clone, __clone2 — create a child process
#define _GNU_SOURCE #include <sched.h>
int
clone( |
int | (*fn)(void
*), |
| void * | child_stack, | |
| int | flags, | |
| void * | arg, | |
... /*
pid_t *pid, 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
towards 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 downwards 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_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_FSIf CLONE_FS is set,
the caller and the child processes 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_FILESIf CLONE_FILES is set,
the calling process and the child processes 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_NEWNS (since Linux
2.4.19)Start the child in a new namespace.
Every process lives in a 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 namespace as the
parent. The system calls mount(2) and
umount(2) change the
namespace of the calling process, and hence affect all
processes that live in the same namespace, but do not
affect processes in a different namespace.
After a clone() where
the CLONE_NEWNS flag is
set, the cloned child is started in a new 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_SIGHANDIf CLONE_SIGHAND is
set, the calling process and the child processes 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_PTRACEIf CLONE_PTRACE is
specified, and the calling process is being traced,
then trace the child also (see ptrace(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_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.
CLONE_VFORKIf 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_VMIf CLONE_VM is set,
the calling process and the child processes 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).
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_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_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_SETTLS (since Linux
2.5.32)The newtls
parameter is the new TLS (Thread Local Storage)
descriptor. (See set_thread_area(2).)
CLONE_PARENT_SETTID (since Linux
2.5.49)Store child thread ID at location parent_tidptr in parent
and child memory. (In Linux 2.5.32-2.5.48 there was a
flag CLONE_SETTID that
did this.)
CLONE_CHILD_SETTID (since Linux
2.5.49)Store child thread ID at location child_tidptr in child
memory.
CLONE_CHILD_CLEARTID (since Linux
2.5.49)Erase child thread ID at location child_tidptr 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.
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.
Since Linux 2.5.49 the system call has five parameters.
The two new parameters are parent_tidptr which points
to the location (in parent and child memory) where the
child thread ID will be written in case CLONE_PARENT_SETTID was specified, and
child_tidptr
which points to the location (in child memory) where the
child thread ID will be written in case CLONE_CHILD_SETTID was specified.
On success, the thread ID of the child process is returned
in the caller's thread of execution. On failure, a −1
will be returned in the caller's context, no child process
will be created, and errno will
be set appropriately.
Too many processes are already running.
CLONE_SIGHAND was
specified, but CLONE_VM
was not. (Since Linux 2.6.0-test6.)
CLONE_THREAD was
specified, but CLONE_SIGHAND was not. (Since Linux
2.5.35.)
Both CLONE_FS and
CLONE_NEWNS were
specified in flags.
Returned by clone()
when a zero value is specified for child_stack.
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.
CLONE_NEWNS was
specified by a non-root process (process without
CAP_SYS_ADMIN).
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 *pid, 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. In programs linked against such libraries,
calls to getpid(2) may return the
same value, even when the threads were not created using
CLONE_THREAD (and thus are not
in the same thread group). 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 2.79 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/.
|
Copyright (c) 1992 Drew Eckhardt <drewcs.colorado.edu>, March 28, 1992 and Copyright (c) Michael Kerrisk, 2001, 2002, 2005 May be distributed under the GNU General Public License. Modified by Michael Haardt <michaelmoria.de> Modified 24 Jul 1993 by Rik Faith <faithcs.unc.edu> Modified 21 Aug 1994 by Michael Chastain <mecshell.portal.com>: New man page (copied from 'fork.2'). Modified 10 June 1995 by Andries Brouwer <aebcwi.nl> Modified 25 April 1998 by Xavier Leroy <Xavier.Leroyinria.fr> Modified 26 Jun 2001 by Michael Kerrisk Mostly upgraded to 2.4.x Added prototype for sys_clone() plus description Added CLONE_THREAD with a brief description of thread groups Added CLONE_PARENT and revised entire page remove ambiguity between "calling process" and "parent process" Added CLONE_PTRACE and CLONE_VFORK Added EPERM and EINVAL error codes Renamed "__clone" to "clone" (which is the prototype in <sched.h>) various other minor tidy ups and clarifications. Modified 26 Jun 2001 by Michael Kerrisk <mtk.manpagesgmail.com> Updated notes for 2.4.7+ behavior of CLONE_THREAD Modified 15 Oct 2002 by Michael Kerrisk <mtk.manpagesgmail.com> Added description for CLONE_NEWNS, which was added in 2.4.19 Slightly rephrased, aeb. Modified 1 Feb 2003 - added CLONE_SIGHAND restriction, aeb. Modified 1 Jan 2004 - various updates, aeb Modified 2004-09-10 - added CLONE_PARENT_SETTID etc. - aeb. 2005-04-12, mtk, noted the PID caching behavior of NPTL's getpid() wrapper under BUGS. 2005-05-10, mtk, added CLONE_SYSVSEM, CLONE_UNTRACED, CLONE_STOPPED. 2005-05-17, mtk, Substantially enhanced discussion of CLONE_THREAD. FIXME Document CLONE_NEWIPC, which is new in 2.6.18 (also supported for unshare()?) FIXME Document CLONE_NEWUTS, which is new in 2.6.19 (also supported for unshare()?) FIXME Document CLONE_NEWUSER, which is new in 2.6.23 (also supported for unshare()?) FIXME 2.6.25 marks the unused CLONE_STOPPED as obsolete, and it will probably be removed in the future. FIXME 2.6.25: CLONE_IO flag to clone() causes I/O contexts (used in the CFQ block I/O scheduler) to be shared with the new child process. |