clone(2) System Calls Manual clone(2)
NAME
clone, __clone2, clone3 - create a child process
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
/* Prototype for the glibc wrapper function */
#define _GNU_SOURCE
#include <sched.h>
int clone(int (*fn)(void *_Nullable), void *stack, int flags,
void *_Nullable arg, ... /* pid_t *_Nullable parent_tid,
void *_Nullable tls,
pid_t *_Nullable child_tid */ );
/* For the prototype of the raw clone() system call, see NOTES */
#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 *cl_args, size_t size);
Note: glibc provides no wrapper for clone3(), necessitating the use of
syscall(2).
DESCRIPTION
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 con-
trol over what pieces of execution context are shared between the call-
ing 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 proces-
sors), 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 allo-
cated 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 individu-
ally, in the newer clone3() interface the arguments are packaged into
the clone_args structure shown above. This structure allows for a su-
perset 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 sig-
naled 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 creat-
ing 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 cre-
ated 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 en-
tries. The first entry defines the PID in the most deeply nested PID
namespace and each of the following entries contains the PID in the cor-
responding 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 hierar-
chy:
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 speci-
fied, 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 be-
havior and allows the caller to specify what is shared between the call-
ing 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 con-
stants 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 mem-
ory when the child exits, and do a wakeup on the futex at that
address. The address involved may be changed by the set_tid_ad-
dress(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 con-
trol 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. 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 call-
ing 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 cre-
ated by the calling process or by the child process is also valid
in the other process. Similarly, if one of the processes closes
a file descriptor, or changes its associated flags (using the fc-
ntl(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 corre-
sponding 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 ap-
ply.
Among the possible use cases for CLONE_INTO_CGROUP are the fol-
lowing:
• 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 ac-
cess. 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_name-
spaces(7).
Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWC-
GROUP.
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
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)) the
process is created in the same network namespace as the calling
process.
For further information on network namespaces, see network_name-
spaces(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 par-
ent. 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_NEW-
PID. 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)) 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_SET-
GID. 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 cre-
ated 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 re-
turned 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 par-
ent_tid (clone()) or cl_args.parent_tid (clone3()) in the par-
ent's memory. (In Linux 2.5.32-2.5.48 there was a flag
CLONE_SETTID that did this.) The store operation completes be-
fore 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 pur-
poses 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 argu-
ment 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 architec-
ture 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 behav-
ior associated with a signal, the behavior is changed in the
other process as well. However, the calling process and child
processes still have distinct signal masks and sets of pending
signals. So, one of them may block or unblock signals using sig-
procmask(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 re-
moved 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 sema-
phore 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 sepa-
rate 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 dis-
cussion of CLONE_THREAD more readable, the term "thread" is used
to refer to the processes within a thread group.
Thread groups were a feature added in Linux 2.4 to support the
POSIX threads notion of a set of threads that share a single PID.
Internally, this shared PID is the so-called thread group identi-
fier (TGID) for the thread group. Since Linux 2.4, calls to get-
pid(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 termina-
tion) signal.
If any of the threads in a thread group performs an execve(2),
then all threads other than the thread group leader are termi-
nated, and the new program is executed in the thread group
leader.
If one of the threads in a thread group creates a child using
fork(2), then any thread in the group can wait(2) for that child.
Since Linux 2.5.35, the flags mask must also include CLONE_SIG-
HAND 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-di-
rected 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 trig-
gering 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.
RETURN VALUE
On success, the thread ID of the child process is returned in the
caller's thread of execution. On failure, -1 is returned in the
caller's context, no child process is created, and errno is set to indi-
cate the error.
ERRORS
EACCES (clone3() only)
CLONE_INTO_CGROUP was specified in cl_args.flags, but the re-
strictions (described in cgroups(7)) on placing the child process
into the version 2 cgroup referred to by cl_args.cgroup are not
met.
EAGAIN Too many processes are already running; see fork(2).
EBUSY (clone3() only)
CLONE_INTO_CGROUP was specified in cl_args.flags, but the file
descriptor specified in cl_args.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 set_tid 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) with the CLONE_NEWPID flag
or used setns(2) to reassociate itself with a PID namespace.
EINVAL Both CLONE_FS and CLONE_NEWNS were specified in 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 architec-
ture. 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)
set_tid_size is greater than the number of nested PID namespaces.
EINVAL (clone3() only)
One of the PIDs specified in set_tid was an invalid.
EINVAL (clone3() only)
CLONE_THREAD or CLONE_PARENT was specified in the flags mask, but
a signal was specified in exit_signal.
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).
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 ex-
ceeded. See user_namespaces(7).
From Linux 3.11 to Linux 4.8, the error diagnosed in this case
was EUSERS.
ENOSPC (since Linux 4.9)
One of the values in the flags mask specified the creation of a
new user namespace, but doing so would have caused the limit de-
fined by the corresponding file in /proc/sys/user to be exceeded.
For further details, see namespaces(7).
EOPNOTSUPP (clone3() only)
CLONE_INTO_CGROUP was specified in cl_args.flags, but the file
descriptor specified in cl_args.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 ef-
fective user ID or the effective group ID of the caller does not
have a mapping in the parent namespace (see user_namespaces(7)).
EPERM (since Linux 3.9)
CLONE_NEWUSER was specified in 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)
set_tid_size 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
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 cl_args.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 speci-
fied. (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 *parent_tid, int *child_tid,
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 *parent_tid, unsigned long tls,
int *child_tid);
On the cris and s390 architectures, the order of the first two arguments
is reversed:
long clone(void *stack, unsigned long flags,
int *parent_tid, int *child_tid,
unsigned long tls);
On the microblaze architecture, an additional argument is supplied:
long clone(unsigned long flags, void *stack,
int stack_size, /* Size of stack */
int *parent_tid, int *child_tid,
unsigned long tls);
blackfin, m68k, and sparc
The argument-passing conventions on blackfin, m68k, and sparc are dif-
ferent from the descriptions above. For details, see the kernel (and
glibc) source.
ia64
On ia64, a different interface is used:
int __clone2(int (*fn)(void *),
void *stack_base, size_t stack_size,
int flags, void *arg, ...
/* pid_t *parent_tid, struct user_desc *tls,
pid_t *child_tid */ );
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 *stack_base,
int stack_size, /* Size of stack */
int *parent_tid, int *child_tid,
unsigned long tls);
__clone2() operates in the same way as clone(), except that stack_base
points to the lowest address of the child's stack area, and stack_size
specifies the size of the stack pointed to by stack_base.
STANDARDS
Linux.
HISTORY
clone3()
Linux 5.3.
Linux 2.4 and earlier
In the Linux 2.4.x series, CLONE_THREAD generally does not make the par-
ent 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 parent_tid,
tls, and child_tid.
NOTES
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) system call can be used to test whether two processes share
various resources such as a file descriptor table, System V semaphore
undo operations, or a virtual address space.
Handlers registered using pthread_atfork(3) are not executed during a
clone call.
BUGS
GNU C library versions 2.3.4 up to and including 2.24 contained a wrap-
per function for getpid(2) that performed caching of PIDs. This caching
relied on support in the glibc wrapper for clone(), but limitations in
the implementation meant that the cache was not up to date in some cir-
cumstances. In particular, if a signal was delivered to the child imme-
diately after the clone() call, then a call to getpid(2) in a handler
for the signal could return the PID of the calling process ("the par-
ent"), if the clone wrapper had not yet had a chance to update the PID
cache in the child. (This discussion ignores the case where the child
was created using CLONE_THREAD, when getpid(2) should return the same
value in the child and in the process that called clone(), since the
caller and the child are in the same thread group. The stale-cache
problem also does not occur if the flags argument includes CLONE_VM.)
To get the truth, it was sometimes necessary to use code such as the
following:
#include <syscall.h>
pid_t mypid;
mypid = syscall(SYS_getpid);
Because of the stale-cache problem, as well as other problems noted in
getpid(2), the PID caching feature was removed in glibc 2.25.
EXAMPLES
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).
Within the sample program, we allocate the memory that is to be used for
the child's stack using mmap(2) rather than malloc(3) for the following
reasons:
• mmap(2) 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 es-
tablish a guard page (a page with protection PROT_NONE) at the end of
the stack using mprotect(2).
• We can specify the MAP_STACK flag to request a mapping that is suit-
able 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
fork(2), futex(2), getpid(2), gettid(2), kcmp(2), mmap(2),
pidfd_open(2), set_thread_area(2), set_tid_address(2), setns(2),
tkill(2), unshare(2), wait(2), capabilities(7), namespaces(7),
pthreads(7)
Linux man-pages 6.9.1 2024-06-15 clone(2)
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