| NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON | The Linux Programming Interface |
CLONE(2) Linux Programmer's Manual CLONE(2)
clone, __clone2 - create a child process
#define _GNU_SOURCE /* See feature_test_macros(7) */
#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 */ );
clone() creates a new process, in a manner similar to fork(2). It is
actually a library function layered on top of the underlying clone() system
call, hereinafter referred to as sys_clone. A description of sys_clone is
given toward the end of this page.
Unlike fork(2), these calls allow 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
application 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
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
If CLONE_FS is set, the caller and the child process share the same
file system information. This includes the root of the file system,
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
file system 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 consists of the set of identifiers for System V IPC
objects. (These objects are created using msgctl(2), semctl(2), and
shmctl(2)). 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 only completed 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
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
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
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
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 sys_clone system call corresponds more closely to fork(2) in that
execution in the child continues from the point of the call. Thus,
sys_clone only requires the flags and child_stack arguments, which have the
same meaning as for clone(). (Note that the order of these arguments
differs from clone().)
Another difference for sys_clone 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.
In Linux 2.4 and earlier, clone() does not take arguments ptid, tls, and
ctid.
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.
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.
There is no entry for clone() in libc5. glibc2 provides clone() as
described in this manual page.
The clone() and sys_clone calls are Linux-specific and should not be used
in programs intended to be portable.
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.
On ia64, a different system call 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 __clone2() system call 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.
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);
fork(2), futex(2), getpid(2), gettid(2), set_thread_area(2),
set_tid_address(2), tkill(2), unshare(2), wait(2), capabilities(7),
pthreads(7)
This page is part of release 3.41 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/.
Linux 2011-09-08 CLONE(2)
HTML rendering created 2012-05-11 by Michael Kerrisk,
author of
The Linux Programming Interface,
maintainer of the
Linux man-pages project