clone(2) - Linux manual page (original) (raw)
clone(2) System Calls Manual clone(2)
NAME top
clone, __clone2, clone3 - create a child processLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
/* Prototype for the glibc wrapper function */
**#define _GNU_SOURCE**
**#include <sched.h>**
**int clone(typeof(int (void *_Nullable)) ***_fn_**,**
**void ***_stack_**,**
**int** _flags_**,**
**void *_Nullable** _arg_**, ...**
/* **pid_t *_Nullable** _parenttid_**,**
**void *_Nullable** _tls_**,**
**pid_t *_Nullable** _childtid_ */ **);**
/* For the prototype of the raw clone() system call, see VERSIONS. */
**#include <linux/sched.h>** /* Definition of **struct clone_args** */
**#include <sched.h>** /* Definition of **CLONE_*** constants */
**#include <sys/syscall.h>** /* Definition of **SYS_*** constants */
**#include <unistd.h>**
**long syscall(SYS_clone3, struct clone_args ***_clargs_**, size_t** _size_**);**
_Note_: glibc provides no wrapper for **clone3**(), necessitating the
use of [syscall(2)](../man2/syscall.2.html).DESCRIPTION top
These system calls create a new ("child") process, in a manner
similar to [fork(2)](../man2/fork.2.html).
By contrast with [fork(2)](../man2/fork.2.html), 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)](../man7/namespaces.7.html).
Note that in this manual page, "calling process" normally
corresponds to "parent process". But see the descriptions of
**CLONE_PARENT** and **CLONE_THREAD** below.
This page describes the following interfaces:
• The glibc **clone**() wrapper function and the underlying system
call on which it is based. The main text describes the wrapper
function; the differences for the raw system call are described
toward the end of this page.
• The newer **clone3**() system call.
In the remainder of this page, the terminology "the clone call" is
used when noting details that apply to all of these interfaces.The clone() wrapper function When the child process is created with the clone() wrapper function, it commences execution by calling the function pointed to by the argument fn. (This differs from fork(2), where execution continues in the child from the point of the fork(2) call.) The arg argument is passed as the argument of the function fn.
When the _fn_(_arg_) function returns, the child process terminates.
The integer returned by _fn_ is the exit status for the child
process. The child process may also terminate explicitly by
calling [exit(2)](../man2/exit.2.html) 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)](../man2/fork.2.html), **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 _clargs_ 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
(_int *_) */
u64 child_tid; /* Where to store child TID,
in child's memory (_pidt *_) */
u64 parent_tid; /* Where to store child TID,
in parent's memory (_pidt *_) */
u64 exit_signal; /* Signal to deliver to parent on
child termination */
u64 stack; /* Pointer to lowest byte of stack */
u64 stack_size; /* Size of stack */
u64 tls; /* Location of new TLS */
u64 set_tid; /* Pointer to a _pidt_ array
(since Linux 5.5) */
u64 set_tid_size; /* Number of elements in _settid_
(since Linux 5.5) */
u64 cgroup; /* File descriptor for target cgroup
of child (since Linux 5.7) */
};
The _size_ argument that is supplied to **clone3**() should be
initialized to the size of this structure. (The existence of the
_size_ argument permits future extensions to the _cloneargs_
structure.)
The stack for the child process is specified via _clargs.stack_,
which points to the lowest byte of the stack area, and
_clargs.stacksize_, which specifies the size of the stack in
bytes. In the case where the **CLONE_VM** flag (see below) is
specified, a stack must be explicitly allocated and specified.
Otherwise, these two fields can be specified as NULL and 0, which
causes the child to use the same stack area as the parent (in the
child's own virtual address space).
The remaining fields in the _clargs_ argument are discussed below.Equivalence between clone() and clone3() arguments Unlike the older clone() interface, where arguments are passed individually, in the newer clone3() interface the arguments are packaged into the cloneargs 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 _cloneargs_ argument supplied to
**clone3**():
**clone() clone3() Notes**
_clargs_ field
_flags & ~0xff_ _flags_ For most flags; details
below
_parenttid_ _pidfd_ See CLONE_PIDFD
_childtid_ _childtid_ See CLONE_CHILD_SETTID
_parenttid_ _parenttid_ See CLONE_PARENT_SETTID
_flags & 0xff_ _exitsignal_
_stack_ _stack_
--- _stacksize_
_tls_ _tls_ See CLONE_SETTLS
--- _settid_ See below for details
--- _settidsize_
--- _cgroup_ See CLONE_INTO_CGROUPThe child termination signal When the child process terminates, a signal may be sent to the parent. The termination signal is specified in the low byte of flags (clone()) or in clargs.exitsignal (clone3()). If this signal is specified as anything other than SIGCHLD, then the parent process must specify the __WALL or __WCLONE options when waiting for the child with wait(2). If no signal (i.e., zero) is specified, then the parent process is not signaled when the child terminates.
The set_tid array By default, the kernel chooses the next sequential PID for the new process in each of the PID namespaces where it is present. When creating a process with clone3(), the settid array (available since Linux 5.5) can be used to select specific PIDs for the process in some or all of the PID namespaces where it is present. If the PID of the newly created process should be set only for the current PID namespace or in the newly created PID namespace (if flags contains CLONE_NEWPID) then the first element in the settid array has to be the desired PID and settidsize needs to be 1.
If the PID of the newly created process should have a certain
value in multiple PID namespaces, then the _settid_ array can have
multiple entries. The first entry defines the PID in the most
deeply nested PID namespace and each of the following entries
contains the PID in the corresponding ancestor PID namespace. The
number of PID namespaces in which a PID should be set is defined
by _settidsize_ which cannot be larger than the number of
currently nested PID namespaces.
To create a process with the following PIDs in a PID namespace
hierarchy:
**PID NS level Requested PID Notes**
0 31496 Outermost PID namespace
1 42
2 7 Innermost PID namespace
Set the array to:
set_tid[0] = 7;
set_tid[1] = 42;
set_tid[2] = 31496;
set_tid_size = 3;
If only the PIDs in the two innermost PID namespaces need to be
specified, set the array to:
set_tid[0] = 7;
set_tid[1] = 42;
set_tid_size = 2;
The PID in the PID namespaces outside the two innermost PID
namespaces is selected the same way as any other PID is selected.
The _settid_ feature requires **CAP_SYS_ADMIN** or (since Linux 5.9)
**CAP_CHECKPOINT_RESTORE** in all owning user namespaces of the target
PID namespaces.
Callers may only choose a PID greater than 1 in a given PID
namespace if an **init** process (i.e., a process with PID 1) already
exists in that namespace. Otherwise the PID entry for this PID
namespace must be 1.The flags mask Both clone() and clone3() allow a flags bit mask that modifies their behavior and allows the caller to specify what is shared between the calling process and the child process. This bit mask— the flags argument of clone() or the clargs.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 _childtid_ (**clone**()) or _clargs.childtid_ (**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)](../man2/set%5Ftid%5Faddress.2.html) 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
_childtid_ (**clone**()) or _clargs.childtid_ (**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 is relevant if the **CLONE_VM** flag is also
employed.)
**CLONE_CLEAR_SIGHAND** (since Linux 5.5)
By default, signal dispositions in the child thread are the
same as in the parent. If this flag is specified, then all
signals that are handled in the parent (and not set to
**SIG_IGN**) are reset to their default dispositions (**SIG_DFL**)
in the child.
Specifying this flag together with **CLONE_SIGHAND** is
nonsensical and disallowed.
**CLONE_DETACHED** (historical)
For a while (during the Linux 2.5 development series) there
was a **CLONE_DETACHED** flag, which caused the parent not to
receive a signal when the child terminated. Ultimately,
the effect of this flag was subsumed under the **CLONE_THREAD**
flag and by the time Linux 2.6.0 was released, this flag
had no effect. Since 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)](../man2/fcntl.2.html) **F_SETFD**
operation), the other process is also affected. If a
process sharing a file descriptor table calls [execve(2)](../man2/execve.2.html),
its file descriptor table is duplicated (unshared).
If **CLONE_FILES** is not set, the child process inherits a
copy of all file descriptors opened in the calling process
at the time of the clone call. Subsequent operations that
open or close file descriptors, or change file descriptor
flags, performed by either the calling process or the child
process do not affect the other process. Note, however,
that the duplicated file descriptors in the child refer to
the same open file descriptions as the corresponding file
descriptors in the calling process, and thus share file
offsets and file status flags (see [open(2)](../man2/open.2.html)).
**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)](../man2/chroot.2.html), [chdir(2)](../man2/chdir.2.html), or [umask(2)](../man2/umask.2.html)
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)](../man2/chroot.2.html), [chdir(2)](../man2/chdir.2.html), or
[umask(2)](../man2/umask.2.html) performed later by one of the processes do not
affect the other process.
**CLONE_INTO_CGROUP** (since Linux 5.7)
By default, a child process is placed in the same version 2
cgroup as its parent. The **CLONE_INTO_CGROUP** flag allows
the child process to be created in a different version 2
cgroup. (Note that **CLONE_INTO_CGROUP** has effect only for
version 2 cgroups.)
In order to place the child process in a different cgroup,
the caller specifies **CLONE_INTO_CGROUP** in _clargs.flags_ and
passes a file descriptor that refers to a version 2 cgroup
in the _clargs.cgroup_ field. (This file descriptor can be
obtained by opening a cgroup v2 directory using either the
**O_RDONLY** or the **O_PATH** flag.) Note that all of the usual
restrictions (described in [cgroups(7)](../man7/cgroups.7.html)) on placing a process
into a version 2 cgroup apply.
Among the possible use cases for **CLONE_INTO_CGROUP** are the
following:
• Spawning a process into a cgroup different from the
parent's cgroup makes it possible for a service manager
to directly spawn new services into dedicated cgroups.
This eliminates the accounting jitter that would be
caused if the child process was first created in the
same cgroup as the parent and then moved into the target
cgroup. Furthermore, spawning the child process
directly into a target cgroup is significantly cheaper
than moving the child process into the target cgroup
after it has been created.
• The **CLONE_INTO_CGROUP** flag also allows the creation of
frozen child processes by spawning them into a frozen
cgroup. (See [cgroups(7)](../man7/cgroups.7.html) for a description of the
freezer controller.)
• For threaded applications (or even thread
implementations which make use of cgroups to limit
individual threads), it is possible to establish a fixed
cgroup layout before spawning each thread directly into
its target cgroup.
**CLONE_IO** (since Linux 2.6.25)
If **CLONE_IO** is set, then the new process shares an I/O
context with the calling process. If this flag is not set,
then (as with [fork(2)](../man2/fork.2.html)) 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)](../man3/aio%5Fread.3.html), 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)](../man2/fork.2.html)) the process is created
in the same cgroup namespaces as the calling process.
For further information on cgroup namespaces, see
[cgroup_namespaces(7)](../man7/cgroup%5Fnamespaces.7.html).
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)](../man2/fork.2.html)), the process is created in the same IPC namespace
as the calling process.
For further information on IPC namespaces, see
[ipc_namespaces(7)](../man7/ipc%5Fnamespaces.7.html).
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 Linux 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)](../man2/fork.2.html)) the process is created in the same network
namespace as the calling process.
For further information on network namespaces, see
[network_namespaces(7)](../man7/network%5Fnamespaces.7.html).
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)](../man7/namespaces.7.html) and [mount_namespaces(7)](../man7/mount%5Fnamespaces.7.html).
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)](../man2/fork.2.html)) the process is created in the same PID namespace
as the calling process.
For further information on PID namespaces, see
[namespaces(7)](../man7/namespaces.7.html) and [pid_namespaces(7)](../man7/pid%5Fnamespaces.7.html).
Only a privileged process (**CAP_SYS_ADMIN**) can employ
**CLONE_NEWPID**. This flag can't be specified in conjunction
with **CLONE_THREAD**.
**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)](../man2/fork.2.html)) the process is created in the same user namespace
as the calling process.
For further information on user namespaces, see
[namespaces(7)](../man7/namespaces.7.html) and [user_namespaces(7)](../man7/user%5Fnamespaces.7.html).
Before Linux 3.8, use of **CLONE_NEWUSER** required that the
caller have three capabilities: **CAP_SYS_ADMIN**, **CAP_SETUID**,
and **CAP_SETGID**. Starting with Linux 3.8, no privileges are
needed to create a user namespace.
This flag can't be specified in conjunction with
**CLONE_THREAD** or **CLONE_PARENT**. For security reasons,
**CLONE_NEWUSER** cannot be specified in conjunction with
**CLONE_FS**.
**CLONE_NEWUTS** (since Linux 2.6.19)
If **CLONE_NEWUTS** is set, then create the process in a new
UTS namespace, whose identifiers are initialized by
duplicating the identifiers from the UTS namespace of the
calling process. If this flag is not set, then (as with
[fork(2)](../man2/fork.2.html)) the process is created in the same UTS namespace
as the calling process.
For further information on UTS namespaces, see
[uts_namespaces(7)](../man7/uts%5Fnamespaces.7.html).
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)](../man2/getppid.2.html)) will be the same as that of the
calling process.
If **CLONE_PARENT** is not set, then (as with [fork(2)](../man2/fork.2.html)) the
child's parent is the calling process.
Note that it is the parent process, as returned by
[getppid(2)](../man2/getppid.2.html), 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, is
signaled.
The **CLONE_PARENT** flag can't be used in clone calls by the
global init process (PID 1 in the initial PID namespace)
and init processes in other PID namespaces. This
restriction prevents the creation of multi-rooted process
trees as well as the creation of unreapable zombies in the
initial PID namespace.
**CLONE_PARENT_SETTID** (since Linux 2.5.49)
Store the child thread ID at the location pointed to by
_parenttid_ (**clone**()) or _clargs.parenttid_ (**clone3**()) in
the parent's memory. (In Linux 2.5.32-2.5.48 there was a
flag **CLONE_SETTID** that did this.) The store operation
completes before the clone call returns control to user
space.
**CLONE_PID** (Linux 2.0 to Linux 2.5.15)
If **CLONE_PID** is set, the child process is created with the
same process ID as the calling process. This is good for
hacking the system, but otherwise of not much use. From
Linux 2.3.21 onward, this flag could be specified only by
the system boot process (PID 0). The flag disappeared
completely from the kernel sources in Linux 2.5.16.
Subsequently, the kernel silently ignored this bit if it
was specified in the _flags_ mask. Much later, the same bit
was recycled for use as the **CLONE_PIDFD** flag.
**CLONE_PIDFD** (since Linux 5.2)
If this flag is specified, a PID file descriptor referring
to the child process is allocated and placed at a specified
location in the parent's memory. The close-on-exec flag is
set on this new file descriptor. PID file descriptors can
be used for the purposes described in [pidfd_open(2)](../man2/pidfd%5Fopen.2.html).
• When using **clone3**(), the PID file descriptor is placed
at the location pointed to by _clargs.pidfd_.
• When using **clone**(), the PID file descriptor is placed at
the location pointed to by _parenttid_. Since the
_parenttid_ argument is used to return the PID file
descriptor, **CLONE_PIDFD** cannot be used with
**CLONE_PARENT_SETTID** when calling **clone**().
It is currently not possible to use this flag together with
**CLONE_THREAD.** This means that the process identified by
the PID file descriptor will always be a thread group
leader.
If the obsolete **CLONE_DETACHED** flag is specified alongside
**CLONE_PIDFD** when calling **clone**(), an error is returned. An
error also results if **CLONE_DETACHED** is specified when
calling **clone3**(). This error behavior ensures that the bit
corresponding to **CLONE_DETACHED** can be reused for further
PID file descriptor features in the future.
**CLONE_PTRACE** (since Linux 2.2)
If **CLONE_PTRACE** is specified, and the calling process is
being traced, then trace the child also (see [ptrace(2)](../man2/ptrace.2.html)).
**CLONE_SETTLS** (since Linux 2.5.32)
The TLS (Thread Local Storage) descriptor is set to _tls_.
The interpretation of _tls_ and the resulting effect is
architecture dependent. On x86, _tls_ is interpreted as a
_struct userdesc *_ (see [set_thread_area(2)](../man2/set%5Fthread%5Farea.2.html)). 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)](../man2/arch%5Fprctl.2.html)). On
architectures with a dedicated TLS register, it is the new
value of that register.
Use of this flag requires detailed knowledge and generally
it should not be used except in libraries implementing
threading.
**CLONE_SIGHAND** (since Linux 2.0)
If **CLONE_SIGHAND** is set, the calling process and the child
process share the same table of signal handlers. If the
calling process or child process calls [sigaction(2)](../man2/sigaction.2.html) 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)](../man2/sigprocmask.2.html) 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)](../man2/sigaction.2.html) 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)](../man2/semop.2.html)). 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)](../man2/unshare.2.html)). 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)](../man2/getpid.2.html) return the TGID of the
caller.
The threads within a group can be distinguished by their
(system-wide) unique thread IDs (TID). A new thread's TID
is available as the function result returned to the caller,
and a thread can obtain its own TID using [gettid(2)](../man2/gettid.2.html).
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)](../man2/getppid.2.html) 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)](../man2/wait.2.html).
(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)](../man2/execve.2.html), 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)](../man2/fork.2.html), then any thread in the group can [wait(2)](../man2/wait.2.html) for
that child.
Since Linux 2.5.35, the _flags_ mask must also include
**CLONE_SIGHAND** if **CLONE_THREAD** is specified (and note that,
since Linux 2.6.0, **CLONE_SIGHAND** also requires **CLONE_VM** to
be included).
Signal dispositions and actions are process-wide: if an
unhandled signal is delivered to a thread, then it will
affect (terminate, stop, continue, be ignored in) all
members of the thread group.
Each thread has its own signal mask, as set by
[sigprocmask(2)](../man2/sigprocmask.2.html).
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)](../man2/kill.2.html) or [sigqueue(3)](../man3/sigqueue.3.html). 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)](../man2/tgkill.2.html) or [pthread_sigqueue(3)](../man3/pthread%5Fsigqueue.3.html), 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)](../man2/sigpending.2.html) 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 is 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)](../man2/sigwaitinfo.2.html), 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)](../man2/execve.2.html) or [_exit(2)](../man2/%5Fexit.2.html) (as with
[vfork(2)](../man2/vfork.2.html)).
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)](../man2/mmap.2.html) or [munmap(2)](../man2/munmap.2.html) 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)](../man2/fork.2.html).
If the **CLONE_VM** flag is specified and the **CLONE_VFORK** flag
is not specified, then any alternate signal stack that was
established by [sigaltstack(2)](../man2/sigaltstack.2.html) is cleared in the child
process.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 is created, and _[errno](../man3/errno.3.html)_ is set to
indicate the error.ERRORS top
**EACCES** (**clone3**() only)
**CLONE_INTO_CGROUP** was specified in _clargs.flags_, but the
restrictions (described in [cgroups(7)](../man7/cgroups.7.html)) on placing the child
process into the version 2 cgroup referred to by
_clargs.cgroup_ are not met.
**EAGAIN** Too many processes are already running; see [fork(2)](../man2/fork.2.html).
**EBUSY** (**clone3**() only)
**CLONE_INTO_CGROUP** was specified in _clargs.flags_, but the
file descriptor specified in _clargs.cgroup_ refers to a
version 2 cgroup in which a domain controller is enabled.
**EEXIST** (**clone3**() only)
One (or more) of the PIDs specified in _settid_ already
exists in the corresponding PID namespace.
**EINVAL** Both **CLONE_SIGHAND** and **CLONE_CLEAR_SIGHAND** were specified
in the _flags_ mask.
**EINVAL CLONE_SIGHAND** was specified in the _flags_ mask, but **CLONE_VM**
was not. (Since Linux 2.6.0.)
**EINVAL CLONE_THREAD** was specified in the _flags_ mask, but
**CLONE_SIGHAND** was not. (Since Linux 2.5.35.)
**EINVAL CLONE_THREAD** was specified in the _flags_ mask, but the
current process previously called [unshare(2)](../man2/unshare.2.html) with the
**CLONE_NEWPID** flag or used [setns(2)](../man2/setns.2.html) 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 CLONE_NEWPID** and one (or both) of **CLONE_THREAD** or
**CLONE_PARENT** were specified in the _flags_ mask.
**EINVAL CLONE_NEWUSER** and **CLONE_THREAD** were specified in the _flags_
mask.
**EINVAL** (since Linux 2.6.32)
**CLONE_PARENT** was specified, and the caller is an init
process.
**EINVAL** Returned by the glibc **clone**() wrapper function when _fn_ or
_stack_ is specified as NULL.
**EINVAL CLONE_NEWIPC** was specified in the _flags_ mask, but the
kernel was not configured with the **CONFIG_SYSVIPC** and
**CONFIG_IPC_NS** options.
**EINVAL CLONE_NEWNET** was specified in the _flags_ mask, but the
kernel was not configured with the **CONFIG_NET_NS** option.
**EINVAL CLONE_NEWPID** was specified in the _flags_ mask, but the
kernel was not configured with the **CONFIG_PID_NS** option.
**EINVAL CLONE_NEWUSER** was specified in the _flags_ mask, but the
kernel was not configured with the **CONFIG_USER_NS** option.
**EINVAL CLONE_NEWUTS** was specified in the _flags_ mask, but the
kernel was not configured with the **CONFIG_UTS_NS** option.
**EINVAL** _stack_ is not aligned to a suitable boundary for this
architecture. For example, on aarch64, _stack_ must be a
multiple of 16.
**EINVAL** (**clone3**() only)
**CLONE_DETACHED** was specified in the _flags_ mask.
**EINVAL** (**clone**() only)
**CLONE_PIDFD** was specified together with **CLONE_DETACHED** in
the _flags_ mask.
**EINVAL CLONE_PIDFD** was specified together with **CLONE_THREAD** in the
_flags_ mask.
**EINVAL** (**clone**() only)
**CLONE_PIDFD** was specified together with **CLONE_PARENT_SETTID**
in the _flags_ mask.
**EINVAL** (**clone3**() only)
_settidsize_ is greater than the number of nested PID
namespaces.
**EINVAL** (**clone3**() only)
One of the PIDs specified in _settid_ was an invalid.
**EINVAL** (**clone3**() only)
**CLONE_THREAD** or **CLONE_PARENT** was specified in the _flags_
mask, but a signal was specified in _exitsignal_.
**EINVAL** (AArch64 only, Linux 4.6 and earlier)
_stack_ was not aligned to a 128-bit boundary.
**ENOMEM** Cannot allocate sufficient memory to allocate a task
structure for the child, or to copy those parts of the
caller's context that need to be copied.
**ENOSPC** (since Linux 3.7)
**CLONE_NEWPID** was specified in the _flags_ mask, but the limit
on the nesting depth of PID namespaces would have been
exceeded; see [pid_namespaces(7)](../man7/pid%5Fnamespaces.7.html).
**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)](../man7/user%5Fnamespaces.7.html).
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)](../man7/namespaces.7.html).
**EOPNOTSUPP** (**clone3**() only)
**CLONE_INTO_CGROUP** was specified in _clargs.flags_, but the
file descriptor specified in _clargs.cgroup_ refers to a
version 2 cgroup that is in the _domain invalid_ state.
**EPERM CLONE_NEWCGROUP**, **CLONE_NEWIPC**, **CLONE_NEWNET**, **CLONE_NEWNS**,
**CLONE_NEWPID**, or **CLONE_NEWUTS** was specified by an
unprivileged process (process without **CAP_SYS_ADMIN**).
**EPERM CLONE_PID** was specified by a process other than process 0.
(This error occurs only on Linux 2.5.15 and earlier.)
**EPERM CLONE_NEWUSER** was specified in the _flags_ mask, but either
the effective user ID or the effective group ID of the
caller does not have a mapping in the parent namespace (see
[user_namespaces(7)](../man7/user%5Fnamespaces.7.html)).
**EPERM** (since Linux 3.9)
**CLONE_NEWUSER** was specified in the _flags_ mask and the
caller is in a chroot environment (i.e., the caller's root
directory does not match the root directory of the mount
namespace in which it resides).
**EPERM** (**clone3**() only)
_settidsize_ was greater than zero, and the caller lacks
the **CAP_SYS_ADMIN** capability in one or more of the user
namespaces that own the corresponding PID namespaces.
**ERESTARTNOINTR** (since Linux 2.6.17)
System call was interrupted by a signal and will be
restarted. (This can be seen only during a trace.)
**EUSERS** (Linux 3.11 to Linux 4.8)
**CLONE_NEWUSER** was specified in the _flags_ mask, and the
limit on the number of nested user namespaces would be
exceeded. See the discussion of the **ENOSPC** error above.VERSIONS top
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.
On i386, **clone**() should not be called through vsyscall, but
directly through _int $0x80_.C library/kernel differences The raw clone() system call corresponds more closely to fork(2) in that execution in the child continues from the point of the call. As such, the fn and arg arguments of the clone() wrapper function are omitted.
In contrast to the glibc wrapper, the raw **clone**() system call
accepts NULL as a _stack_ argument (and **clone3**() likewise allows
_clargs.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 ***_parenttid_**, int ***_childtid_**,**
**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 ***_parenttid_**, unsigned long** _tls_**,**
**int ***_childtid_**);**
On the cris and s390 architectures, the order of the first two
arguments is reversed:
**long clone(void ***_stack_**, unsigned long** _flags_**,**
**int ***_parenttid_**, int ***_childtid_**,**
**unsigned long** _tls_**);**
On the microblaze architecture, an additional argument is
supplied:
**long clone(unsigned long** _flags_**, void ***_stack_**,**
**int** _stacksize_**,** /* Size of stack */
**int ***_parenttid_**, int ***_childtid_**,**
**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(typeof(int (void *)) ***_fn_**,**
**void ***_stackbase_**, size_t** _stacksize_**,**
**int** _flags_**, void ***_arg_**, ...**
/* **pid_t ***_parenttid_**, struct user_desc ***_tls_**,**
**pid_t ***_childtid_ */ **);**
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 ***_stackbase_**,**
**int** _stacksize_**,** /* Size of stack */
**int ***_parenttid_**, int ***_childtid_**,**
**unsigned long** _tls_**);**
**__clone2**() operates in the same way as **clone**(), except that
_stackbase_ points to the lowest address of the child's stack area,
and _stacksize_ specifies the size of the stack pointed to by
_stackbase_.STANDARDS top
Linux.HISTORY top
**clone3**()
Linux 5.3.Linux 2.4 and earlier 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, from Linux 2.4.7 to Linux 2.4.18 the CLONE_THREAD flag implied the CLONE_PARENT flag (as in Linux 2.6.0 and later).
In Linux 2.4 and earlier, **clone**() does not take arguments
_parenttid_, _tls_, and _childtid_.NOTES top
One use of these system calls is to implement threads: multiple
flows of control in a program that run concurrently in a shared
address space.
The [kcmp(2)](../man2/kcmp.2.html) 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)](../man3/pthread%5Fatfork.3.html) are not executed
during a clone call.BUGS top
GNU C library versions 2.3.4 up to and including 2.24 contained a
wrapper function for [getpid(2)](../man2/getpid.2.html) 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)](../man2/getpid.2.html) 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)](../man2/getpid.2.html) _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)](../man2/getpid.2.html), the PID caching feature was removed in glibc
2.25.EXAMPLES 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)](../man2/setns.2.html).
Within the sample program, we allocate the memory that is to be
used for the child's stack using [mmap(2)](../man2/mmap.2.html) rather than [malloc(3)](../man3/malloc.3.html) for
the following reasons:
• [mmap(2)](../man2/mmap.2.html) 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)](../man2/mprotect.2.html).
• 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 <err.h> #include <sched.h> #include <signal.h> #include <stdint.h> #include <stdio.h> #include <stdlib.h> #include <string.h> #include <sys/mman.h> #include <sys/types.h> #include <sys/utsname.h> #include <sys/wait.h> #include <unistd.h>
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)
err(EXIT_FAILURE, "sethostname");
/* Retrieve and display hostname. */
if (uname(&uts) == -1)
err(EXIT_FAILURE, "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)
err(EXIT_FAILURE, "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)
err(EXIT_FAILURE, "clone");
printf("clone() returned %jd\n", (intmax_t) 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)
err(EXIT_FAILURE, "uname");
printf("uts.nodename in parent: %s\n", uts.nodename);
if (waitpid(pid, NULL, 0) == -1) /* Wait for child */
err(EXIT_FAILURE, "waitpid");
printf("child has terminated\n");
exit(EXIT_SUCCESS);
}SEE ALSO top
[fork(2)](../man2/fork.2.html), [futex(2)](../man2/futex.2.html), [getpid(2)](../man2/getpid.2.html), [gettid(2)](../man2/gettid.2.html), [kcmp(2)](../man2/kcmp.2.html), [mmap(2)](../man2/mmap.2.html),
[pidfd_open(2)](../man2/pidfd%5Fopen.2.html), [set_thread_area(2)](../man2/set%5Fthread%5Farea.2.html), [set_tid_address(2)](../man2/set%5Ftid%5Faddress.2.html), [setns(2)](../man2/setns.2.html),
[tkill(2)](../man2/tkill.2.html), [unshare(2)](../man2/unshare.2.html), [wait(2)](../man2/wait.2.html), [capabilities(7)](../man7/capabilities.7.html), [namespaces(7)](../man7/namespaces.7.html),
[pthreads(7)](../man7/pthreads.7.html)COLOPHON top
This page is part of the _man-pages_ (Linux kernel and C library
user-space interface documentation) project. Information about
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