clone creates a new process, just like fork(2). clone is 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
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 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
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 (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
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.
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
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.
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 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, 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 (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 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 (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.
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 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
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.
If CLONE_PTRACE is specified, and the calling
process is being traced, then trace the child also (see ptrace(2)).
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
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.
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 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).
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
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.
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 since 2.4, calls
to getpid(2) return the thread group ID of the caller.)
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
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
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
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 parent 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
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