For review: documentation of clone3() system call

From: Michael Kerrisk (man-pages)
Date: Fri Oct 25 2019 - 13:01:26 EST

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Hello Christian and all,

I've made a first shot at adding documentation for clone3(). You can
see the diff here:
https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=faa0e55ae9e490d71c826546bbdef954a1800969

In the end, I decided that the most straightforward approach was to
add the documentation as part of the existing clone(2) page. This has
the advantage of avoiding duplication of information across two pages,
and perhaps also makes it easier to see the commonality of the two
APIs.

Because the new text is integrated into the existing page, I think it
makes most sense to just show that page text for review purposes. I
welcome input on the below.

The notable changes are:
* In the first part of the page, up to and including the paragraph
with the subheading "The flags bit mask"
* Minor changes in the description of CLONE_CHILD_CLEARTID,
CLONE_CHILD_SETTID, CLONE_PARENT_SETTID, and CLONE_PIDFD, to reflect
the argument differences between clone() and clone2()

Most of the resy of page is unchanged.

I welcome fixes, suggestions for improvements, etc.

Thanks,

Michael

CLONE(2) Linux Programmer's Manual CLONE(2)

NAME
clone, __clone2 - create a child process

SYNOPSIS
/* Prototype for the glibc wrapper function */

#define _GNU_SOURCE
#include <sched.h>

int clone(int (*fn)(void *), void *stack, int flags, void *arg, ...
/* pid_t *parent_tid, void *tls, pid_t *child_tid */ );

/* For the prototype of the raw clone() system call, see NOTES */

long clone3(struct clone_args *cl_args, size_t size);

Note: There is not yet a glibc wrapper for clone3(); see NOTES.

DESCRIPTION
These system calls create a new process, in a manner similar to
fork(2).

Unlike fork(2), these system calls allow the child process to
share parts of its execution context with the calling process,
such as the virtual address space, the table of file descriptors,
and the table of signal handlers. (Note that on this manual page,
"calling process" normally corresponds to "parent process". But
see the description of CLONE_PARENT below.)

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.

The clone() wrapper function
When the child process is created with the clone() wrapper funcâ
tion, it commences execution by calling the function pointed to by
the argument fn. (This differs from fork(2), where execution conâ
tinues 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 callâ
ing 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 memâ
ory, it is not possible for the child process to execute in the
same stack as the calling process. The calling process must
therefore set up memory space for the child stack and pass a
pointer to this space to clone(). Stacks grow downward on all
processors that run Linux (except the HP PA processors), so 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 (int *) */
u64 parent_tid; /* Where to store child TID,
in parent's memory (int *) */
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 */
};

The size argument that is supplied to clone3() should be initialâ
ized 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 speciâ
fied, a stack must be explicitly allocated and specified. Otherâ
wise, 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 strucâ
ture 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() clone(3) Notes
cl_args field
flags & ~0xff flags
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

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 parâ
ent process must specify the __WALL or __WCLONE options when waitâ
ing 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 flags bit 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
is specified as a bitwise-OR of zero or more of the constants
listed below. Except as otherwise 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
clone() returns control to user space in the child process.
(Note that the store operation may not have completed
before clone() returns in the parent process, which will be
relevant if the CLONE_VM flag is also employed.)

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 operaâ
tion), 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 clone(). 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) perâ
formed 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_IO (since Linux 2.6.25)
If CLONE_IO is set, then the new process shares an I/O conâ
text with the calling process. If this flag is not set,
then (as with fork(2)) the new process has its own I/O conâ
text.

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 conseâ
quence, they get to share disk time. For some I/O schedâ
ulers, 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. This
flag is intended for the implementation of containers.

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. This flag is intended for the
implementation of containers.

An IPC namespace provides an isolated view of System V IPC
objects (see sysvipc(7)) and (since Linux 2.6.30) POSIX
message queues (see mq_overview(7)). The common characterâ
istic of these IPC mechanisms is that IPC objects are idenâ
tified by mechanisms other than filesystem pathnames.

Objects created in an IPC namespace are visible to all
other processes that are members of that namespace, but are
not visible to processes in other IPC namespaces.

When an IPC namespace is destroyed (i.e., when the last
process that is a member of the namespace terminates), all
IPC objects in the namespace are automatically destroyed.

Only a privileged process (CAP_SYS_ADMIN) can employ
CLONE_NEWIPC. This flag can't be specified in conjunction
with CLONE_SYSVSEM.

For further information on IPC namespaces, see namesâ
paces(7).

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 namesâ
pace as the calling process. This flag is intended for the
implementation of containers.

A network namespace provides an isolated view of the netâ
working stack (network device interfaces, IPv4 and IPv6
protocol stacks, IP routing tables, firewall rules, the
/proc/net and /sys/class/net directory trees, sockets,
etc.). A physical network device can live in exactly one
network namespace. A virtual network (veth(4)) device pair
provides a pipe-like abstraction that can be used to create
tunnels between network namespaces, and can be used to creâ
ate a bridge to a physical network device in another namesâ
pace.

When a network namespace is freed (i.e., when the last
process in the namespace terminates), its physical network
devices are moved back to the initial network namespace
(not to the parent of the process). For further informaâ
tion on network namespaces, see 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.

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.

For further information on mount namespaces, see namesâ
paces(7) and mount_namespaces(7).

CLONE_NEWPID (since Linux 2.6.24)
If CLONE_NEWPID is set, then create the process in a new
PID namespace. If this flag is not set, then (as with
fork(2)) the process is created in the same PID namespace
as the calling process. This flag is intended for the
implementation of containers.

For further information on PID namespaces, see namesâ
paces(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 comâ
pletely 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.

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.

For further information on user namespaces, see namesâ
paces(7) and user_namespaces(7).

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 dupliâ
cating the identifiers from the UTS namespace of the callâ
ing process. If this flag is not set, then (as with
fork(2)) the process is created in the same UTS namespace
as the calling process. This flag is intended for the
implementation of containers.

A UTS namespace is the set of identifiers returned by
uname(2); among these, the domain name and the hostname can
be modified by setdomainname(2) and sethostname(2), respecâ
tively. Changes made to the identifiers in a UTS namespace
are visible to all other processes in the same namespace,
but are not visible to processes in other UTS namespaces.

Only a privileged process (CAP_SYS_ADMIN) can employ
CLONE_NEWUTS.

For further information on UTS namespaces, see namesâ
paces(7).

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 getpâ
pid(2), which is signaled when the child terminates, so
that if CLONE_PARENT is set, then the parent of the calling
process, rather than the calling process itself, will be
signaled.

CLONE_PARENT_SETTID (since Linux 2.5.49)
Store the child thread ID at the location pointed to by
parent_tid (clone()) or cl_args.child_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 clone() 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 comâ
pletely from the kernel sources in Linux 2.5.16. Since
then, the kernel silently ignores this bit if it is speciâ
fied in flags.

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 parâ
ent_tid argument is used to return the PID file descripâ
tor, 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.

For a while there was a CLONE_DETACHED flag. This flag is
usually ignored when passed along with other flags. Howâ
ever, when passed alongside CLONE_PIDFD, an error is
returned. This ensures that this flag 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 architecâ
tures with a dedicated TLS register, it is the new value of
that register.

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 clone() is called. Calls to sigaction(2) performed
later by one of the processes have no effect on the other
process.

Since Linux 2.6.0, flags 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 adjustâ
ment (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 termiâ
nates (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 callâ
er.

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
of clone(), and a thread can obtain its own TID using getâ
tid(2).

When a call is made to clone() without specifying
CLONE_THREAD, then the resulting thread is placed in a new
thread group whose TGID is the same as the thread's TID.
This thread is the leader of the new thread group.

A new thread created with CLONE_THREAD has the same parent
process as the caller of clone() (i.e., like CLONE_PARENT),
so that calls to getppid(2) return the same value for all
of the threads in a thread group. When a CLONE_THREAD
thread terminates, the thread that created it using clone()
is not sent a SIGCHLD (or other termination) signal; nor
can the status of such a thread be obtained using wait(2).
(The thread is said to be detached.)

After all of the threads in a thread group terminate the
parent process of the thread group is sent a SIGCHLD (or
other 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, flags 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 memâ
bers of the thread group.

Each thread has its own signal mask, as set by sigprocâ
mask(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 sigâ
nals 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 will be invoked in exactly one,
arbitrarily selected member of the thread group that has
not blocked the signal. If multiple threads in a group are
waiting to accept the same 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 canâ
not 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 appliâ
cation should not rely on execution occurring in any parâ
ticular 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, memâ
ory 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 sepaâ
rate copy of the memory space of the calling process at the
time of clone(). Memory writes or file mappings/unmappings
performed by one of the processes do not affect the other,
as with fork(2).

NOTES
One use of these systems calls is to implement threads: multiple
flows of control in a program that run concurrently in a shared
address space.

Glibc does not provide a wrapper for clone(3); call it using
syscall(2).

Note that 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.

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.

Another difference for the raw clone() system call is that the
stack argument may be NULL, in which case the child uses a dupliâ
cate 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 architecâ
tures (including sh, tile, ia-64, 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 supâ
plied:

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
different from the descriptions above. For details, see the kerâ
nel (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.

Linux 2.4 and earlier
In Linux 2.4 and earlier, clone() does not take arguments parâ
ent_tid, tls, and child_tid.

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 will be created, and errno will
be set appropriately.

ERRORS
EAGAIN Too many processes are already running; see fork(2).

EINVAL CLONE_SIGHAND was specified, but CLONE_VM was not. (Since
Linux 2.6.0.)

EINVAL CLONE_THREAD was specified, but CLONE_SIGHAND was not.
(Since Linux 2.5.35.)

EINVAL CLONE_THREAD was specified, but the current process previâ
ously 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 flags.

EINVAL (since Linux 3.9)
Both CLONE_NEWUSER and CLONE_FS were specified in flags.

EINVAL Both CLONE_NEWIPC and CLONE_SYSVSEM were specified in
flags.

EINVAL One (or both) of CLONE_NEWPID or CLONE_NEWUSER and one (or
both) of CLONE_THREAD or CLONE_PARENT were specified in
flags.

EINVAL Returned by the glibc clone() wrapper function when fn or
stack is specified as NULL.

EINVAL CLONE_NEWIPC was specified in flags, but the kernel was not
configured with the CONFIG_SYSVIPC and CONFIG_IPC_NS
options.

EINVAL CLONE_NEWNET was specified in flags, but the kernel was not
configured with the CONFIG_NET_NS option.

EINVAL CLONE_NEWPID was specified in flags, but the kernel was not
configured with the CONFIG_PID_NS option.

EINVAL CLONE_NEWUSER was specified in flags, but the kernel was
not configured with the CONFIG_USER_NS option.

EINVAL CLONE_NEWUTS was specified in flags, but the kernel was not
configured with the CONFIG_UTS_NS option.

EINVAL stack is not aligned to a suitable boundary for this archiâ
tecture. For example, on aarch64, stack must be a multiple
of 16.

EINVAL CLONE_PIDFD was specified together with CLONE_DETACHED.

EINVAL CLONE_PIDFD was specified together with CLONE_THREAD.

EINVAL (clone() only)
CLONE_PIDFD was specified together with CLONE_PARENT_SETâ
TID.

ENOMEM Cannot allocate sufficient memory to allocate a task strucâ
ture 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 flags, 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 flags, and the call would
cause the limit on the number of nested user namespaces to
be exceeded. 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 flags 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).

EPERM CLONE_NEWCGROUP, CLONE_NEWIPC, CLONE_NEWNET, CLONE_NEWNS,
CLONE_NEWPID, or CLONE_NEWUTS was specified by an unpriviâ
leged 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 flags, but either the effecâ
tive user ID or the effective group ID of the caller does
not have a mapping in the parent namespace (see user_namesâ
paces(7)).

EPERM (since Linux 3.9)
CLONE_NEWUSER was specified in flags 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).

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 flags, and the limit on the
number of nested user namespaces would be exceeded. See
the discussion of the ENOSPC error above.

VERSIONS
The clone3() system call first appeared in Linux 5.3.

CONFORMING TO
These system calls are Linux-specific and should not be used in
programs intended to be portable.

NOTES
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 durâ
ing a call to clone().

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, for kernel versions 2.4.7 to 2.4.18 the
CLONE_THREAD flag implied the CLONE_PARENT flag (as in Linux 2.6.0
and later).

For a while there was CLONE_DETACHED (introduced in 2.5.32): parâ
ent wants no child-exit signal. In Linux 2.6.2, the need to give
this flag together with CLONE_THREAD disappeared. This flag is
still defined, but has no effect.

On i386, clone() should not be called through vsyscall, but
directly through int $0x80.

BUGS
GNU C library versions 2.3.4 up to and including 2.24 contained a
wrapper 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 circumstances. In particular, if a signal was
delivered to the child immediately 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 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) 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.

EXAMPLE
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).

Program source
#define _GNU_SOURCE
#include <sys/wait.h>
#include <sys/utsname.h>
#include <sched.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
} while (0)

static int /* Start function for cloned child */
childFunc(void *arg)
{
struct utsname uts;

/* Change hostname in UTS namespace of child */

if (sethostname(arg, strlen(arg)) == -1)
errExit("sethostname");

/* Retrieve and display hostname */

if (uname(&uts) == -1)
errExit("uname");
printf("uts.nodename in child: %s\n", uts.nodename);

/* Keep the namespace open for a while, by sleeping.
This allows some experimentation--for example, another
process might join the namespace. */

sleep(200);

return 0; /* Child terminates now */
}

#define STACK_SIZE (1024 * 1024) /* Stack size for cloned child */

int
main(int argc, char *argv[])
{
char *stack; /* Start of stack buffer */
char *stackTop; /* End of stack buffer */
pid_t pid;
struct utsname uts;

if (argc < 2) {
fprintf(stderr, "Usage: %s <child-hostname>\n", argv[0]);
exit(EXIT_SUCCESS);
}

/* Allocate stack for child */

stack = malloc(STACK_SIZE);
if (stack == NULL)
errExit("malloc");
stackTop = stack + STACK_SIZE; /* Assume stack grows downward */

/* Create child that has its own UTS namespace;
child commences execution in childFunc() */

pid = clone(childFunc, stackTop, CLONE_NEWUTS | SIGCHLD, argv[1]);
if (pid == -1)
errExit("clone");
printf("clone() returned %ld\n", (long) pid);

/* Parent falls through to here */

sleep(1); /* Give child time to change its hostname */

/* Display hostname in parent's UTS namespace. This will be
different from hostname in child's UTS namespace. */

if (uname(&uts) == -1)
errExit("uname");
printf("uts.nodename in parent: %s\n", uts.nodename);

if (waitpid(pid, NULL, 0) == -1) /* Wait for child */
errExit("waitpid");
printf("child has terminated\n");

exit(EXIT_SUCCESS);
}

SEE ALSO
fork(2), futex(2), getpid(2), gettid(2), kcmp(2), 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)

--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/

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