Many pages: Use \[ti] instead of \(ti

[thirdparty/man-pages.git] / man2 / clone.2

'\" t
.\" Copyright (c) 1992 Drew Eckhardt <drew@cs.colorado.edu>, March 28, 1992
.\" and Copyright (c) Michael Kerrisk, 2001, 2002, 2005, 2013, 2019
.\"
.\" SPDX-License-Identifier: GPL-1.0-or-later
.\"
.\" Modified by Michael Haardt <michael@moria.de>
.\" Modified 24 Jul 1993 by Rik Faith <faith@cs.unc.edu>
.\" Modified 21 Aug 1994 by Michael Chastain <mec@shell.portal.com>:
.\"   New man page (copied from 'fork.2').
.\" Modified 10 June 1995 by Andries Brouwer <aeb@cwi.nl>
.\" Modified 25 April 1998 by Xavier Leroy <Xavier.Leroy@inria.fr>
.\" Modified 26 Jun 2001 by Michael Kerrisk
.\"     Mostly upgraded to Linux 2.4.x
.\"     Added prototype for sys_clone() plus description
.\"     Added CLONE_THREAD with a brief description of thread groups
.\"     Added CLONE_PARENT and revised entire page remove ambiguity
.\"             between "calling process" and "parent process"
.\"     Added CLONE_PTRACE and CLONE_VFORK
.\"     Added EPERM and EINVAL error codes
.\"     Renamed "__clone" to "clone" (which is the prototype in <sched.h>)
.\"     various other minor tidy ups and clarifications.
.\" Modified 26 Jun 2001 by Michael Kerrisk <mtk.manpages@gmail.com>
.\"     Updated notes for 2.4.7+ behavior of CLONE_THREAD
.\" Modified 15 Oct 2002 by Michael Kerrisk <mtk.manpages@gmail.com>
.\"     Added description for CLONE_NEWNS, which was added in Linux 2.4.19
.\" Slightly rephrased, aeb.
.\" Modified 1 Feb 2003 - added CLONE_SIGHAND restriction, aeb.
.\" Modified 1 Jan 2004 - various updates, aeb
.\" Modified 2004-09-10 - added CLONE_PARENT_SETTID etc. - aeb.
.\" 2005-04-12, mtk, noted the PID caching behavior of NPTL's getpid()
.\"     wrapper under BUGS.
.\" 2005-05-10, mtk, added CLONE_SYSVSEM, CLONE_UNTRACED, CLONE_STOPPED.
.\" 2005-05-17, mtk, Substantially enhanced discussion of CLONE_THREAD.
.\" 2008-11-18, mtk, order CLONE_* flags alphabetically
.\" 2008-11-18, mtk, document CLONE_NEWPID
.\" 2008-11-19, mtk, document CLONE_NEWUTS
.\" 2008-11-19, mtk, document CLONE_NEWIPC
.\" 2008-11-19, Jens Axboe, mtk, document CLONE_IO
.\"
.TH clone 2 (date) "Linux man-pages (unreleased)"
.SH NAME
clone, __clone2, clone3 \- create a child process
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
/* Prototype for the glibc wrapper function */
.PP
.B #define _GNU_SOURCE
.B #include <sched.h>
.PP
.BI "int clone(int (*" "fn" ")(void *_Nullable), void *" stack \
", int " flags ,
.BI "          void *_Nullable " "arg" ", ..." \
"  \fR/*\fP" " pid_t *_Nullable " parent_tid ,
.BI "                                       void *_Nullable " tls ,
.BI "                                       pid_t *_Nullable " child_tid " \fR*/\fP );"
.PP
/* For the prototype of the raw clone() system call, see NOTES */
.PP
.BR "#include <linux/sched.h>" "    /* Definition of " "struct clone_args" " */"
.BR "#include <sched.h>" "          /* Definition of " CLONE_* " constants */"
.BR "#include <sys/syscall.h>" "    /* Definition of " SYS_* " constants */"
.B #include <unistd.h>
.PP
.BI "long syscall(SYS_clone3, struct clone_args *" cl_args ", size_t " size );
.fi
.PP
.IR Note :
glibc provides no wrapper for
.BR clone3 (),
necessitating the use of
.BR syscall (2).
.SH DESCRIPTION
These system calls
create a new ("child") process, in a manner similar to
.BR fork (2).
.PP
By contrast with
.BR 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
.BR namespaces (7).
.PP
Note that in this manual
page, "calling process" normally corresponds to "parent process".
But see the descriptions of
.B CLONE_PARENT
and
.B CLONE_THREAD
below.
.PP
This page describes the following interfaces:
.IP \(bu 3
The glibc
.BR 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.
.IP \(bu
The newer
.BR clone3 ()
system call.
.PP
In the remainder of this page, the terminology "the clone call" is used
when noting details that apply to all of these interfaces,
.\"
.SS The clone() wrapper function
When the child process is created with the
.BR clone ()
wrapper function,
it commences execution by calling the function pointed to by the argument
.IR fn .
(This differs from
.BR fork (2),
where execution continues in the child from the point
of the
.BR fork (2)
call.)
The
.I arg
argument is passed as the argument of the function
.IR fn .
.PP
When the
.IR fn ( arg )
function returns, the child process terminates.
The integer returned by
.I fn
is the exit status for the child process.
The child process may also terminate explicitly by calling
.BR exit (2)
or after receiving a fatal signal.
.PP
The
.I 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
.BR clone ().
Stacks grow downward on all processors that run Linux
(except the HP PA processors), so
.I stack
usually points to the topmost address of the memory space set up for
the child stack.
Note that
.BR clone ()
does not provide a means whereby the caller can inform the kernel of the
size of the stack area.
.PP
The remaining arguments to
.BR clone ()
are discussed below.
.\"
.SS clone3()
The
.BR clone3 ()
system call provides a superset of the functionality of the older
.BR 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.
.PP
As with
.BR fork (2),
.BR 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.
.PP
The
.I cl_args
argument of
.BR clone3 ()
is a structure of the following form:
.PP
.in +4n
.EX
struct clone_args {
    u64 flags;        /* Flags bit mask */
    u64 pidfd;        /* Where to store PID file descriptor
                         (\fIint *\fP) */
    u64 child_tid;    /* Where to store child TID,
                         in child\[aq]s memory (\fIpid_t *\fP) */
    u64 parent_tid;   /* Where to store child TID,
                         in parent\[aq]s memory (\fIpid_t *\fP) */
    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 \fIpid_t\fP array
                         (since Linux 5.5) */
    u64 set_tid_size; /* Number of elements in \fIset_tid\fP
                         (since Linux 5.5) */
    u64 cgroup;       /* File descriptor for target cgroup
                         of child (since Linux 5.7) */
};
.EE
.in
.PP
The
.I size
argument that is supplied to
.BR clone3 ()
should be initialized to the size of this structure.
(The existence of the
.I size
argument permits future extensions to the
.I clone_args
structure.)
.PP
The stack for the child process is specified via
.IR cl_args.stack ,
which points to the lowest byte of the stack area,
and
.IR cl_args.stack_size ,
which specifies the size of the stack in bytes.
In the case where the
.B 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).
.PP
The remaining fields in the
.I cl_args
argument are discussed below.
.\"
.SS Equivalence between clone() and clone3() arguments
Unlike the older
.BR clone ()
interface, where arguments are passed individually, in the newer
.BR clone3 ()
interface the arguments are packaged into the
.I clone_args
structure shown above.
This structure allows for a superset of the information passed via the
.BR clone ()
arguments.
.PP
The following table shows the equivalence between the arguments of
.BR clone ()
and the fields in the
.I clone_args
argument supplied to
.BR clone3 ():
.RS 4
.TS
lb lb lb
l l l
li li l.
clone() clone3()        Notes
        \fIcl_args\fP field
flags & \[ti]0xff       flags   T{
For most flags; details below
T}
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
\fP---\fP       stack_size
tls     tls     See CLONE_SETTLS
\fP---\fP       set_tid See below for details
\fP---\fP       set_tid_size
\fP---\fP       cgroup  See CLONE_INTO_CGROUP
.TE
.RE
.\"
.SS 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
.I flags
.RB ( clone ())
or in
.I cl_args.exit_signal
.RB ( clone3 ()).
If this signal is specified as anything other than
.BR SIGCHLD ,
then the parent process must specify the
.B __WALL
or
.B __WCLONE
options when waiting for the child with
.BR wait (2).
If no signal (i.e., zero) is specified, then the parent process is not signaled
when the child terminates.
.\"
.SS 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
.BR clone3 (),
the
.I 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
.I flags
contains
.BR CLONE_NEWPID )
then the first element in the
.I set_tid
array has to be the desired PID and
.I set_tid_size
needs to be 1.
.PP
If the PID of the newly created process should have a certain value in
multiple PID namespaces, then the
.I 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
.I set_tid_size
which cannot be larger than the number of currently nested PID namespaces.
.PP
To create a process with the following PIDs in a PID namespace hierarchy:
.RS 4
.TS
lb lb lb
l l l.
PID NS level    Requested PID   Notes
0       31496   Outermost PID namespace
1       42
2       7       Innermost PID namespace
.TE
.RE
.PP
Set the array to:
.PP
.in +4n
.EX
set_tid[0] = 7;
set_tid[1] = 42;
set_tid[2] = 31496;
set_tid_size = 3;
.EE
.in
.PP
If only the PIDs in the two innermost PID namespaces
need to be specified, set the array to:
.PP
.in +4n
.EX
set_tid[0] = 7;
set_tid[1] = 42;
set_tid_size = 2;
.EE
.in
.PP
The PID in the PID namespaces outside the two innermost PID namespaces
is selected the same way as any other PID is selected.
.PP
The
.I set_tid
feature requires
.B CAP_SYS_ADMIN
or
(since Linux 5.9)
.\" commit 124ea650d3072b005457faed69909221c2905a1f
.\" commit 1caef81da05a84a40dbf02110e967ce6d1135ff6
.B CAP_CHECKPOINT_RESTORE
in all owning user namespaces of the target PID namespaces.
.PP
Callers may only choose a PID greater than 1 in a given PID namespace
if an
.B 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.
.\"
.SS The flags mask
Both
.BR clone ()
and
.BR 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\[em]the
.I flags
argument of
.BR clone ()
or the
.I cl_args.flags
field passed to
.BR clone3 ()\[em]is
referred to as the
.I flags
mask in the remainder of this page.
.PP
The
.I 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
.BR clone ()
and
.BR clone3 ().
.TP
.BR CLONE_CHILD_CLEARTID " (since Linux 2.5.49)"
Clear (zero) the child thread ID at the location pointed to by
.I child_tid
.RB ( clone ())
or
.I cl_args.child_tid
.RB ( 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
.BR set_tid_address (2)
system call.
This is used by threading libraries.
.TP
.BR CLONE_CHILD_SETTID " (since Linux 2.5.49)"
Store the child thread ID at the location pointed to by
.I child_tid
.RB ( clone ())
or
.I cl_args.child_tid
.RB ( 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
.B CLONE_VM
flag is also employed.)
.TP
.BR CLONE_CLEAR_SIGHAND " (since Linux 5.5)"
.\" commit b612e5df4587c934bd056bf05f4a1deca4de4f75
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
.RB ( SIG_DFL )
in the child.
.IP
Specifying this flag together with
.B CLONE_SIGHAND
is nonsensical and disallowed.
.TP
.BR CLONE_DETACHED " (historical)"
For a while (during the Linux 2.5 development series)
.\" added in Linux 2.5.32; removed in Linux 2.6.0-test4
there was a
.B 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
.B 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
.B CLONE_THREAD
disappeared.
.IP
This flag is still defined, but it is usually ignored when calling
.BR clone ().
However, see the description of
.B CLONE_PIDFD
for some exceptions.
.TP
.BR CLONE_FILES " (since Linux 2.0)"
If
.B 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
.BR fcntl (2)
.B F_SETFD
operation), the other process is also affected.
If a process sharing a file descriptor table calls
.BR execve (2),
its file descriptor table is duplicated (unshared).
.IP
If
.B CLONE_FILES
is not set, the child process inherits a copy of all file descriptors
opened in the calling process at the time of 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
.BR open (2)).
.TP
.BR CLONE_FS " (since Linux 2.0)"
If
.B 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
.BR chroot (2),
.BR chdir (2),
or
.BR umask (2)
performed by the calling process or the child process also affects the
other process.
.IP
If
.B 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
.BR chroot (2),
.BR chdir (2),
or
.BR umask (2)
performed later by one of the processes do not affect the other process.
.TP
.BR CLONE_INTO_CGROUP " (since Linux 5.7)"
.\" commit ef2c41cf38a7559bbf91af42d5b6a4429db8fc68
By default, a child process is placed in the same version 2
cgroup as its parent.
The
.B CLONE_INTO_CGROUP
flag allows the child process to be created in a different version 2 cgroup.
(Note that
.B CLONE_INTO_CGROUP
has effect only for version 2 cgroups.)
.IP
In order to place the child process in a different cgroup,
the caller specifies
.B CLONE_INTO_CGROUP
in
.I cl_args.flags
and passes a file descriptor that refers to a version 2 cgroup in the
.I cl_args.cgroup
field.
(This file descriptor can be obtained by opening a cgroup v2 directory
using either the
.B O_RDONLY
or the
.B O_PATH
flag.)
Note that all of the usual restrictions (described in
.BR cgroups (7))
on placing a process into a version 2 cgroup apply.
.IP
Among the possible use cases for
.B CLONE_INTO_CGROUP
are the following:
.RS
.IP \(bu 3
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.
.IP \(bu
The
.B CLONE_INTO_CGROUP
flag also allows the creation of
frozen child processes by spawning them into a frozen cgroup.
(See
.BR cgroups (7)
for a description of the freezer controller.)
.IP \(bu
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.
.RE
.TP
.BR CLONE_IO " (since Linux 2.6.25)"
If
.B 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
.BR fork (2))
the new process has its own I/O context.
.IP
.\" The following based on text from Jens Axboe
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,
.\" the anticipatory and CFQ scheduler
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
.RB ( aio_read (3),
for instance), they should employ
.B CLONE_IO
to get better I/O performance.
.\" with CFQ and AS.
.IP
If the kernel is not configured with the
.B CONFIG_BLOCK
option, this flag is a no-op.
.TP
.BR CLONE_NEWCGROUP " (since Linux 4.6)"
Create the process in a new cgroup namespace.
If this flag is not set, then (as with
.BR fork (2))
the process is created in the same cgroup namespaces as the calling process.
.IP
For further information on cgroup namespaces, see
.BR cgroup_namespaces (7).
.IP
Only a privileged process
.RB ( CAP_SYS_ADMIN )
can employ
.BR CLONE_NEWCGROUP .
.\"
.TP
.BR CLONE_NEWIPC " (since Linux 2.6.19)"
If
.B CLONE_NEWIPC
is set, then create the process in a new IPC namespace.
If this flag is not set, then (as with
.BR fork (2)),
the process is created in the same IPC namespace as
the calling process.
.IP
For further information on IPC namespaces, see
.BR ipc_namespaces (7).
.IP
Only a privileged process
.RB ( CAP_SYS_ADMIN )
can employ
.BR CLONE_NEWIPC .
This flag can't be specified in conjunction with
.BR CLONE_SYSVSEM .
.TP
.BR CLONE_NEWNET " (since Linux 2.6.24)"
(The implementation of this flag was completed only
by about Linux 2.6.29.)
.IP
If
.B CLONE_NEWNET
is set, then create the process in a new network namespace.
If this flag is not set, then (as with
.BR fork (2))
the process is created in the same network namespace as
the calling process.
.IP
For further information on network namespaces, see
.BR network_namespaces (7).
.IP
Only a privileged process
.RB ( CAP_SYS_ADMIN )
can employ
.BR CLONE_NEWNET .
.TP
.BR CLONE_NEWNS " (since Linux 2.4.19)"
If
.B 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
.B CLONE_NEWNS
is not set, the child lives in the same mount
namespace as the parent.
.IP
For further information on mount namespaces, see
.BR namespaces (7)
and
.BR mount_namespaces (7).
.IP
Only a privileged process
.RB ( CAP_SYS_ADMIN )
can employ
.BR CLONE_NEWNS .
It is not permitted to specify both
.B CLONE_NEWNS
and
.B CLONE_FS
.\" See https://lwn.net/Articles/543273/
in the same clone call.
.TP
.BR CLONE_NEWPID " (since Linux 2.6.24)"
.\" This explanation draws a lot of details from
.\" http://lwn.net/Articles/259217/
.\" Authors: Pavel Emelyanov <xemul@openvz.org>
.\" and Kir Kolyshkin <kir@openvz.org>
.\"
.\" The primary kernel commit is 30e49c263e36341b60b735cbef5ca37912549264
.\" Author: Pavel Emelyanov <xemul@openvz.org>
If
.B CLONE_NEWPID
is set, then create the process in a new PID namespace.
If this flag is not set, then (as with
.BR fork (2))
the process is created in the same PID namespace as
the calling process.
.IP
For further information on PID namespaces, see
.BR namespaces (7)
and
.BR pid_namespaces (7).
.IP
Only a privileged process
.RB ( CAP_SYS_ADMIN )
can employ
.BR CLONE_NEWPID .
This flag can't be specified in conjunction with
.B CLONE_THREAD
or
.BR CLONE_PARENT .
.TP
.B CLONE_NEWUSER
(This flag first became meaningful for
.BR clone ()
in Linux 2.6.23,
the current
.BR clone ()
semantics were merged in Linux 3.5,
and the final pieces to make the user namespaces completely usable were
merged in Linux 3.8.)
.IP
If
.B CLONE_NEWUSER
is set, then create the process in a new user namespace.
If this flag is not set, then (as with
.BR fork (2))
the process is created in the same user namespace as the calling process.
.IP
For further information on user namespaces, see
.BR namespaces (7)
and
.BR user_namespaces (7).
.IP
Before Linux 3.8, use of
.B CLONE_NEWUSER
required that the caller have three capabilities:
.BR CAP_SYS_ADMIN ,
.BR CAP_SETUID ,
and
.BR CAP_SETGID .
.\" Before Linux 2.6.29, it appears that only CAP_SYS_ADMIN was needed
Starting with Linux 3.8,
no privileges are needed to create a user namespace.
.IP
This flag can't be specified in conjunction with
.B CLONE_THREAD
or
.BR CLONE_PARENT .
For security reasons,
.\" commit e66eded8309ebf679d3d3c1f5820d1f2ca332c71
.\" https://lwn.net/Articles/543273/
.\" The fix actually went into Linux 3.9 and into Linux 3.8.3. However, user namespaces
.\" were, for practical purposes, unusable in earlier Linux 3.8.x because of the
.\" various filesystems that didn't support userns.
.B CLONE_NEWUSER
cannot be specified in conjunction with
.BR CLONE_FS .
.TP
.BR CLONE_NEWUTS " (since Linux 2.6.19)"
If
.B 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
.BR fork (2))
the process is created in the same UTS namespace as
the calling process.
.IP
For further information on UTS namespaces, see
.BR uts_namespaces (7).
.IP
Only a privileged process
.RB ( CAP_SYS_ADMIN )
can employ
.BR CLONE_NEWUTS .
.TP
.BR CLONE_PARENT " (since Linux 2.3.12)"
If
.B CLONE_PARENT
is set, then the parent of the new child (as returned by
.BR getppid (2))
will be the same as that of the calling process.
.IP
If
.B CLONE_PARENT
is not set, then (as with
.BR fork (2))
the child's parent is the calling process.
.IP
Note that it is the parent process, as returned by
.BR getppid (2),
which is signaled when the child terminates, so that
if
.B CLONE_PARENT
is set, then the parent of the calling process, rather than the
calling process itself, is signaled.
.IP
The
.B 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.
.TP
.BR CLONE_PARENT_SETTID " (since Linux 2.5.49)"
Store the child thread ID at the location pointed to by
.I parent_tid
.RB ( clone ())
or
.I cl_args.parent_tid
.RB ( clone3 ())
in the parent's memory.
(In Linux 2.5.32-2.5.48 there was a flag
.B CLONE_SETTID
that did this.)
The store operation completes before the clone call
returns control to user space.
.TP
.BR CLONE_PID " (Linux 2.0 to Linux 2.5.15)"
If
.B 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
.I flags
mask.
Much later, the same bit was recycled for use as the
.B CLONE_PIDFD
flag.
.TP
.BR CLONE_PIDFD " (since Linux 5.2)"
.\" commit b3e5838252665ee4cfa76b82bdf1198dca81e5be
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
.BR pidfd_open (2).
.RS
.IP \(bu 3
When using
.BR clone3 (),
the PID file descriptor is placed at the location pointed to by
.IR cl_args.pidfd .
.IP \(bu
When using
.BR clone (),
the PID file descriptor is placed at the location pointed to by
.IR parent_tid .
Since the
.I parent_tid
argument is used to return the PID file descriptor,
.B CLONE_PIDFD
cannot be used with
.B CLONE_PARENT_SETTID
when calling
.BR clone ().
.RE
.IP
It is currently not possible to use this flag together with
.B CLONE_THREAD.
This means that the process identified by the PID file descriptor
will always be a thread group leader.
.IP
If the obsolete
.B CLONE_DETACHED
flag is specified alongside
.B CLONE_PIDFD
when calling
.BR clone (),
an error is returned.
An error also results if
.B CLONE_DETACHED
is specified when calling
.BR clone3 ().
This error behavior ensures that the bit corresponding to
.B CLONE_DETACHED
can be reused for further PID file descriptor features in the future.
.TP
.BR CLONE_PTRACE " (since Linux 2.2)"
If
.B CLONE_PTRACE
is specified, and the calling process is being traced,
then trace the child also (see
.BR ptrace (2)).
.TP
.BR CLONE_SETTLS " (since Linux 2.5.32)"
The TLS (Thread Local Storage) descriptor is set to
.IR tls .
.IP
The interpretation of
.I tls
and the resulting effect is architecture dependent.
On x86,
.I tls
is interpreted as a
.I struct user_desc\~*
(see
.BR set_thread_area (2)).
On x86-64 it is the new value to be set for the %fs base register
(see the
.B ARCH_SET_FS
argument to
.BR arch_prctl (2)).
On architectures with a dedicated TLS register, it is the new value
of that register.
.IP
Use of this flag requires detailed knowledge and generally it
should not be used except in libraries implementing threading.
.TP
.BR CLONE_SIGHAND " (since Linux 2.0)"
If
.B 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
.BR 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
.BR sigprocmask (2)
without affecting the other process.
.IP
If
.B 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
.BR sigaction (2)
performed later by one of the processes have no effect on the other
process.
.IP
Since Linux 2.6.0,
.\" Precisely: Linux 2.6.0-test6
the
.I flags
mask must also include
.B CLONE_VM
if
.B CLONE_SIGHAND
is specified.
.TP
.BR CLONE_STOPPED " (since Linux 2.6.0)"
.\" Precisely: Linux 2.6.0-test2
If
.B CLONE_STOPPED
is set, then the child is initially stopped (as though it was sent a
.B SIGSTOP
signal), and must be resumed by sending it a
.B SIGCONT
signal.
.IP
This flag was
.I deprecated
from Linux 2.6.25 onward,
and was
.I removed
altogether in Linux 2.6.38.
Since then, the kernel silently ignores it without error.
.\" glibc 2.8 removed this defn from bits/sched.h
Starting with Linux 4.6, the same bit was reused for the
.B CLONE_NEWCGROUP
flag.
.TP
.BR CLONE_SYSVSEM " (since Linux 2.5.10)"
If
.B CLONE_SYSVSEM
is set, then the child and the calling process share
a single list of System V semaphore adjustment
.RI ( semadj )
values (see
.BR semop (2)).
In this case, the shared list accumulates
.I semadj
values across all processes sharing the list,
and semaphore adjustments are performed only when the last process
that is sharing the list terminates (or ceases sharing the list using
.BR unshare (2)).
If this flag is not set, then the child has a separate
.I semadj
list that is initially empty.
.TP
.BR CLONE_THREAD " (since Linux 2.4.0)"
.\" Precisely: Linux 2.6.0-test8
If
.B 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
.B CLONE_THREAD
more readable, the term "thread" is used to refer to the
processes within a thread group.
.IP
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
.BR getpid (2)
return the TGID of the caller.
.IP
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
.BR gettid (2).
.IP
When a clone call is made without specifying
.BR 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
.I leader
of the new thread group.
.IP
A new thread created with
.B CLONE_THREAD
has the same parent process as the process that made the clone call
(i.e., like
.BR CLONE_PARENT ),
so that calls to
.BR getppid (2)
return the same value for all of the threads in a thread group.
When a
.B CLONE_THREAD
thread terminates, the thread that created it is not sent a
.B SIGCHLD
(or other termination) signal;
nor can the status of such a thread be obtained
using
.BR wait (2).
(The thread is said to be
.IR detached .)
.IP
After all of the threads in a thread group terminate
the parent process of the thread group is sent a
.B SIGCHLD
(or other termination) signal.
.IP
If any of the threads in a thread group performs an
.BR execve (2),
then all threads other than the thread group leader are terminated,
and the new program is executed in the thread group leader.
.IP
If one of the threads in a thread group creates a child using
.BR fork (2),
then any thread in the group can
.BR wait (2)
for that child.
.IP
Since Linux 2.5.35, the
.I flags
mask must also include
.B CLONE_SIGHAND
if
.B CLONE_THREAD
is specified
(and note that, since Linux 2.6.0,
.\" Precisely: Linux 2.6.0-test6
.B CLONE_SIGHAND
also requires
.B CLONE_VM
to be included).
.IP
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.
.IP
Each thread has its own signal mask, as set by
.BR sigprocmask (2).
.IP
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
.BR kill (2)
or
.BR 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
.BR tgkill (2)
or
.BR pthread_sigqueue (3),
or because the thread executed a machine language instruction that triggered
a hardware exception
(e.g., invalid memory access triggering
.B SIGSEGV
or a floating-point exception triggering
.BR SIGFPE ).
.IP
A call to
.BR 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.
.IP
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
.BR sigwaitinfo (2),
the kernel will arbitrarily select one of these threads
to receive the signal.
.TP
.BR CLONE_UNTRACED " (since Linux 2.5.46)"
If
.B CLONE_UNTRACED
is specified, then a tracing process cannot force
.B CLONE_PTRACE
on this child process.
.TP
.BR CLONE_VFORK " (since Linux 2.2)"
If
.B CLONE_VFORK
is set, the execution of the calling process is suspended
until the child releases its virtual memory
resources via a call to
.BR execve (2)
or
.BR _exit (2)
(as with
.BR vfork (2)).
.IP
If
.B 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.
.TP
.BR CLONE_VM " (since Linux 2.0)"
If
.B 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
.BR mmap (2)
or
.BR munmap (2)
by the child or calling process also affects the other process.
.IP
If
.B 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
.BR fork (2).
.IP
If the
.B CLONE_VM
flag is specified and the
.B CLONE_VFORK
flag is not specified,
then any alternate signal stack that was established by
.BR sigaltstack (2)
is cleared in the child process.
.SH RETURN VALUE
.\" gettid(2) returns current->pid;
.\" getpid(2) returns current->tgid;
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
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.BR EACCES " (" clone3 "() only)"
.B CLONE_INTO_CGROUP
was specified in
.IR cl_args.flags ,
but the restrictions (described in
.BR cgroups (7))
on placing the child process into the version 2 cgroup referred to by
.I cl_args.cgroup
are not met.
.TP
.B EAGAIN
Too many processes are already running; see
.BR fork (2).
.TP
.BR EBUSY " (" clone3 "() only)"
.B CLONE_INTO_CGROUP
was specified in
.IR cl_args.flags ,
but the file descriptor specified in
.I cl_args.cgroup
refers to a version 2 cgroup in which a domain controller is enabled.
.TP
.BR EEXIST " (" clone3 "() only)"
One (or more) of the PIDs specified in
.I set_tid
already exists in the corresponding PID namespace.
.TP
.B EINVAL
Both
.B CLONE_SIGHAND
and
.B CLONE_CLEAR_SIGHAND
were specified in the
.I flags
mask.
.TP
.B EINVAL
.B CLONE_SIGHAND
was specified in the
.I flags
mask, but
.B CLONE_VM
was not.
(Since Linux 2.6.0.)
.\" Precisely: Linux 2.6.0-test6
.TP
.B EINVAL
.B CLONE_THREAD
was specified in the
.I flags
mask, but
.B CLONE_SIGHAND
was not.
(Since Linux 2.5.35.)
.\" .TP
.\" .B EINVAL
.\" Precisely one of
.\" .B CLONE_DETACHED
.\" and
.\" .B CLONE_THREAD
.\" was specified.
.\" (Since Linux 2.6.0-test6.)
.TP
.B EINVAL
.B CLONE_THREAD
was specified in the
.I flags
mask, but the current process previously called
.BR unshare (2)
with the
.B CLONE_NEWPID
flag or used
.BR setns (2)
to reassociate itself with a PID namespace.
.TP
.B EINVAL
.\" commit e66eded8309ebf679d3d3c1f5820d1f2ca332c71
Both
.B CLONE_FS
and
.B CLONE_NEWNS
were specified in the
.I flags
mask.
.TP
.BR EINVAL " (since Linux 3.9)"
Both
.B CLONE_NEWUSER
and
.B CLONE_FS
were specified in the
.I flags
mask.
.TP
.B EINVAL
Both
.B CLONE_NEWIPC
and
.B CLONE_SYSVSEM
were specified in the
.I flags
mask.
.TP
.B EINVAL
One (or both) of
.B CLONE_NEWPID
or
.B CLONE_NEWUSER
and one (or both) of
.B CLONE_THREAD
or
.B CLONE_PARENT
were specified in the
.I flags
mask.
.TP
.BR EINVAL " (since Linux 2.6.32)"
.\" commit 123be07b0b399670a7cc3d82fef0cb4f93ef885c
.B CLONE_PARENT
was specified, and the caller is an init process.
.TP
.B EINVAL
Returned by the glibc
.BR clone ()
wrapper function when
.I fn
or
.I stack
is specified as NULL.
.TP
.B EINVAL
.B CLONE_NEWIPC
was specified in the
.I flags
mask,
but the kernel was not configured with the
.B CONFIG_SYSVIPC
and
.B CONFIG_IPC_NS
options.
.TP
.B EINVAL
.B CLONE_NEWNET
was specified in the
.I flags
mask,
but the kernel was not configured with the
.B CONFIG_NET_NS
option.
.TP
.B EINVAL
.B CLONE_NEWPID
was specified in the
.I flags
mask,
but the kernel was not configured with the
.B CONFIG_PID_NS
option.
.TP
.B EINVAL
.B CLONE_NEWUSER
was specified in the
.I flags
mask,
but the kernel was not configured with the
.B CONFIG_USER_NS
option.
.TP
.B EINVAL
.B CLONE_NEWUTS
was specified in the
.I flags
mask,
but the kernel was not configured with the
.B CONFIG_UTS_NS
option.
.TP
.B EINVAL
.I stack
is not aligned to a suitable boundary for this architecture.
For example, on aarch64,
.I stack
must be a multiple of 16.
.TP
.BR EINVAL " (" clone3 "() only)"
.B  CLONE_DETACHED
was specified in the
.I flags
mask.
.TP
.BR EINVAL " (" clone "() only)"
.B CLONE_PIDFD
was specified together with
.B CLONE_DETACHED
in the
.I flags
mask.
.TP
.B EINVAL
.B CLONE_PIDFD
was specified together with
.B CLONE_THREAD
in the
.I flags
mask.
.TP
.BR "EINVAL " "("  clone "() only)"
.B CLONE_PIDFD
was specified together with
.B CLONE_PARENT_SETTID
in the
.I flags
mask.
.TP
.BR EINVAL " (" clone3 "() only)"
.I set_tid_size
is greater than the number of nested PID namespaces.
.TP
.BR EINVAL " (" clone3 "() only)"
One of the PIDs specified in
.I set_tid
was an invalid.
.TP
.BR EINVAL " (AArch64 only, Linux 4.6 and earlier)"
.I stack
was not aligned to a 128-bit boundary.
.TP
.B ENOMEM
Cannot allocate sufficient memory to allocate a task structure for the
child, or to copy those parts of the caller's context that need to be
copied.
.TP
.BR ENOSPC " (since Linux 3.7)"
.\" commit f2302505775fd13ba93f034206f1e2a587017929
.B CLONE_NEWPID
was specified in the
.I flags
mask,
but the limit on the nesting depth of PID namespaces
would have been exceeded; see
.BR pid_namespaces (7).
.TP
.BR ENOSPC " (since Linux 4.9; beforehand " EUSERS )
.B CLONE_NEWUSER
was specified in the
.I flags
mask, and the call would cause the limit on the number of
nested user namespaces to be exceeded.
See
.BR user_namespaces (7).
.IP
From Linux 3.11 to Linux 4.8, the error diagnosed in this case was
.BR EUSERS .
.TP
.BR ENOSPC " (since Linux 4.9)"
One of the values in the
.I flags
mask specified the creation of a new user namespace,
but doing so would have caused the limit defined by the corresponding file in
.I /proc/sys/user
to be exceeded.
For further details, see
.BR namespaces (7).
.TP
.BR EOPNOTSUPP " (" clone3 "() only)"
.B CLONE_INTO_CGROUP
was specified in
.IR cl_args.flags ,
but the file descriptor specified in
.I cl_args.cgroup
refers to a version 2 cgroup that is in the
.I domain invalid
state.
.TP
.B EPERM
.BR CLONE_NEWCGROUP ,
.BR CLONE_NEWIPC ,
.BR CLONE_NEWNET ,
.BR CLONE_NEWNS ,
.BR CLONE_NEWPID ,
or
.B CLONE_NEWUTS
was specified by an unprivileged process (process without \fBCAP_SYS_ADMIN\fP).
.TP
.B EPERM
.B CLONE_PID
was specified by a process other than process 0.
(This error occurs only on Linux 2.5.15 and earlier.)
.TP
.B EPERM
.B CLONE_NEWUSER
was specified in the
.I flags
mask,
but either the effective user ID or the effective group ID of the caller
does not have a mapping in the parent namespace (see
.BR user_namespaces (7)).
.TP
.BR EPERM " (since Linux 3.9)"
.\" commit 3151527ee007b73a0ebd296010f1c0454a919c7d
.B CLONE_NEWUSER
was specified in the
.I flags
mask and the caller is in a chroot environment
.\" FIXME What is the rationale for this restriction?
(i.e., the caller's root directory does not match the root directory
of the mount namespace in which it resides).
.TP
.BR EPERM " (" clone3 "() only)"
.I set_tid_size
was greater than zero, and the caller lacks the
.B CAP_SYS_ADMIN
capability in one or more of the user namespaces that own the
corresponding PID namespaces.
.TP
.BR ERESTARTNOINTR " (since Linux 2.6.17)"
.\" commit 4a2c7a7837da1b91468e50426066d988050e4d56
System call was interrupted by a signal and will be restarted.
(This can be seen only during a trace.)
.TP
.BR EUSERS " (Linux 3.11 to Linux 4.8)"
.B CLONE_NEWUSER
was specified in the
.I flags
mask,
and the limit on the number of nested user namespaces would be exceeded.
See the discussion of the
.B ENOSPC
error above.
.SH VERSIONS
The
.BR clone3 ()
system call first appeared in Linux 5.3.
.\" There is no entry for
.\" .BR clone ()
.\" in libc5.
.\" glibc2 provides
.\" .BR clone ()
.\" as described in this manual page.
.SH STANDARDS
These system calls
are Linux-specific and should not be used in programs
intended to be portable.
.SH 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.
.PP
Note that the glibc
.BR clone ()
wrapper function makes some changes
in the memory pointed to by
.I stack
(changes required to set the stack up correctly for the child)
.I before
invoking the
.BR clone ()
system call.
So, in cases where
.BR clone ()
is used to recursively create children,
do not use the buffer employed for the parent's stack
as the stack of the child.
.PP
The
.BR kcmp (2)
system call can be used to test whether two processes share various
resources such as a file descriptor table,
System V semaphore undo operations, or a virtual address space.
.PP
Handlers registered using
.BR pthread_atfork (3)
are not executed during a clone call.
.PP
In the Linux 2.4.x series,
.B CLONE_THREAD
generally does not make the parent of the new thread the same
as the parent of the calling process.
However, from Linux 2.4.7 to Linux 2.4.18 the
.B CLONE_THREAD
flag implied the
.B CLONE_PARENT
flag (as in Linux 2.6.0 and later).
.PP
On i386,
.BR clone ()
should not be called through vsyscall, but directly through
.IR "int $0x80" .
.\"
.SS C library/kernel differences
The raw
.BR clone ()
system call corresponds more closely to
.BR fork (2)
in that execution in the child continues from the point of the
call.
As such, the
.I fn
and
.I arg
arguments of the
.BR clone ()
wrapper function are omitted.
.PP
In contrast to the glibc wrapper, the raw
.BR clone ()
system call accepts NULL as a
.I stack
argument (and
.BR clone3 ()
likewise allows
.I 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
.B CLONE_VM
option should not be specified.
(If the child
.I shares
the parent's memory because of the use of the
.B CLONE_VM
flag,
then no copy-on-write duplication occurs and chaos is likely to result.)
.PP
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.
.PP
The raw system call interface on x86-64 and some other architectures
(including sh, tile, and alpha) is:
.PP
.in +4n
.EX
.BI "long clone(unsigned long " flags ", void *" stack ,
.BI "           int *" parent_tid ", int *" child_tid ,
.BI "           unsigned long " tls );
.EE
.in
.PP
On x86-32, and several other common architectures
(including score, ARM, ARM 64, PA-RISC, arc, Power PC, xtensa,
and MIPS),
.\" CONFIG_CLONE_BACKWARDS
the order of the last two arguments is reversed:
.PP
.in +4n
.EX
.BI "long clone(unsigned long " flags ", void *" stack ,
.BI "          int *" parent_tid ", unsigned long " tls ,
.BI "          int *" child_tid );
.EE
.in
.PP
On the cris and s390 architectures,
.\" CONFIG_CLONE_BACKWARDS2
the order of the first two arguments is reversed:
.PP
.in +4n
.EX
.BI "long clone(void *" stack ", unsigned long " flags ,
.BI "           int *" parent_tid ", int *" child_tid ,
.BI "           unsigned long " tls );
.EE
.in
.PP
On the microblaze architecture,
.\" CONFIG_CLONE_BACKWARDS3
an additional argument is supplied:
.PP
.in +4n
.EX
.BI "long clone(unsigned long " flags ", void *" stack ,
.BI "           int " stack_size , "\fR         /* Size of stack */"
.BI "           int *" parent_tid ", int *" child_tid ,
.BI "           unsigned long " tls );
.EE
.in
.\"
.SS blackfin, m68k, and sparc
.\" Mike Frysinger noted in a 2013 mail:
.\"     these arches don't define __ARCH_WANT_SYS_CLONE:
.\"     blackfin ia64 m68k sparc
The argument-passing conventions on
blackfin, m68k, and sparc are different from the descriptions above.
For details, see the kernel (and glibc) source.
.SS ia64
On ia64, a different interface is used:
.PP
.in +4n
.EX
.BI "int __clone2(int (*" "fn" ")(void *),"
.BI "             void *" stack_base ", size_t " stack_size ,
.BI "             int " flags ", void *" "arg" ", ..."
.BI "          /* pid_t *" parent_tid ", struct user_desc *" tls ,
.BI "             pid_t *" child_tid " */ );"
.EE
.in
.PP
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
.BR clone ()
prototype on microblaze):
.PP
.in +4n
.EX
.BI "long clone2(unsigned long " flags ", void *" stack_base ,
.BI "            int " stack_size , "\fR         /* Size of stack */"
.BI "            int *" parent_tid ", int *" child_tid ,
.BI "            unsigned long " tls );
.EE
.in
.PP
.BR __clone2 ()
operates in the same way as
.BR clone (),
except that
.I stack_base
points to the lowest address of the child's stack area,
and
.I stack_size
specifies the size of the stack pointed to by
.IR stack_base .
.SS Linux 2.4 and earlier
In Linux 2.4 and earlier,
.BR clone ()
does not take arguments
.IR parent_tid ,
.IR tls ,
and
.IR child_tid .
.SH BUGS
GNU C library versions 2.3.4 up to and including 2.24
contained a wrapper function for
.BR getpid (2)
that performed caching of PIDs.
This caching relied on support in the glibc wrapper for
.BR 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
.BR clone ()
call, then a call to
.BR 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
.BR CLONE_THREAD ,
when
.BR getpid (2)
.I should
return the same value in the child and in the process that called
.BR clone (),
since the caller and the child are in the same thread group.
The stale-cache problem also does not occur if the
.I flags
argument includes
.BR CLONE_VM .)
To get the truth, it was sometimes necessary to use code such as the following:
.PP
.in +4n
.EX
#include <syscall.h>

pid_t mypid;

mypid = syscall(SYS_getpid);
.EE
.in
.\" See also the following bug reports
.\" https://bugzilla.redhat.com/show_bug.cgi?id=417521
.\" http://sourceware.org/bugzilla/show_bug.cgi?id=6910
.PP
Because of the stale-cache problem, as well as other problems noted in
.BR getpid (2),
the PID caching feature was removed in glibc 2.25.
.SH EXAMPLES
The following program demonstrates the use of
.BR 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
.BR setns (2).
.PP
Within the sample program, we allocate the memory that is to
be used for the child's stack using
.BR mmap (2)
rather than
.BR malloc (3)
for the following reasons:
.IP \(bu 3
.BR 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 establish a guard page (a page with protection
.BR PROT_NONE )
at the end of the stack using
.BR mprotect (2).
.IP \(bu
We can specify the
.B MAP_STACK
flag to request a mapping that is suitable for a stack.
For the moment, this flag is a no-op on Linux,
but it exists and has effect on some other systems,
so we should include it for portability.
.SS Program source
.\" SRC BEGIN (clone.c)
.EX
#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/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\en", 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>\en", 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\en", (intmax_t) pid);

    /* Parent falls through to here */

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

    /* Display hostname in parent\[aq]s UTS namespace. This will be
       different from hostname in child\[aq]s UTS namespace. */

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

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

    exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.SH SEE ALSO
.BR fork (2),
.BR futex (2),
.BR getpid (2),
.BR gettid (2),
.BR kcmp (2),
.BR mmap (2),
.BR pidfd_open (2),
.BR set_thread_area (2),
.BR set_tid_address (2),
.BR setns (2),
.BR tkill (2),
.BR unshare (2),
.BR wait (2),
.BR capabilities (7),
.BR namespaces (7),
.BR pthreads (7)