NAME¶
clone, __clone2 - create a child process
SYNOPSIS¶
#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 */ );
DESCRIPTION¶
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).
sys_clone¶
The
sys_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 is roughly:
long clone(unsigned long flags, void *child_stack,
void *ptid, void *ctid,
struct pt_regs *regs);
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.
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.
The
clone() and
sys_clone calls are 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.
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.
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);
SEE ALSO¶
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)
COLOPHON¶
This page is part of release 3.44 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/.