These system calls create a new ("child") process, in a manner
similar to fork(2).
By contrast with fork(2), these system calls provide more precise
control over what pieces of execution context are shared between
the calling process and the child process. For example, using
these system calls, the caller can control whether or not the two
processes share the virtual address space, the table of file
descriptors, and the table of signal handlers. These system
calls also allow the new child process to be placed in separate
namespaces(7).
Note that in this manual page, "calling process" normally
corresponds to "parent process". But see the 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) or after receiving a fatal signal.
The stack argument specifies the location of the stack used by
the child process. Since the child and calling process may share
memory, it is not possible for the child process to execute in
the same stack as the calling process. The calling process must
therefore set up memory space for the child stack and pass a
pointer to this space to clone(). Stacks grow downward on all
processors that run Linux (except the HP PA processors), so stack
usually points to the topmost address of the memory space set up
for the child stack. Note that clone() does not provide a means
whereby the caller can inform the kernel of the size of the stack
area.
The remaining arguments to clone() are discussed below.
clone3()
The clone3() system call provides a superset of the functionality
of the older clone() interface. It also provides a number of API
improvements, including: space for additional flags bits; cleaner
separation in the use of various arguments; and the ability to
specify the size of the child's stack area.
As with fork(2), clone3() returns in both the parent and the
child. It returns 0 in the child process and returns the PID of
the child in the parent.
The cl_args argument of clone3() is a structure of the following
form:
struct clone_args {
u64 flags; /* Flags bit mask */
u64 pidfd; /* Where to store PID file descriptor
(int *) */
u64 child_tid; /* Where to store child TID,
in child's memory (pid_t *) */
u64 parent_tid; /* Where to store child TID,
in parent's memory (pid_t *) */
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 pid_t array
(since Linux 5.5) */
u64 set_tid_size; /* Number of elements in set_tid
(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 clone_args
structure.)
The stack for the child process is specified via cl_args.stack,
which points to the lowest byte of the stack area, and
cl_args.stack_size, which specifies the size of the stack in
bytes. In the case where the CLONE_VM flag (see below) is
specified, a stack must be 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 cl_args 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 clone_args structure shown above. This
structure allows for a superset of the information passed via the
clone() arguments.
The following table shows the equivalence between the arguments
of clone() and the fields in the clone_args argument supplied to
clone3():
clone() clone3() Notes
cl_args field
flags & ~0xff flags For most flags; details
below
parent_tid pidfd See CLONE_PIDFD
child_tid child_tid See CLONE_CHILD_SETTID
parent_tid parent_tid See CLONE_PARENT_SETTID
flags & 0xff exit_signal
stack stack
--- stack_size
tls tls See CLONE_SETTLS
--- set_tid See below for details
--- set_tid_size
--- cgroup See CLONE_INTO_CGROUP
The 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 cl_args.exit_signal (clone3()). If this
signal is specified as anything other than SIGCHLD, then the
parent process must specify the __WALL or __WCLONE options when
waiting for the child with wait(2). If no signal (i.e., zero) is
specified, then the parent process is not signaled when the child
terminates.
The 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 set_tid 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 set_tid array has to be the desired PID and set_tid_size
needs to be 1.
If the PID of the newly created process should have a certain
value in multiple PID namespaces, then the set_tid 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 set_tid_size 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 set_tid 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 cl_args.flags field
passed to clone3()—is referred to as the flags mask in the
remainder of this page.
The flags mask is specified as a bitwise-OR of zero or more of
the constants listed below. Except as noted below, these flags
are available (and have the same effect) in both clone() and
clone3().
CLONE_CHILD_CLEARTID (since Linux 2.5.49)
Clear (zero) the child thread ID at the location pointed
to by child_tid (clone()) or cl_args.child_tid (clone3())
in child memory when the child exits, and do a wakeup on
the futex at that address. The address involved may be
changed by the set_tid_address(2) system call. This is
used by threading libraries.
CLONE_CHILD_SETTID (since Linux 2.5.49)
Store the child thread ID at the location pointed to by
child_tid (clone()) or cl_args.child_tid (clone3()) in the
child's memory. The store operation completes before the
clone call returns control to user space in the child
process. (Note that the store operation may not have
completed before the clone call returns in the parent
process, which 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 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. Starting in Linux
2.6.2, the need to give this flag together with
CLONE_THREAD disappeared.
This flag is still defined, but it is usually ignored when
calling clone(). However, see the description of
CLONE_PIDFD for some exceptions.
CLONE_FILES (since Linux 2.0)
If CLONE_FILES is set, the calling process and the child
process share the same file descriptor table. Any file
descriptor created by the calling process or by the child
process is also valid in the other process. Similarly, if
one of the processes closes a file descriptor, or changes
its associated flags (using the fcntl(2) F_SETFD
operation), the other process is also affected. If a
process sharing a file descriptor table calls execve(2),
its file descriptor table is duplicated (unshared).
If CLONE_FILES is not set, the child process inherits a
copy of all file descriptors opened in the calling process
at the time of the clone call. Subsequent operations that
open or close file descriptors, or change file descriptor
flags, performed by either the calling process or the
child process do not affect the other process. Note,
however, that the duplicated file descriptors in the child
refer to the same open file descriptions as the
corresponding file descriptors in the calling process, and
thus share file offsets and file status flags (see
open(2)).
CLONE_FS (since Linux 2.0)
If CLONE_FS is set, the caller and the child process share
the same filesystem information. This includes the root
of the filesystem, the current working directory, and the
umask. Any call to chroot(2), chdir(2), or umask(2)
performed by the calling process or the child process also
affects the other process.
If CLONE_FS is not set, the child process works on a copy
of the filesystem information of the calling process at
the time of the clone call. Calls to chroot(2), chdir(2),
or umask(2) performed later by one of the processes do not
affect the other process.
CLONE_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 cl_args.flags
and passes a file descriptor that refers to a version 2
cgroup in the cl_args.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)) 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) 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)) the new process has its own
I/O context.
The I/O context is the I/O scope of the disk scheduler
(i.e., what the I/O scheduler uses to model scheduling of
a process's I/O). If processes share the same I/O
context, they are treated as one by the I/O scheduler. As
a consequence, they get to share disk time. For some I/O
schedulers, if two processes share an I/O context, they
will be allowed to interleave their disk access. If
several threads are doing I/O on behalf of the same
process (aio_read(3), for instance), they should employ
CLONE_IO to get better I/O performance.
If the kernel is not configured with the CONFIG_BLOCK
option, this flag is a no-op.
CLONE_NEWCGROUP (since Linux 4.6)
Create the process in a new cgroup namespace. If this
flag is not set, then (as with fork(2)) the process is
created in the same cgroup namespaces as the calling
process.
For further information on cgroup namespaces, see
cgroup_namespaces(7).
Only a privileged process (CAP_SYS_ADMIN) can employ
CLONE_NEWCGROUP.
CLONE_NEWIPC (since Linux 2.6.19)
If CLONE_NEWIPC is set, then create the process in a new
IPC namespace. If this flag is not set, then (as with
fork(2)), the process is created in the same IPC namespace
as the calling process.
For further information on IPC namespaces, see
ipc_namespaces(7).
Only a privileged process (CAP_SYS_ADMIN) can employ
CLONE_NEWIPC. This flag can't be specified in conjunction
with CLONE_SYSVSEM.
CLONE_NEWNET (since Linux 2.6.24)
(The implementation of this flag was completed only by
about kernel version 2.6.29.)
If CLONE_NEWNET is set, then create the process in a new
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.
For further information on network namespaces, see
network_namespaces(7).
Only a privileged process (CAP_SYS_ADMIN) can employ
CLONE_NEWNET.
CLONE_NEWNS (since Linux 2.4.19)
If CLONE_NEWNS is set, the cloned child is started in a
new mount namespace, initialized with a copy of the
namespace of the parent. If CLONE_NEWNS is not set, the
child lives in the same mount namespace as the parent.
For further information on mount namespaces, see
namespaces(7) and mount_namespaces(7).
Only a privileged process (CAP_SYS_ADMIN) can employ
CLONE_NEWNS. It is not permitted to specify both
CLONE_NEWNS and CLONE_FS in the same clone call.
CLONE_NEWPID (since Linux 2.6.24)
If CLONE_NEWPID is set, then create the process in a new
PID namespace. If this flag is not set, then (as with
fork(2)) the process is created in the same PID namespace
as the calling process.
For further information on PID namespaces, see
namespaces(7) and pid_namespaces(7).
Only a privileged process (CAP_SYS_ADMIN) can employ
CLONE_NEWPID. This flag can't be specified in conjunction
with CLONE_THREAD or CLONE_PARENT.
CLONE_NEWUSER
(This flag first became meaningful for clone() in Linux
2.6.23, the current clone() semantics were merged in Linux
3.5, and the final pieces to make the user namespaces
completely usable were merged in Linux 3.8.)
If CLONE_NEWUSER is set, then create the process in a new
user namespace. If this flag is not set, then (as with
fork(2)) the process is created in the same user namespace
as the calling process.
For further information on user namespaces, see
namespaces(7) and user_namespaces(7).
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)) the process is created in the same UTS namespace
as the calling process.
For further information on UTS namespaces, see
uts_namespaces(7).
Only a privileged process (CAP_SYS_ADMIN) can employ
CLONE_NEWUTS.
CLONE_PARENT (since Linux 2.3.12)
If CLONE_PARENT is set, then the parent of the new child
(as returned by getppid(2)) will be the same as that of
the 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,
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
parent_tid (clone()) or cl_args.parent_tid (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 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).
* When using clone3(), the PID file descriptor is placed
at the location pointed to by cl_args.pidfd.
* When using clone(), the PID file descriptor is placed
at the location pointed to by parent_tid. Since the
parent_tid argument is used to return the PID file
descriptor, CLONE_PIDFD cannot be used with
CLONE_PARENT_SETTID when calling clone().
It is currently not possible to use this flag together
with CLONE_THREAD. This means that the process identified
by the PID file descriptor will always be a thread group
leader.
If the obsolete CLONE_DETACHED flag is specified alongside
CLONE_PIDFD when calling clone(), an error is returned.
An error also results if CLONE_DETACHED is specified when
calling clone3(). This error behavior ensures that the
bit 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)).
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 user_desc * (see set_thread_area(2)). On x86-64 it
is the new value to be set for the %fs base register (see
the ARCH_SET_FS argument to arch_prctl(2)). On
architectures with a dedicated TLS register, it is the new
value of that register.
Use of this flag requires detailed knowledge and generally
it should not be used except in libraries implementing
threading.
CLONE_SIGHAND (since Linux 2.0)
If CLONE_SIGHAND is set, the calling process and the child
process share the same table of signal handlers. If the
calling process or child process calls sigaction(2) to
change the behavior associated with a signal, the behavior
is changed in the other process as well. However, the
calling process and child processes still have distinct
signal masks and sets of pending signals. So, one of them
may block or unblock signals using sigprocmask(2) without
affecting the other process.
If CLONE_SIGHAND is not set, the child process inherits a
copy of the signal handlers of the calling process at the
time of the clone call. Calls to sigaction(2) performed
later by one of the processes have no effect on the other
process.
Since Linux 2.6.0, the flags mask must also include
CLONE_VM if CLONE_SIGHAND is specified.
CLONE_STOPPED (since Linux 2.6.0)
If CLONE_STOPPED is set, then the child is initially
stopped (as though it was sent a SIGSTOP signal), and must
be resumed by sending it a SIGCONT signal.
This flag was deprecated from Linux 2.6.25 onward, and was
removed altogether in Linux 2.6.38. Since then, the
kernel silently ignores it without error. Starting with
Linux 4.6, the same bit was reused for the CLONE_NEWCGROUP
flag.
CLONE_SYSVSEM (since Linux 2.5.10)
If CLONE_SYSVSEM is set, then the child and the calling
process share a single list of System V semaphore
adjustment (semadj) values (see semop(2)). In this case,
the shared list accumulates semadj values across all
processes sharing the list, and semaphore adjustments are
performed only when the last process that is sharing the
list terminates (or ceases sharing the list using
unshare(2)). If this flag is not set, then the child has
a separate semadj list that is initially empty.
CLONE_THREAD (since Linux 2.4.0)
If CLONE_THREAD is set, the child is placed in the same
thread group as the calling process. To make the
remainder of the discussion of CLONE_THREAD more readable,
the term "thread" is used to refer to the processes within
a thread group.
Thread groups were a feature added in Linux 2.4 to support
the POSIX threads notion of a set of threads that share a
single PID. Internally, this shared PID is the so-called
thread group identifier (TGID) for the thread group.
Since Linux 2.4, calls to getpid(2) return the TGID of the
caller.
The threads within a group can be distinguished by their
(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).
When a clone call is made without specifying CLONE_THREAD,
then the resulting thread is placed in a new thread group
whose TGID is the same as the thread's TID. This thread
is the leader of the new thread group.
A new thread created with CLONE_THREAD has the same parent
process as the process that made the clone call (i.e.,
like CLONE_PARENT), so that calls to getppid(2) return the
same value for all of the threads in a thread group. When
a CLONE_THREAD thread terminates, the thread that created
it is not sent a SIGCHLD (or other termination) signal;
nor can the status of such a thread be obtained using
wait(2). (The thread is said to be detached.)
After all of the threads in a thread group terminate the
parent process of the thread group is sent a SIGCHLD (or
other termination) signal.
If any of the threads in a thread group performs an
execve(2), then all threads other than the thread group
leader are terminated, and the new program is executed in
the thread group leader.
If one of the threads in a thread group creates a child
using fork(2), then any thread in the group can wait(2)
for that child.
Since Linux 2.5.35, 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).
A signal may be process-directed or thread-directed. A
process-directed signal is targeted at a thread group
(i.e., a TGID), and is delivered to an arbitrarily
selected thread from among those that are not blocking the
signal. A signal may be process-directed because it was
generated by the kernel for reasons other than a hardware
exception, or because it was sent using kill(2) or
sigqueue(3). A thread-directed signal is targeted at
(i.e., delivered to) a specific thread. A signal may be
thread directed because it was sent using tgkill(2) or
pthread_sigqueue(3), or because the thread executed a
machine language instruction that triggered a hardware
exception (e.g., invalid memory access triggering SIGSEGV
or a floating-point exception triggering SIGFPE).
A call to sigpending(2) returns a signal set that is the
union of the pending process-directed signals and the
signals that are pending for the calling thread.
If a process-directed signal is delivered to a thread
group, and the thread group has installed a handler for
the signal, then the handler 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), the kernel will arbitrarily select one of
these threads to receive the signal.
CLONE_UNTRACED (since Linux 2.5.46)
If CLONE_UNTRACED is specified, then a tracing process
cannot force CLONE_PTRACE on this child process.
CLONE_VFORK (since Linux 2.2)
If CLONE_VFORK is set, the execution of the calling
process is suspended until the child releases its virtual
memory resources via a call to execve(2) or _exit(2) (as
with vfork(2)).
If CLONE_VFORK is not set, then both the calling process
and the child are schedulable after the call, and an
application should not rely on execution occurring in any
particular order.
CLONE_VM (since Linux 2.0)
If CLONE_VM is set, the calling process and the child
process run in the same memory space. In particular,
memory writes performed by the calling process or by the
child process are also visible in the other process.
Moreover, any memory mapping or unmapping performed with
mmap(2) or munmap(2) by the child or calling process also
affects the other process.
If CLONE_VM is not set, the child process runs in a
separate copy of the memory space of the calling process
at the time of the clone call. Memory writes or file
mappings/unmappings performed by one of the processes do
not affect the other, as with fork(2).
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) is cleared in the child
process.
