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NAME
clone, __clone2 - create a child process
SYNOPSIS
/* Prototype for the glibc wrapper function */
#include <sched.h>
int clone(int (*fn)(void *), void *child_stack,
int flags, void *arg, ...
/* pid_t *ptid, struct user_desc *tls, pid_t *ctid */ );
/* Prototype for the raw system call */
long clone(unsigned long flags, void *child_stack,
void *ptid, void *ctid,
struct pt_regs *regs);
Feature Test Macro Requirements for glibc wrapper function (see feature_test_macros(7)):
clone():
Since glibc 2.14:
_GNU_SOURCE
Before glibc 2.14:
_BSD_SOURCE || _SVID_SOURCE
/* _GNU_SOURCE also suffices */
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 memory 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.)
The main use of clone() is to implement threads: multiple threads of control in a program
that run concurrently in a shared memory space.
When the child process is created with clone(), it executes the function fn(arg). (This
differs from fork(2), where execution continues in the child from the point of the fork(2)
call.) The fn argument is a pointer to a function that is called by the child process at
the beginning of its execution. The arg argument is passed to the fn function.
When the fn(arg) function application returns, the child process terminates. The integer
returned by fn is the exit code 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 memory, it is not possible for the child
process to execute in the same stack as the calling process. The calling process must
therefore set up memory space for the child stack and pass a pointer to this space to
clone(). Stacks grow downward on all processors that run Linux (except the HP PA
processors), so 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-or'ed with zero or more of the following constants, in order to
specify what is shared between the calling process and the child process:
CLONE_CHILD_CLEARTID (since Linux 2.5.49)
Erase child thread ID at 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 child thread ID at location ctid in child memory.
CLONE_FILES (since Linux 2.0)
If CLONE_FILES is set, the calling process and the child process share the same
file descriptor table. Any file descriptor created by the calling process or by
the child process is also valid in the other process. Similarly, if one of the
processes closes a file descriptor, or changes its associated flags (using the
fcntl(2) F_SETFD operation), the other process is also affected.
If 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(). (The duplicated
file descriptors in the child refer to the same open file descriptions (see
open(2)) as the corresponding file descriptors in the calling process.) 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.
CLONE_FS (since Linux 2.0)
If CLONE_FS is set, the caller and the child process share the same filesystem
information. This includes the root of the filesystem, the current working
directory, and the umask. Any call to chroot(2), chdir(2), or umask(2) performed
by the calling process or the child process also affects the other process.
If CLONE_FS is not set, the child process works on a copy of the filesystem
information of the calling process at the time of the clone() call. Calls to
chroot(2), chdir(2), 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 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_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 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 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.
Use of this flag requires: a kernel configured with the CONFIG_SYSVIPC and
CONFIG_IPC_NS options and that the process be privileged (CAP_SYS_ADMIN). This
flag can't be specified in conjunction with CLONE_SYSVSEM.
CLONE_NEWNET (since Linux 2.6.24)
(The implementation of this flag was completed only by about kernel version
2.6.29.)
If CLONE_NEWNET is set, then create the process in a new network namespace. If
this flag is not set, then (as with fork(2)), the process is created in the same
network namespace as the calling process. This flag is intended for the
implementation of containers.
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 network device can live in exactly one network namespace. A virtual
network device ("veth") pair provides a pipe-like abstraction that can be used to
create tunnels between network namespaces, 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).
Use of this flag requires: a kernel configured with the CONFIG_NET_NS option and
that the process be privileged (CAP_SYS_ADMIN).
CLONE_NEWNS (since Linux 2.4.19)
Start the child in a new mount namespace.
Every process lives in a mount namespace. The namespace of a process is the data
(the set of mounts) describing the file hierarchy as seen by that process. After a
fork(2) or clone() where the CLONE_NEWNS flag is not set, the child lives in the
same mount namespace as the parent. The system calls mount(2) and umount(2) change
the mount namespace of the calling process, and hence affect all processes that
live in the same namespace, but do not affect processes in a different mount
namespace.
After a clone() where the CLONE_NEWNS flag is set, the cloned child is started in a
new mount namespace, initialized with a copy of the namespace of the parent.
Only a privileged process (one having the CAP_SYS_ADMIN capability) may specify the
CLONE_NEWNS flag. It is not permitted to specify both CLONE_NEWNS and CLONE_FS in
the same clone() call.
CLONE_NEWPID (since Linux 2.6.24)
If CLONE_NEWPID is set, then create the process in a new PID namespace. If this
flag is not set, then (as with fork(2)), the process is created in the same PID
namespace as the calling process. This flag is intended for the implementation of
containers.
A PID namespace provides an isolated environment for PIDs: PIDs in a new namespace
start at 1, somewhat like a standalone system, and calls to fork(2), vfork(2), or
clone() will produce processes with PIDs that are unique within the namespace.
The first process created in a new namespace (i.e., the process created using the
CLONE_NEWPID flag) has the PID 1, and is the "init" process for the namespace.
Children that are orphaned within the namespace will be reparented to this process
rather than init(8). Unlike the traditional init process, the "init" process of a
PID namespace can terminate, and if it does, all of the processes in the namespace
are terminated.
PID namespaces form a hierarchy. When a new PID namespace is created, the
processes in that namespace are visible in the PID namespace of the process that
created the new namespace; analogously, if the parent PID namespace is itself the
child of another PID namespace, then processes in the child and parent PID
namespaces will both be visible in the grandparent PID namespace. Conversely, the
processes in the "child" PID namespace do not see the processes in the parent
namespace. The existence of a namespace hierarchy means that each process may now
have multiple PIDs: one for each namespace in which it is visible; each of these
PIDs is unique within the corresponding namespace. (A call to getpid(2) always
returns the PID associated with the namespace in which the process lives.)
After creating the new namespace, it is useful for the child to change its root
directory and mount a new procfs instance at /proc so that tools such as ps(1) work
correctly. (If CLONE_NEWNS is also included in flags, then it isn't necessary to
change the root directory: a new procfs instance can be mounted directly over
/proc.)
Use of this flag requires: a kernel configured with the CONFIG_PID_NS option and
that the process be privileged (CAP_SYS_ADMIN). This flag can't be specified in
conjunction with CLONE_THREAD.
CLONE_NEWUTS (since Linux 2.6.19)
If CLONE_NEWUTS is set, then create the process in a new UTS namespace, whose
identifiers are initialized by duplicating the identifiers from the UTS namespace
of the calling process. If this flag is not set, then (as with fork(2)), the
process is created in the same UTS namespace as the calling process. 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 host name 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
processes in other UTS namespaces.
Use of this flag requires: a kernel configured with the CONFIG_UTS_NS option and
that the process be privileged (CAP_SYS_ADMIN).
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 child thread ID at location ptid in parent and child memory. (In Linux
2.5.32-2.5.48 there was a flag CLONE_SETTID that did this.)
CLONE_PID (obsolete)
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. Since 2.3.21 this flag can be specified only by the system boot process (PID
0). It disappeared in Linux 2.5.16.
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 newtls argument is the new TLS (Thread Local Storage) descriptor. (See
set_thread_area(2).)
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 some 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.
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 undo values (see semop(2)). If this flag is not set, then
the child has a separate undo list, which 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 discussion of CLONE_THREAD more readable,
the term "thread" is used to refer to the processes within a thread group.
Thread groups were a feature added in Linux 2.4 to support the POSIX threads notion
of a set of threads that share a single PID. Internally, this shared PID is the
so-called thread group identifier (TGID) for the thread group. Since Linux 2.4,
calls to getpid(2) return the TGID of the caller.
The threads within a group can be distinguished by their (system-wide) unique
thread IDs (TID). A new thread's TID is available as the function result returned
to the caller 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
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.
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 unhandled signal is
delivered to a thread, then it will affect (terminate, stop, continue, be ignored
in) all members of the thread group.
Each thread has its own signal mask, as set by sigprocmask(2), 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 signal 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 performed by the calling process or by
the child process are also visible in the other process. Moreover, any memory
mapping or unmapping performed with mmap(2) or munmap(2) by the child or calling
process also affects the other process.
If CLONE_VM is not set, the child process runs in a separate copy of the memory
space of the calling process at the time of clone(). Memory writes or file
mappings/unmappings performed by one of the processes do not affect the other, as
with fork(2).
The raw system call interface
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. Furthermore, the argument order changes. The raw
system call interface on x86 and many other architectures is roughly:
long clone(unsigned long flags, void *child_stack,
void *ptid, void *ctid,
struct pt_regs *regs);
Another difference for the raw system call is that the child_stack argument may be zero,
in which case 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.
For some architectures, the order of the arguments for the system call differs from that
shown above. On the score, microblaze, ARM, ARM 64, PA-RISC, arc, Power PC, xtensa, and
MIPS architectures, the order of the fourth and fifth arguments is reversed. On the cris
and s390 architectures, the order of the first and second arguments is reversed.
blackfin, m68k, and sparc
The argument-passing conventions on blackfin, m68k, and sparc are different from
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 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.
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 Both CLONE_NEWIPC and CLONE_SYSVSEM were specified in flags.
EINVAL Both CLONE_NEWPID and CLONE_THREAD were specified in flags.
EINVAL Returned by clone() when a zero value is specified for child_stack.
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_NEWUTS was specified in flags, but the kernel was not configured with the
CONFIG_UTS option.
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.
EPERM 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.
VERSIONS
There is no entry for clone() in libc5. glibc2 provides clone() as described in this
manual page.
CONFORMING TO
clone() is Linux-specific and should not be used in programs intended to be portable.
NOTES
In the kernel 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 kernel 2.6).
For a while there was CLONE_DETACHED (introduced in 2.5.32): parent wants no child-exit
signal. In 2.6.2 the need to give this 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
Versions of the GNU C library that include the NPTL threading library contain a wrapper
function for getpid(2) that performs caching of PIDs. This caching relies on support in
the glibc wrapper for clone(), but as currently implemented, the cache may not be up to
date in some circumstances. In particular, if a signal is delivered to the child
immediately after the clone() call, then a call to getpid(2) in a handler for the signal
may return the PID of the calling process ("the parent"), if the clone wrapper has 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 may be necessary to use code such as
the following:
#include <syscall.h>
pid_t mypid;
mypid = syscall(SYS_getpid);
EXAMPLE
Create a child that executes in a separate UTS namespace
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).
#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), capabilities(7), pthreads(7)
COLOPHON
This page is part of release 3.54 of the Linux man-pages project. A description of the
project, and information about reporting bugs, can be found at
http://www.kernel.org/doc/man-pages/.