clone
CLONE(2) Linux Programmer’s Manual CLONE(2)
NAME
clone - create a child process
SYNOPSIS
#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 */ );
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 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 han-
dlers. (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 func-
tion 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 pro-
cess 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 ter-
minates. 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 mem-
ory, it is not possible for the child process to execute in the same
stack as the calling process. The calling process must therefore set
up memory space for the child stack and pass a pointer to this space
to clone(). Stacks grow 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 pro-
cess 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_FS
If 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_FILES
If CLONE_FILES is set, the calling process and the child pro-
cesses share the same file descriptor table. Any file descrip-
tor created by the calling process or by the child process is
also valid in the other process. Similarly, if one of the pro-
cesses 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.) Subse-
quent 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(2) where the
CLONE_NEWNS flag is not set, the child lives in the same names-
pace 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(2) 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 capa-
bility) may specify the CLONE_NEWNS flag. It is not permitted
to specify both CLONE_NEWNS and CLONE_FS in the same clone()
call.
CLONE_SIGHAND
If CLONE_SIGHAND is set, the calling process and the child pro-
cesses 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 pro-
cess.
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_PTRACE
If 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_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 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 map-
ping 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 (sys-
tem-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 pro-
cess as the caller of clone() (i.e., like CLONE_PARENT), so
that calls to getppid(2) return the same value for all of the
threads in a thread group. When a CLONE_THREAD thread termi-
nates, the thread that created it using clone() is not sent a
SIGCHLD (or other termination) signal; nor can the status of
such a thread be obtained using wait(2). (The thread is said
to be detached.)
After all of the threads in a thread group terminate the parent
process of the thread group is sent a SIGCHLD (or other termi-
nation) signal.
If any of the threads in a thread group performs an execve(2),
then all threads other than the thread group leader are termi-
nated, and the new program is executed in the thread group
leader.
If one of the threads in a thread group creates a child using
fork(2), then any thread in the group can wait(2) for that
child.
Since Linux 2.5.35, flags must also include CLONE_SIGHAND if
CLONE_THREAD is specified.
Signals may be sent to a thread group as a whole (i.e., a TGID)
using kill(2), or to a specific thread (i.e., TID) using
tgkill(2).
Signal dispositions and actions are process-wide: if an unhan-
dled signal is delivered to a thread, then it will affect (ter-
minate, stop, continue, be ignored in) all members of the
thread group.
Each thread has its own signal mask, as set by sigprocmask(2),
but signals can be pending either: for the whole process (i.e.,
deliverable to any member of the thread group), when sent with
kill(2); or for an individual thread, when sent with tgkill(2).
A call to sigpending(2) returns a signal set that is the union
of the signals pending for the whole process and the signals
that are pending for the calling thread.
If kill(2) is used to send a signal to a thread group, and the
thread group has installed a handler for the signal, then the
handler will be invoked in exactly one, arbitrarily selected
member of the thread group that has not blocked the signal. If
multiple threads in a group are waiting to accept the same sig-
nal using sigwaitinfo(2), the kernel will arbitrarily select
one of these threads to receive a signal sent using kill(2).
CLONE_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 sepa-
rate 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.
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. 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 writ-
ten in case CLONE_CHILD_SETTID was specified.
RETURN VALUE
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.
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 Returned by clone() when a zero value is specified for
child_stack.
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_NEWNS was specified by a non-root process (process with-
out 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
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 pro-
cess. 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 x86, clone() should not be called through vsyscall, but directly
through int $0x80.
On IA-64, 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.
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. 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);
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)
Linux 2.6 2007-06-01 CLONE(2)
