CLONE(2) Linux Programmer's Manual CLONE(2)
NAME
clone, __clone2 - create a child process
SYNOPSIS
/* Prototype for the glibc wrapper function */
#include <sched.h>
int clone(int (*fn)(void *), void *child_stack,
int flags, void *arg, ...
/* pid_t *ptid, struct user_desc *tls, pid_t *ctid */ );
/* Prototype for the raw system call */
long clone(unsigned long flags, void *child_stack,
void *ptid, void *ctid,
struct pt_regs *regs);
Feature Test Macro Requirements for glibc wrapper function (see fea-
ture_test_macros(7)):
clone():
Since glibc 2.14:
_GNU_SOURCE
Before glibc 2.14:
_BSD_SOURCE || _SVID_SOURCE
/* _GNU_SOURCE also suffices */
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 memory 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.)
The main use of clone() is to implement threads: multiple threads of
control in a program that run concurrently in a shared memory space.
When the child process is created with clone(), it executes the func-
tion fn(arg). (This differs from fork(2), where execution continues in
the child from the point of the fork(2) call.) The fn argument is a
pointer to a function that is called by the child process at the begin-
ning of its execution. The arg argument is passed to the fn function.
When the fn(arg) function application returns, the child process termi-
nates. The integer returned by fn is the exit code 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 mem-
ory, 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-or'ed with zero or more of the following con-
stants, in order to specify what is shared between the calling process
and the child process:
CLONE_CHILD_CLEARTID (since Linux 2.5.49)
Erase child thread ID at 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 child thread ID at location ctid in child memory.
CLONE_FILES (since Linux 2.0)
If CLONE_FILES is set, the calling process and the child process
share the same file descriptor table. Any file descriptor cre-
ated by the calling process or by the child process is also
valid in the other process. Similarly, if one of the processes
closes a file descriptor, or changes its associated flags (using
the fcntl(2) F_SETFD operation), the other process is also
affected.
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(). (The duplicated file descriptors in the child refer
to the same open file descriptions (see open(2)) as the corre-
sponding file descriptors in the calling process.) 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.
CLONE_FS (since Linux 2.0)
If CLONE_FS is set, the caller and the child process share the
same file system information. This includes the root of the
file system, the current working directory, and the umask. Any
call to chroot(2), chdir(2), or umask(2) performed by the call-
ing process or the child process also affects the other process.
If CLONE_FS is not set, the child process works on a copy of the
file system information of the calling process at the time of
the clone() call. Calls to chroot(2), chdir(2), umask(2) per-
formed 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_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 con-
tainers.
An IPC namespace provides an isolated view of System V IPC
objects (see svipc(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 visi-
ble 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.
Use of this flag requires: a kernel configured with the CON-
FIG_SYSVIPC and CONFIG_IPC_NS options and that the process be
privileged (CAP_SYS_ADMIN). 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. This flag is intended for the implementation of con-
tainers.
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 net-
work device can live in exactly one network namespace. A vir-
tual network device ("veth") pair provides a pipe-like abstrac-
tion that can be used to create tunnels between network names-
paces, 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).
Use of this flag requires: a kernel configured with the CON-
FIG_NET_NS option and that the process be privileged
(CAP_SYS_ADMIN).
CLONE_NEWNS (since Linux 2.4.19)
Start the child in a new mount namespace.
Every process lives in a mount namespace. The namespace of a
process is the data (the set of mounts) describing the file
hierarchy as seen by that process. After a fork(2) or clone()
where the CLONE_NEWNS flag is not set, the child lives in the
same mount namespace as the parent. The system calls mount(2)
and umount(2) change the mount namespace of the calling process,
and hence affect all processes that live in the same namespace,
but do not affect processes in a different mount namespace.
After a clone() where the CLONE_NEWNS flag is set, the cloned
child is started in a new mount namespace, initialized with a
copy of the namespace of the parent.
Only a privileged process (one having the CAP_SYS_ADMIN capabil-
ity) may specify the CLONE_NEWNS flag. 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. This flag is intended for the implementation of con-
tainers.
A PID namespace provides an isolated environment for PIDs: PIDs
in a new namespace start at 1, somewhat like a standalone sys-
tem, and calls to fork(2), vfork(2), or clone() will produce
processes with PIDs that are unique within the namespace.
The first process created in a new namespace (i.e., the process
created using the CLONE_NEWPID flag) has the PID 1, and is the
"init" process for the namespace. Children that are orphaned
within the namespace will be reparented to this process rather
than init(8). Unlike the traditional init process, the "init"
process of a PID namespace can terminate, and if it does, all of
the processes in the namespace are terminated.
PID namespaces form a hierarchy. When a new PID namespace is
created, the processes in that namespace are visible in the PID
namespace of the process that created the new namespace; analo-
gously, if the parent PID namespace is itself the child of
another PID namespace, then processes in the child and parent
PID namespaces will both be visible in the grandparent PID
namespace. Conversely, the processes in the "child" PID names-
pace do not see the processes in the parent namespace. The
existence of a namespace hierarchy means that each process may
now have multiple PIDs: one for each namespace in which it is
visible; each of these PIDs is unique within the corresponding
namespace. (A call to getpid(2) always returns the PID associ-
ated with the namespace in which the process lives.)
After creating the new namespace, it is useful for the child to
change its root directory and mount a new procfs instance at
/proc so that tools such as ps(1) work correctly. (If
CLONE_NEWNS is also included in flags, then it isn't necessary
to change the root directory: a new procfs instance can be
mounted directly over /proc.)
Use of this flag requires: a kernel configured with the CON-
FIG_PID_NS option and that the process be privileged
(CAP_SYS_ADMIN). This flag can't be specified in conjunction
with CLONE_THREAD.
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 host name 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.
Use of this flag requires: a kernel configured with the CON-
FIG_UTS_NS option and that the process be privileged
(CAP_SYS_ADMIN).
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 child thread ID at location ptid in parent and child mem-
ory. (In Linux 2.5.32-2.5.48 there was a flag CLONE_SETTID that
did this.)
CLONE_PID (obsolete)
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. Since 2.3.21 this flag
can be specified only by the system boot process (PID 0). It
disappeared in Linux 2.5.16.
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 newtls argument is the new TLS (Thread Local Storage)
descriptor. (See set_thread_area(2).)
CLONE_SIGHAND (since Linux 2.0)
If CLONE_SIGHAND is set, the calling process and the child
process share the same table of signal handlers. If the calling
process or child process calls sigaction(2) to change the behav-
ior associated with a signal, the behavior is changed in the
other process as well. However, the calling process and child
processes still have distinct signal masks and sets of pending
signals. So, one of them may block or unblock some 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-test6, flags must also include CLONE_VM if
CLONE_SIGHAND is specified
CLONE_STOPPED (since Linux 2.6.0-test2)
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.
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 undo values (see
semop(2)). If this flag is not set, then the child has a sepa-
rate undo list, which is initially empty.
CLONE_THREAD (since Linux 2.4.0-test8)
If CLONE_THREAD is set, the child is placed in the same thread
group as the calling process. To make the remainder of the dis-
cussion of CLONE_THREAD more readable, the term "thread" is used
to refer to the processes within a thread group.
Thread groups were a feature added in Linux 2.4 to support the
POSIX threads notion of a set of threads that share a single
PID. Internally, this shared PID is the so-called thread group
identifier (TGID) for the thread group. Since Linux 2.4, calls
to getpid(2) return the TGID of the caller.
The threads within a group can be distinguished by their (sys-
tem-wide) unique thread IDs (TID). A new thread's TID is avail-
able as the function result returned to the caller 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 termi-
nates, 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 termina-
tion) signal.
If any of the threads in a thread group performs an execve(2),
then all threads other than the thread group leader are termi-
nated, and the new program is executed in the thread group
leader.
If one of the threads in a thread group creates a child using
fork(2), then any thread in the group can wait(2) for that
child.
Since Linux 2.5.35, flags must also include CLONE_SIGHAND if
CLONE_THREAD is specified.
Signals may be sent to a thread group as a whole (i.e., a TGID)
using kill(2), or to a specific thread (i.e., TID) using
tgkill(2).
Signal dispositions and actions are process-wide: if an unhan-
dled signal is delivered to a thread, then it will affect (ter-
minate, stop, continue, be ignored in) all members of the thread
group.
Each thread has its own signal mask, as set by sigprocmask(2),
but signals can be pending either: for the whole process (i.e.,
deliverable to any member of the thread group), when sent with
kill(2); or for an individual thread, when sent with tgkill(2).
A call to sigpending(2) returns a signal set that is the union
of the signals pending for the whole process and the signals
that are pending for the calling thread.
If kill(2) is used to send a signal to a thread group, and the
thread group has installed a handler for the signal, then the
handler will be invoked in exactly one, arbitrarily selected
member of the thread group that has not blocked the signal. If
multiple threads in a group are waiting to accept the same sig-
nal using sigwaitinfo(2), the kernel will arbitrarily select one
of these threads to receive a signal sent using kill(2).
CLONE_UNTRACED (since Linux 2.5.46)
If CLONE_UNTRACED is specified, then a tracing process cannot
force CLONE_PTRACE on this child process.
CLONE_VFORK (since Linux 2.2)
If CLONE_VFORK is set, the execution of the calling process is
suspended until the child releases its virtual memory resources
via a call to execve(2) or _exit(2) (as with vfork(2)).
If CLONE_VFORK is not set then both the calling process and the
child are schedulable after the call, and an application should
not rely on execution occurring in any particular order.
CLONE_VM (since Linux 2.0)
If CLONE_VM is set, the calling process and the child process
run in the same memory space. In particular, memory writes per-
formed by the calling process or by the child process are also
visible in the other process. Moreover, any memory mapping or
unmapping performed with mmap(2) or munmap(2) by the child or
calling process also affects the other process.
If CLONE_VM is not set, the child process runs in a separate
copy of the memory space of the calling process at the time of
clone(). Memory writes or file mappings/unmappings performed by
one of the processes do not affect the other, as with fork(2).
The raw system call interface
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.
Furthermore, the argument order changes. The raw system call interface
on x86 and many other architectures is roughly:
long clone(unsigned long flags, void *child_stack,
void *ptid, void *ctid,
struct pt_regs *regs);
Another difference for the raw system call is that the child_stack
argument may be zero, in which case copy-on-write semantics ensure that
the child gets separate copies of stack pages when either process modi-
fies the stack. In this case, for correct operation, the CLONE_VM
option should not be specified.
For some architectures, the order of the arguments for the system call
differs from that shown above. On the score, microblaze, ARM, ARM 64,
PA-RISC, arc, Power PC, xtensa, and MIPS architectures, the order of
the fourth and fifth arguments is reversed. On the cris and s390
architectures, the order of the first and second arguments is reversed.
blackfin, m68k, and sparc
The argument-passing conventions on blackfin, m68k, and sparc are dif-
ferent from 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; the raw
system call interface has no fn or arg argument, and changes the order
of the arguments so that flags is the first argument, and tls is the
last argument.
__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 call-
er'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 appro-
priately.
ERRORS
EAGAIN Too many processes are already running.
EINVAL CLONE_SIGHAND was specified, but CLONE_VM was not. (Since Linux
2.6.0-test6.)
EINVAL CLONE_THREAD was specified, but CLONE_SIGHAND was not. (Since
Linux 2.5.35.)
EINVAL Both CLONE_FS and CLONE_NEWNS were specified in flags.
EINVAL Both CLONE_NEWIPC and CLONE_SYSVSEM were specified in flags.
EINVAL Both CLONE_NEWPID and CLONE_THREAD were specified in flags.
EINVAL Returned by clone() when a zero value is specified for
child_stack.
EINVAL CLONE_NEWIPC was specified in flags, but the kernel was not con-
figured with the CONFIG_SYSVIPC and CONFIG_IPC_NS options.
EINVAL CLONE_NEWNET was specified in flags, but the kernel was not con-
figured with the CONFIG_NET_NS option.
EINVAL CLONE_NEWPID was specified in flags, but the kernel was not con-
figured with the CONFIG_PID_NS option.
EINVAL CLONE_NEWUTS was specified in flags, but the kernel was not con-
figured with the CONFIG_UTS option.
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.
EPERM 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.
VERSIONS
There is no entry for clone() in libc5. glibc2 provides clone() as
described in this manual page.
CONFORMING TO
clone() is Linux-specific and should not be used in programs intended
to be portable.
NOTES
In the kernel 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 kernel 2.6).
For a while there was CLONE_DETACHED (introduced in 2.5.32): parent
wants no child-exit signal. In 2.6.2 the need to give this 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
Versions of the GNU C library that include the NPTL threading library
contain a wrapper function for getpid(2) that performs caching of PIDs.
This caching relies on support in the glibc wrapper for clone(), but as
currently implemented, the cache may not be up to date in some circum-
stances. In particular, if a signal is delivered to the child immedi-
ately after the clone() call, then a call to getpid(2) in a handler for
the signal may return the PID of the calling process ("the parent"), if
the clone wrapper has not yet had a chance to update the PID cache in
the child. (This discussion ignores the case where the child was cre-
ated 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 may be necessary to use code such as the following:
#include <syscall.h>
pid_t mypid;
mypid = syscall(SYS_getpid);
EXAMPLE
Create a child that executes in a separate UTS namespace
The following program demonstrates the use of clone() to create a child
process that executes in a separate UTS namespace. The child changes
the hostname in its UTS namespace. Both parent and child then display
the system hostname, making it possible to see that the hostname dif-
fers in the UTS namespaces of the parent and child. For an example of
the use of this program, see setns(2).
#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), capabili-
ties(7), pthreads(7)
COLOPHON
This page is part of release 3.53 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
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