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 *child_stack,
int flags, void *arg, ...
/* pid_t *ptid, void *newtls, pid_t *ctid */ );
/* For the prototype of the raw system call, see NOTES */
DESCRIPTION
clone() creates a new process, in a manner similar to fork(2).
This page describes both 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.
Unlike fork(2), clone() allows 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.)
One use of clone() is to implement threads: multiple flows of control
in a program that run concurrently in a shared address space.
When the child process is created with clone(), 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 child_stack argument specifies the location of the stack used by
the child process. Since the child and calling process may share mem-
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 child_stack usually points to the topmost
address of the memory space set up for the child stack.
The low byte of flags contains the number of the termination signal
sent to the parent when the child dies. 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 is specified, then the parent process is not signaled when
the child terminates.
flags may also be bitwise-ORed with zero or more of the following con-
stants, in order to specify what is shared between the calling process
and the child process:
CLONE_CHILD_CLEARTID (since Linux 2.5.49)
Clear (zero) the child thread ID at the location ctid 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 ctid in the child's
memory. The store operation completes before clone() returns
control to user space.
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 fcntl(2) F_SETFD operation), the other process is also
affected. If a process sharing a file descriptor table calls
execve(2), its file descriptor table is duplicated (unshared).
If CLONE_FILES is not set, the child process inherits a copy of
all file descriptors opened in the calling process at the time
of 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 descrip-
tions as the corresponding file descriptors in the calling
process, and thus share file offsets and file status flags (see
open(2)).
CLONE_FS (since Linux 2.0)
If CLONE_FS is set, the caller and the child process share the
same filesystem information. This includes the root of the
filesystem, the current working directory, and the umask. Any
call to chroot(2), chdir(2), or umask(2) performed by the call-
ing 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) per-
formed 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 context
with the calling process. If this flag is not set, then (as
with fork(2)) the new process has its own I/O context.
The I/O context is the I/O scope of the disk scheduler (i.e.,
what the I/O scheduler uses to model scheduling of a process's
I/O). If processes share the same I/O context, they are treated
as one by the I/O scheduler. As a consequence, they get to
share disk time. For some I/O schedulers, if two processes
share an I/O context, they will be allowed to interleave their
disk access. If several threads are doing I/O on behalf of the
same process (aio_read(3), for instance), they should employ
CLONE_IO to get better I/O performance.
If the kernel is not configured with the CONFIG_BLOCK option,
this flag is a no-op.
CLONE_NEWCGROUP (since Linux 4.6)
Create the process in a new cgroup namespace. If this flag is
not set, then (as with fork(2)) the process is created in the
same cgroup namespaces as the calling process. This flag is
intended for the implementation of containers.
For further information on cgroup namespaces, see cgroup_names-
paces(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. This flag is intended for the implementation of con-
tainers.
An IPC namespace provides an isolated view of System V IPC
objects (see svipc(7)) and (since Linux 2.6.30) POSIX message
queues (see mq_overview(7)). The common characteristic of these
IPC mechanisms is that IPC objects are identified 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 visi-
ble 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 namespaces(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 namespace as the calling
process. This flag is intended for the implementation of con-
tainers.
A network namespace provides an isolated view of the networking
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 net-
work device can live in exactly one network namespace. A vir-
tual network (veth(4)) device pair provides a pipe-like abstrac-
tion that can be used to create tunnels between network names-
paces, and can be used to create a bridge to a physical network
device in another namespace.
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 information 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 namespaces(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 con-
tainers.
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 or CLONE_PARENT.
CLONE_NEWUSER
(This flag first became meaningful for clone() in Linux 2.6.23,
the current clone() semantics were merged in Linux 3.5, and the
final pieces to make the user namespaces completely usable were
merged in Linux 3.8.)
If CLONE_NEWUSER is set, then create the process in a new user
namespace. If this flag is not set, then (as with fork(2)) the
process is created in the same user namespace as the calling
process.
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 cre-
ate 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 namespaces(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 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. 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), respectively. Changes made
to the identifiers in a UTS namespace are visible to all other
processes in the same namespace, but are not visible to pro-
cesses in other UTS namespaces.
Only a privileged process (CAP_SYS_ADMIN) can employ
CLONE_NEWUTS.
For further information on UTS namespaces, see namespaces(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 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, will be signaled.
CLONE_PARENT_SETTID (since Linux 2.5.49)
Store the child thread ID at the location ptid 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 completely from the ker-
nel sources in Linux 2.5.16. Since then, the kernel silently
ignores this bit if it is specified in flags.
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 newtls.
The interpretation of newtls and the resulting effect is archi-
tecture dependent. On x86, newtls 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.
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
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-test6, flags must also include CLONE_VM if
CLONE_SIGHAND is specified
CLONE_STOPPED (since Linux 2.6.0-test2)
If CLONE_STOPPED is set, then the child is initially stopped (as
though it was sent a SIGSTOP signal), and must be resumed by
sending it a SIGCONT signal.
This flag was deprecated from Linux 2.6.25 onward, and was
removed altogether in Linux 2.6.38. Since then, the kernel
silently ignores it without error. Starting with Linux 4.6, the
same bit was reused for the CLONE_NEWCGROUP flag.
CLONE_SYSVSEM (since Linux 2.5.10)
If CLONE_SYSVSEM is set, then the child and the calling process
share a single list of System V semaphore adjustment (semadj)
values (see semop(2)). In this case, the shared list accumu-
lates semadj values across all processes sharing the list, and
semaphore adjustments are performed only when the last process
that is sharing the list terminates (or ceases sharing the list
using unshare(2)). If this flag is not set, then the child has
a separate semadj list that is initially empty.
CLONE_THREAD (since Linux 2.4.0-test8)
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
identifier (TGID) for the thread group. Since Linux 2.4, calls
to getpid(2) return the TGID of the caller.
The threads within a group can be distinguished by their (sys-
tem-wide) unique thread IDs (TID). A new thread's TID is avail-
able as the function result returned to the caller of clone(),
and a thread can obtain its own TID using gettid(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 termi-
nates, 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 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, flags must also include CLONE_SIGHAND if
CLONE_THREAD is specified (and note that, since Linux
2.6.0-test6, CLONE_SIGHAND also requires CLONE_VM to be
included).
Signals may be sent to a thread group as a whole (i.e., a TGID)
using kill(2), or to a specific thread (i.e., TID) using
tgkill(2).
Signal dispositions and actions are process-wide: if an unhan-
dled signal is delivered to a thread, then it will affect (ter-
minate, stop, continue, be ignored in) all members of the thread
group.
Each thread has its own signal mask, as set by sigprocmask(2),
but signals can be pending either: for the whole process (i.e.,
deliverable to any member of the thread group), when sent with
kill(2); or for an individual thread, when sent with tgkill(2).
A call to sigpending(2) returns a signal set that is the union
of the signals pending for the whole process and the signals
that are pending for the calling thread.
If kill(2) is used to send a signal 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 sig-
nal using sigwaitinfo(2), the kernel will arbitrarily select one
of these threads to receive a signal sent using kill(2).
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 per-
formed 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
clone(). Memory writes or file mappings/unmappings performed by
one of the processes do not affect the other, as with fork(2).
NOTES
Note that the glibc clone() wrapper function makes some changes in the
memory pointed to by child_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
child_stack argument may be zero, in which case the child uses a dupli-
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 duplica-
tion 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 roughly:
long clone(unsigned long flags, void *child_stack,
int *ptid, int *ctid,
unsigned long newtls);
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 *child_stack,
int *ptid, unsigned long newtls,
int *ctid);
On the cris and s390 architectures, the order of the first two argu-
ments is reversed:
long clone(void *child_stack, unsigned long flags,
int *ptid, int *ctid,
unsigned long newtls);
On the microblaze architecture, an additional argument is supplied:
long clone(unsigned long flags, void *child_stack,
int stack_size, /* Size of stack */
int *ptid, int *ctid,
unsigned long newtls);
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 *child_stack_base, size_t stack_size,
int flags, void *arg, ...
/* pid_t *ptid, struct user_desc *tls, pid_t *ctid */ );
The prototype shown above is for the glibc wrapper function; the raw
system call interface has no fn or arg argument, and changes the order
of the arguments so that flags is the first argument, and tls is the
last argument.
__clone2() operates in the same way as clone(), except that
child_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
child_stack_base.
Linux 2.4 and earlier
In Linux 2.4 and earlier, clone() does not take arguments ptid, tls,
and ctid.
RETURN VALUE
On success, the thread ID of the child process is returned in the call-
er'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 appro-
priately.
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-test6.)
EINVAL CLONE_THREAD was specified, but CLONE_SIGHAND was not. (Since
Linux 2.5.35.)
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
child_stack is specified as NULL.
EINVAL CLONE_NEWIPC was specified in flags, but the kernel was not con-
figured with the CONFIG_SYSVIPC and CONFIG_IPC_NS options.
EINVAL CLONE_NEWNET was specified in flags, but the kernel was not con-
figured with the CONFIG_NET_NS option.
EINVAL CLONE_NEWPID was specified in flags, but the kernel was not con-
figured with the CONFIG_PID_NS option.
EINVAL CLONE_NEWUTS was specified in flags, but the kernel was not con-
figured with the CONFIG_UTS option.
EINVAL child_stack is not aligned to a suitable boundary for this
architecture. For example, on aarch64, child_stack must be a
multiple of 16.
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 flags, but the limit on the nest-
ing 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 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 flags, but either the effective
user ID or the effective group ID of the caller does not have a
mapping in the parent namespace (see user_namespaces(7)).
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 num-
ber of nested user namespaces would be exceeded. See the dis-
cussion of the ENOSPC error above.
CONFORMING TO
clone() is 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 during 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): parent
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 wrap-
per function for getpid(2) that performed caching of PIDs. This
caching relied on support in the glibc wrapper for clone(), but limita-
tions 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 dif-
fers 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), set_thread_area(2),
set_tid_address(2), setns(2), tkill(2), unshare(2), wait(2), capabili-
ties(7), namespaces(7), pthreads(7)
COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 CLONE(2)
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