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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);
_syscall2(int, clone, int, flags, void *, child_stack)
DESCRIPTION
clone creates a new process, just like fork(2). clone is a library function lay-
ered 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 descrip-
tion 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 pro-
cess. 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 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 sig-
naled when the child terminates.
flags may also be bitwise-or’ed with one or several of the following constants, in
order to specify what is shared between the calling process and the child process:
CLONE_PARENT
(Linux 2.4 onwards) 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 sig-
naled 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 takes
effect in 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 processes share the
same file descriptor table. File descriptors always refer to the same files
in the calling process and in the child process. Any file descriptor cre-
ated 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, 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. Operations
on file descriptors performed later by either the calling process or the
child process do not affect the other process.
CLONE_NEWNS
(Linux 2.4.19 onwards) 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 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(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 may specify the CLONE_NEWNS flag. It is not per-
mitted 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 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 pro-
cess and child processes still have distinct signal masks and sets of pend-
ing signals. So, one of them may block or unblock some signals using sig-
procmask(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.
CLONE_PTRACE
If CLONE_PTRACE is specified, and the calling process is being traced, then
trace the child also (see ptrace(2)).
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 mapping or unmapping performed with mmap(2) or mun-
map(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
(Linux 2.4 onwards) If CLONE_THREAD is set, the child is placed in the same
thread group as the calling process.
If CLONE_THREAD is not set, then the child is placed in its own (new) thread
group, whose ID is the same as the process ID.
(Thread groups are feature added in Linux 2.4 to support the POSIX threads
notion of a set of threads sharing a single PID. In Linux 2.4, calls to
getpid(2) return the thread group ID of the caller.)
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.
RETURN VALUE
On success, the PID of the child process is returned in the caller’s thread of exe-
cution. On failure, a -1 will be returned in the caller’s context, no child pro-
cess will be created, and errno will be set appropriately.
ERRORS
EAGAIN Too many processes are already running.
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.
EINVAL Returned by clone when a zero value is specified for child_stack.
EINVAL Both CLONE_FS and CLONE_NEWNS were specified in flags.
EINVAL CLONE_THREAD was specified, but CLONE_SIGHAND was not. (Since Linux 2.5.35.)
EINVAL Precisely one of CLONE_DETACHED and CLONE_THREAD was specified. (Since Linux
2.6.0-test6.)
EINVAL CLONE_SIGHAND was specified, but CLONE_VM was not. (Since Linux
2.6.0-test6.)
EPERM CLONE_NEWNS was specified by a non-root process (process without
CAP_SYS_ADMIN).
EPERM CLONE_PID was specified by a process other than process 0.
BUGS
There is no entry for clone in libc version 5. libc 6 (a.k.a. glibc 2) provides
clone as described in this manual page.
NOTES
For kernel versions 2.4.7-2.4.18 the CLONE_THREAD flag implied the CLONE_PARENT
flag.
CONFORMING TO
The clone and sys_clone calls are Linux-specific and should not be used in programs
intended to be portable. For programming threaded applications (multiple threads
of control in the same memory space), it is better to use a library implementing
the POSIX 1003.1c thread API, such as the LinuxThreads library (included in
glibc2). See pthread_create(3).
SEE ALSO
fork(2), wait(2), pthread_create(3)
Linux 2.4 2001-12-31 CLONE(2)
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On Apache/1.3.41 (Unix) PHP/5.2.5 mod_perl/1.30 mod_gzip/1.3.26.1a
Under GNU General Public License
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