).  The common characteristic of these
              IPC mechanisms is that IPC objects are identified by mechanisms
              other than filesystem pathnames.

              Objects created in an IPC namespace are visible to all other
              processes that are members of that namespace, but are not
              visible to processes in other IPC namespaces.

              When an IPC namespace is destroyed (i.e., when the last process
              that is a member of the namespace terminates), all IPC objects
              in the namespace are automatically destroyed.

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

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

       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.  This flag is intended for the implementation of
              containers.

              A network namespace provides an isolated view of the networking
              stack (network device interfaces, IPv4 and IPv6 protocol stacks,
              IP routing tables, firewall rules, the /proc/net and
              /sys/class/net directory trees, sockets, etc.).  A physical
              network device can live in exactly one network namespace.  A
              virtual network (veth(4)) device pair provides a pipe-like
              abstraction that can be used to create tunnels between network
              namespaces, and can be used to create a bridge to a physical
              network device in another namespace.

              When a network namespace is freed (i.e., when the last process
              in the namespace terminates), its physical network devices are
              moved back to the initial network namespace (not to the parent
              of the process).  For further information on network namespaces,
              see 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.

              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.

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

       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.  This flag is intended for the implementation of
              containers.

              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.

              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.

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

       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.  This
              flag is intended for the implementation of containers.

              A UTS namespace is the set of identifiers returned by uname(2);
              among these, the domain name and the hostname can be modified by
              setdomainname(2) and sethostname(2), respectively.  Changes made
              to the identifiers in a UTS namespace are visible to all other
              processes in the same namespace, but are not visible to
              processes in other UTS namespaces.

              Only a privileged process (CAP_SYS_ADMIN) can employ
              CLONE_NEWUTS.

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

       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.

       CLONE_PARENT_SETTID (since Linux 2.5.49)
              Store the child thread ID at the location ptid 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 clone()
              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.  Since then, the kernel silently
              ignores this bit if it is specified in flags.

       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 newtls.

              The interpretation of newtls and the resulting effect is
              architecture dependent.  On x86, newtls 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.

       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
              clone() is called.  Calls to sigaction(2) performed later by one
              of the processes have no effect on the other process.

              Since Linux 2.6.0, flags 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 of
              clone(), and a thread can obtain its own TID using gettid(2).

              When a call is made to clone() 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 caller of clone() (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 using clone() 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, flags 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
              clone().  Memory writes or file mappings/unmappings performed by
              one of the processes do not affect the other, as with fork(2).

NOTES
       Note that the glibc clone() wrapper function makes some changes in the
       memory pointed to by child_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.

       Another difference for the raw clone() system call is that the
       child_stack argument may be NULL, in which 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, ia-64, and alpha) is:

           long clone(unsigned long flags, void *child_stack,
                      int *ptid, int *ctid,
                      unsigned long newtls);

       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 *child_stack,
                     int *ptid, unsigned long newtls,
                     int *ctid);

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

           long clone(void *child_stack, unsigned long flags,
                      int *ptid, int *ctid,
                      unsigned long newtls);

       On the microblaze architecture, an additional argument is supplied:

           long clone(unsigned long flags, void *child_stack,
                      int stack_size,         /* Size of stack */
                      int *ptid, int *ctid,
                      unsigned long newtls);

   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 *child_stack_base, size_t stack_size,
                        int flags, void *arg, ...
                     /* pid_t *ptid, struct user_desc *tls, pid_t *ctid */ );

       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 *child_stack_base,
                       int stack_size,         /* Size of stack */
                       int *ptid, int *ctid,
                       unsigned long tls);

       __clone2() operates in the same way as clone(), except that
       child_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
       child_stack_base.

   Linux 2.4 and earlier
       In Linux 2.4 and earlier, clone() does not take arguments ptid, tls,
       and ctid.

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).

       EINVAL CLONE_SIGHAND was specified, but CLONE_VM was not.  (Since Linux
              2.6.0.)

       EINVAL CLONE_THREAD was specified, but CLONE_SIGHAND was not.  (Since
              Linux 2.5.35.)

       EINVAL CLONE_THREAD was specified, 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 flags.

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

       EINVAL Both CLONE_NEWIPC and CLONE_SYSVSEM were specified in flags.

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

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

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

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

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

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

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

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

       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 flags, 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 flags, 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 flags 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 flags, 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 flags 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 flags, and the limit on the
              number of nested user namespaces would be exceeded.  See the
              discussion of the ENOSPC error above.

CONFORMING TO
       clone() is Linux-specific and should not be used in programs intended
       to be portable.

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

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

       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).

       For a while there was CLONE_DETACHED (introduced in 2.5.32): parent
       wants no child-exit signal.  In Linux 2.6.2, the need to give this flag
       together with CLONE_THREAD disappeared.  This flag is still defined,
       but has no effect.

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

BUGS
       GNU C library versions 2.3.4 up to and including 2.24 contained a
       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
       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).

   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>

       #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 stack for child */

           stack = malloc(STACK_SIZE);
           if (stack == NULL)
               errExit("malloc");
           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), 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 5.02 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                             2019-08-02                          CLONE(2)