.\" 2008-11-19, mtk, document CLONE_NEWIPC
.\" 2008-11-19, Jens Axboe, mtk, document CLONE_IO
.\"
-.TH CLONE 2 2014-08-19 "Linux" "Linux Programmer's Manual"
+.TH CLONE 2 2016-12-12 "Linux" "Linux Programmer's Manual"
.SH NAME
clone, __clone2 \- create a child process
.SH SYNOPSIS
.nf
/* Prototype for the glibc wrapper function */
+.B #define _GNU_SOURCE
.B #include <sched.h>
.BI "int clone(int (*" "fn" ")(void *), void *" child_stack ,
.BI " int " flags ", void *" "arg" ", ... "
-.BI " /* pid_t *" ptid ", struct user_desc *" tls \
+.BI " /* pid_t *" ptid ", void *" newtls \
", pid_t *" ctid " */ );"
-/* Prototype for the raw system call */
-
-.BI "long clone(unsigned long " flags ", void *" child_stack ,
-.BI " void *" ptid ", void *" ctid ,
-.BI " struct pt_regs *" regs );
+/* For the prototype of the raw system call, see NOTES */
.fi
-.sp
-.in -4n
-Feature Test Macro Requirements for glibc wrapper function (see
-.BR feature_test_macros (7)):
-.in
-.sp
-.BR clone ():
-.ad l
-.RS 4
-.PD 0
-.TP 4
-Since glibc 2.14:
-_GNU_SOURCE
-.TP 4
-.\" See http://sources.redhat.com/bugzilla/show_bug.cgi?id=4749
-Before glibc 2.14:
-_BSD_SOURCE || _SVID_SOURCE
- /* _GNU_SOURCE also suffices */
-.PD
-.RE
-.ad b
.SH DESCRIPTION
.BR clone ()
creates a new process, in a manner similar to
.B CLONE_PARENT
below.)
-The main use of
+One use of
.BR clone ()
is to implement threads: multiple threads of control in a program that
run concurrently in a shared memory space.
and the child process:
.TP
.BR CLONE_CHILD_CLEARTID " (since Linux 2.5.49)"
-Erase child thread ID at location
+Clear (zero) the child thread ID at the location
.I ctid
in child memory when the child exits, and do a wakeup on the futex
at that address.
This is used by threading libraries.
.TP
.BR CLONE_CHILD_SETTID " (since Linux 2.5.49)"
-Store child thread ID at location
+Store the child thread ID at the location
.I ctid
-in child memory.
+in the child's memory.
+The store operation completes before
+.BR clone ()
+returns control to user space.
.TP
.BR CLONE_FILES " (since Linux 2.0)"
If
.BR fcntl (2)
.B F_SETFD
operation), the other process is also affected.
+If a process sharing a file descriptor table calls
+.BR execve (2),
+its file descriptor table is duplicated (unshared).
If
.B CLONE_FILES
is not set, the child process inherits a copy of all file descriptors
opened in the calling process at the time of
.BR clone ().
-(The duplicated file descriptors in the child refer to the
-same open file descriptions (see
-.BR open (2))
-as the corresponding 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.
+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
+.BR open (2)).
.TP
.BR CLONE_FS " (since Linux 2.0)"
If
the new process has its own I/O context.
.\" The following based on text from Jens Axboe
-The I/O context is the I/O scope of the disk scheduler (i.e,
+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.
.B CONFIG_BLOCK
option, this flag is a no-op.
.TP
+.BR CLONE_NEWCGROUP " (since Linux 4.6)"
+Create the process in a new cgroup namespace.
+If this flag is not set, then (as with
+.BR 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
+.BR cgroup_namespaces (7).
+
+Only a privileged process
+.RB ( CAP_SYS_ADMIN )
+can employ
+.BR CLONE_NEWCGROUP .
+.\"
+.TP
.BR CLONE_NEWIPC " (since Linux 2.6.19)"
If
.B CLONE_NEWIPC
(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
-that the process be privileged
-.RB ( CAP_SYS_ADMIN ).
+Only a privileged process
+.RB ( CAP_SYS_ADMIN )
+can employ
+.BR CLONE_NEWIPC .
This flag can't be specified in conjunction with
.BR CLONE_SYSVSEM .
(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
+.BR namespaces (7).
-Use of this flag requires: a kernel configured with the
-.B CONFIG_NET_NS
-option and that the process be privileged
-.RB ( CAP_SYS_ADMIN ).
+Only a privileged process
+.RB ( CAP_SYS_ADMIN )
+can employ
+.BR CLONE_NEWNET .
.TP
.BR CLONE_NEWNS " (since Linux 2.4.19)"
-Start the child in a new mount namespace.
-
-Every process lives in a mount namespace.
-The
-.I namespace
-of a process is the data (the set of mounts) describing the file hierarchy
-as seen by that process.
-After a
-.BR fork (2)
-or
-.BR clone ()
-where the
-.B CLONE_NEWNS
-flag is not set, the child lives in the same mount
-namespace as the parent.
-The system calls
-.BR mount (2)
-and
-.BR 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
-.BR clone ()
-where the
+If
.B CLONE_NEWNS
-flag is set, the cloned child is started in a new mount namespace,
+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 \fBCAP_SYS_ADMIN\fP capability)
-may specify the
+If
.B CLONE_NEWNS
-flag.
+is not set, the child lives in the same mount
+namespace as the parent.
+
+Only a privileged process
+.RB ( CAP_SYS_ADMIN )
+can employ
+.BR CLONE_NEWNS .
It is not permitted to specify both
.B CLONE_NEWNS
and
.B CLONE_FS
+.\" See https://lwn.net/Articles/543273/
in the same
.BR clone ()
call.
+
+For further information on mount namespaces, see
+.BR namespaces (7)
+and
+.BR mount_namespaces (7).
+.TP
+.BR CLONE_NEWPID " (since Linux 2.6.24)"
+.\" This explanation draws a lot of details from
+.\" http://lwn.net/Articles/259217/
+.\" Authors: Pavel Emelyanov <xemul@openvz.org>
+.\" and Kir Kolyshkin <kir@openvz.org>
+.\"
+.\" The primary kernel commit is 30e49c263e36341b60b735cbef5ca37912549264
+.\" Author: Pavel Emelyanov <xemul@openvz.org>
+If
+.B CLONE_NEWPID
+is set, then create the process in a new PID namespace.
+If this flag is not set, then (as with
+.BR 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
+.BR namespaces (7)
+and
+.BR pid_namespaces (7).
+
+Only a privileged process
+.RB ( CAP_SYS_ADMIN )
+can employ
+.BR CLONE_NEWPID .
+This flag can't be specified in conjunction with
+.BR CLONE_THREAD
+or
+.BR CLONE_PARENT .
.TP
.BR CLONE_NEWUSER
(This flag first became meaningful for
.BR clone ()
in Linux 2.6.23,
the current
-.BR clone()
+.BR 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.)
.BR fork (2))
the process is created in the same user namespace as the calling process.
-A user namespace provides an isolated environment for
-security related identifiers, in particular,
-user IDs, group IDs, keys (see
-.BR keyctl (2)),
-and capabilities.
-
-When a user namespace is created,
-it starts out without a mapping of user IDs (group IDs)
-to the parent user namespace.
-The desired mapping of user IDs (group IDs) to the parent user namespace
-may be set by writing into
-.IR /proc/[pid]/uid_map
-.RI ( /proc/[pid]/gid_map );
-see
-.BR proc (5).
-
-The first process in a user namespace starts out with a complete set
-of capabilities with respect to the new user namespace.
-
-System calls that return user IDs (group IDs) will return
-either the user ID (group ID) mapped into the current
-user namespace if there is a mapping, or the overflow user ID (group ID);
-the default value for the overflow user ID (group ID) is 65534.
-See the descriptions of
-.IR /proc/sys/kernel/overflowuid
+For further information on user namespaces, see
+.BR namespaces (7)
and
-.IR /proc/sys/kernel/overflowgid
-in
-.BR proc (5).
+.BR user_namespaces (7)
-Use of this flag requires a kernel configured with the
-.BR CONFIG_USER_NS
-option.
Before Linux 3.8, use of
.BR CLONE_NEWUSER
required that the caller have three capabilities:
.BR CAP_SETGID .
.\" Before Linux 2.6.29, it appears that only CAP_SYS_ADMIN was needed
Starting with Linux 3.8,
-no privileges are needed to create a user namespace,
-and mount, PID, IPC, network, and UTS namespaces can be created with just the
-.B CAP_SYS_ADMIN
-capability in the caller's user namespace.
+no privileges are needed to create a user namespace.
-If
-.BR CLONE_NEWUSER
-is specified along with other
-.B CLONE_NEW*
-flags in a single
-.BR clone()
-call, the user namespace is guaranteed to be created first,
-giving the caller privileges over the remaining
-namespaces created by the call.
-Thus, it possible for an unprivileged caller to specify this combination
-of flags.
-
-Over the years, there have been a lot of features that have been added
-to the Linux kernel that are only available to privileged users
-because of their potential to confuse set-user-ID-root applications.
-In general, it becomes safe to allow the root user in a user namespace to
-use those features because it is impossible, while in a user namespace,
-to gain more privilege than the root user of a user namespace has.
-
-.TP
-.BR CLONE_NEWPID " (since Linux 2.6.24)"
-.\" This explanation draws a lot of details from
-.\" http://lwn.net/Articles/259217/
-.\" Authors: Pavel Emelyanov <xemul@openvz.org>
-.\" and Kir Kolyshkin <kir@openvz.org>
-.\"
-.\" The primary kernel commit is 30e49c263e36341b60b735cbef5ca37912549264
-.\" Author: Pavel Emelyanov <xemul@openvz.org>
-If
-.B CLONE_NEWPID
-is set, then create the process in a new PID namespace.
-If this flag is not set, then (as with
-.BR fork (2))
-the process is created in the same PID namespace as
-the calling process.
-This flag is intended for the implementation of containers.
-
-A PID namespace provides an isolated environment for PIDs:
-PIDs in a new namespace start at 1,
-somewhat like a standalone system, and calls to
-.BR fork (2),
-.BR vfork (2),
+This flag can't be specified in conjunction with
+.BR CLONE_THREAD
or
-.BR 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
-.BR 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
-.BR init (8).
-Unlike the traditional
-.B 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;
-analogously, 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 namespace 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
-.BR getpid (2)
-always returns the PID associated 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
-.I /proc
-so that tools such as
-.BR ps (1)
-work correctly.
-.\" mount -t proc proc /proc
-(If
-.BR CLONE_NEWNS
-is also included in
-.IR flags ,
-then it isn't necessary to change the root directory:
-a new procfs instance can be mounted directly over
-.IR /proc .)
+.BR CLONE_PARENT .
+For security reasons,
+.\" commit e66eded8309ebf679d3d3c1f5820d1f2ca332c71
+.\" https://lwn.net/Articles/543273/
+.\" The fix actually went into 3.9 and into 3.8.3. However, user namespaces
+.\" were, for practical purposes, unusable in earlier 3.8.x because of the
+.\" various filesystems that didn't support userns.
+.BR CLONE_NEWUSER
+cannot be specified in conjunction with
+.BR CLONE_FS .
-Use of this flag requires: a kernel configured with the
-.B CONFIG_PID_NS
-option and that the process be privileged
-.RB ( CAP_SYS_ADMIN ).
-This flag can't be specified in conjunction with
-.BR CLONE_THREAD .
+For further information on user namespaces, see
+.BR user_namespaces (7).
.TP
.BR CLONE_NEWUTS " (since Linux 2.6.19)"
If
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
-.B CONFIG_UTS_NS
-option and that the process be privileged
-.RB ( CAP_SYS_ADMIN ).
+Only a privileged process
+.RB ( CAP_SYS_ADMIN )
+can employ
+.BR CLONE_NEWUTS .
+
+For further information on UTS namespaces, see
+.BR namespaces (7).
.TP
.BR CLONE_PARENT " (since Linux 2.3.12)"
If
calling process itself, will be signaled.
.TP
.BR CLONE_PARENT_SETTID " (since Linux 2.5.49)"
-Store child thread ID at location
+Store the child thread ID at the location
.I ptid
-in parent and child memory.
+in the parent's memory.
(In Linux 2.5.32-2.5.48 there was a flag
.B CLONE_SETTID
that did this.)
+The store operation completes before
+.BR clone ()
+returns control to user space.
.TP
.BR CLONE_PID " (obsolete)"
If
Since 2.3.21 this flag can be
specified only by the system boot process (PID 0).
It disappeared in Linux 2.5.16.
+Since then, the kernel silently ignores it without error.
.TP
.BR CLONE_PTRACE " (since Linux 2.2)"
If
.BR ptrace (2)).
.TP
.BR CLONE_SETTLS " (since Linux 2.5.32)"
-The
+The TLS (Thread Local Storage) descriptor is set to
+.I newtls.
+
+The interpretation of
+.I newtls
+and the resulting effect is architecture dependent.
+On x86,
.I newtls
-argument is the new TLS (Thread Local Storage) descriptor.
+is interpreted as a
+.IR "struct user_desc *"
(See
-.BR set_thread_area (2).)
+.BR set_thread_area (2)).
+On x86_64 it is the new value to be set for the %fs base register
+(See the
+.I ARCH_SET_FS
+argument to
+.BR arch_prctl (2)).
+On architectures with a dedicated TLS register, it is the new value
+of that register.
.TP
.BR CLONE_SIGHAND " (since Linux 2.0)"
If
and was
.I removed
altogether in Linux 2.6.38.
+Since then, the kernel silently ignores it without error.
.\" glibc 2.8 removed this defn from bits/sched.h
+Starting with Linux 4.6, the same bit was reused for the
+.BR CLONE_NEWCGROUP
+flag.
.TP
.BR CLONE_SYSVSEM " (since Linux 2.5.10)"
If
.B CLONE_SYSVSEM
is set, then the child and the calling process share
-a single list of System\ V semaphore undo values (see
+a single list of System V semaphore adjustment
+.RI ( semadj )
+values (see
.BR semop (2)).
-If this flag is not set, then the child has a separate undo list,
-which is initially empty.
+In this case, the shared list accumulates
+.I 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
+.BR unshare (2)).
+If this flag is not set, then the child has a separate
+.I semadj
+list that is initially empty.
.TP
.BR CLONE_THREAD " (since Linux 2.4.0-test8)"
If
Memory writes or file mappings/unmappings performed by one of the
processes do not affect the other, as with
.BR fork (2).
-.SS C library/kernel ABI differences
+.SS C library/kernel differences
The raw
.BR clone ()
system call corresponds more closely to
.BR clone ()
wrapper function are omitted.
Furthermore, the argument order changes.
-The raw system call interface on x86 and many other architectures is roughly:
+In addition, there are variations across architectures.
+
+The raw system call interface on x86-64 and some other architectures
+(including sh, tile, and alpha) is roughly:
+
.in +4
.nf
+.BI "long clone(unsigned long " flags ", void *" child_stack ,
+.BI " int *" ptid ", int *" ctid ,
+.BI " unsigned long " newtls );
+.fi
+.in
+
+On x86-32, and several other common architectures
+(including score, ARM, ARM 64, PA-RISC, arc, Power PC, xtensa,
+and MIPS),
+.\" CONFIG_CLONE_BACKWARDS
+the order of the last two arguments is reversed:
+.in +4
+.nf
.BI "long clone(unsigned long " flags ", void *" child_stack ,
-.BI " void *" ptid ", void *" ctid ,
-.BI " struct pt_regs *" regs );
+.BI " int *" ptid ", unsigned long " newtls ,
+.BI " int *" ctid );
+.fi
+.in
+On the cris and s390 architectures,
+.\" CONFIG_CLONE_BACKWARDS2
+the order of the first two arguments is reversed:
+
+.in +4
+.nf
+.BI "long clone(void *" child_stack ", unsigned long " flags ,
+.BI " int *" ptid ", int *" ctid ,
+.BI " unsigned long " newtls );
.fi
.in
+
+On the microblaze architecture,
+.\" CONFIG_CLONE_BACKWARDS3
+an additional argument is supplied:
+
+.in +4
+.nf
+.BI "long clone(unsigned long " flags ", void *" child_stack ,
+.BI " int " stack_size , "\fR /* Size of stack */"
+.BI " int *" ptid ", int *" ctid ,
+.BI " unsigned long " newtls );
+.fi
+.in
+
Another difference for the raw system call is that the
.I child_stack
argument may be zero, in which case copy-on-write semantics ensure that the
In this case, for correct operation, the
.B 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.
+.\"
.SS blackfin, m68k, and sparc
+.\" Mike Frysinger noted in a 2013 mail:
+.\" these arches don't define __ARCH_WANT_SYS_CLONE:
+.\" blackfin ia64 m68k sparc
The argument-passing conventions on
blackfin, m68k, and sparc are different from the descriptions above.
For details, see the kernel (and glibc) source.
.\" (Since Linux 2.6.0-test6.)
.TP
.B EINVAL
+.\" commit e66eded8309ebf679d3d3c1f5820d1f2ca332c71
Both
.B CLONE_FS
and
were specified in
.IR flags .
.TP
+.BR EINVAL " (since Linux 3.9)"
+Both
+.B CLONE_NEWUSER
+and
+.B CLONE_FS
+were specified in
+.IR flags .
+.TP
.B EINVAL
Both
.B CLONE_NEWIPC
.IR flags .
.TP
.B EINVAL
-Both
+One (or both) of
.BR CLONE_NEWPID
-and
+or
+.BR CLONE_NEWUSER
+and one (or both) of
.BR CLONE_THREAD
+or
+.BR CLONE_PARENT
were specified in
.IR flags .
.TP
.B EINVAL
-Returned by
+Returned by the glibc
.BR clone ()
-when a zero value is specified for
-.IR child_stack .
+wrapper function when
+.IR fn
+or
+.IR child_stack
+is specified as NULL.
.TP
.B EINVAL
.BR CLONE_NEWIPC
.B CONFIG_UTS
option.
.TP
+.B EINVAL
+.I child_stack
+is not aligned to a suitable boundary for this architecture.
+For example, on aarch64,
+.I child_stack
+must be a multiple of 16.
+.TP
.B 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
.B EPERM
.B CLONE_PID
was specified by a process other than process 0.
+.TP
+.B EPERM
+.BR CLONE_NEWUSER
+was specified in
+.IR 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
+.BR user_namespaces (7)).
+.TP
+.BR EPERM " (since Linux 3.9)"
+.\" commit 3151527ee007b73a0ebd296010f1c0454a919c7d
+.B CLONE_NEWUSER
+was specified in
+.I flags
+and the caller is in a chroot environment
+.\" FIXME What is the rationale for this restriction?
+(i.e., the caller's root directory does not match the root directory
+of the mount namespace in which it resides).
+.TP
+.BR ERESTARTNOINTR " (since Linux 2.6.17)"
+.\" commit 4a2c7a7837da1b91468e50426066d988050e4d56
+System call was interrupted by a signal and will be restarted.
+(This can be seen only during a trace.)
+.TP
+.BR EUSERS " (since Linux 3.11)"
+.B CLONE_NEWUSER
+was specified in
+.IR flags ,
+and the call would cause the limit on the number of
+nested user namespaces to be exceeded.
+See
+.BR user_namespaces (7).
+.\" .SH VERSIONS
+.\" There is no entry for
+.\" .BR clone ()
+.\" in libc5.
+.\" glibc2 provides
+.\" .BR clone ()
+.\" as described in this manual page.
.SH CONFORMING TO
.BR clone ()
is Linux-specific and should not be used in programs
intended to be portable.
.SH NOTES
-In the kernel 2.4.x series,
+The
+.BR 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.
+
+In the Linux 2.4.x series,
.B CLONE_THREAD
generally does not make the parent of the new thread the same
as the parent of the calling process.
.B CLONE_THREAD
flag implied the
.B CLONE_PARENT
-flag (as in kernel 2.6).
+flag (as in Linux 2.6.0 and later).
For a while there was
.B CLONE_DETACHED
(introduced in 2.5.32):
parent wants no child-exit signal.
-In 2.6.2 the need to give this
-together with
+In Linux 2.6.2, the need to give this flag together with
.B CLONE_THREAD
disappeared.
This flag is still defined, but has no effect.
.BR tkill (2),
.BR unshare (2),
.BR wait (2),
-.BR proc (5),
.BR capabilities (7),
+.BR namespaces (7),
.BR pthreads (7)
projects / thirdparty / man-pages.git / blobdiff