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);
_syscall2(int, clone, int, flags, void *, child_stack);
DESCRIPTION
clone creates a new process, just like fork(2). clone is a library
function layered on top of the underlying clone system call, here-
inafter 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 con-
trol 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 con-
tinues in the child from the point of the fork(2) call.) The fn argu-
ment 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 termi-
nates. The integer returned by fn is the exit code for the child pro-
cess. 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 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 one or several of the following
constants, in order to specify what is shared between the calling pro-
cess 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 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 call-
ing 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) per-
formed 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. File descriptors
always refer to the same files in the calling process and in the
child process. Any file descriptor created by the calling pro-
cess 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_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 behav-
ior 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.
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 per-
formed 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
If CLONE_PID is set, the child process is created with the same
process ID as the calling process.
If CLONE_PID is not set, the child process possesses a unique
process ID, distinct from that of the calling process.
This flag can only be specified by the system boot process (PID
0).
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.)
If this flag is specified in Linux 2.4.7 onwards, the parent of
the child is made the same as the caller's parent (i.e., this
flag includes the effect of the CLONE_PARENT flag).
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 execution. On failure, a -1 will be returned in the caller's
context, no child process will be created, and errno will be set appro-
priately.
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.
EPERM CLONE_PID was specified by a process with a non-zero PID.
BUGS
As of version 2.1.97 of the kernel, the CLONE_PID flag should not be
used, since other parts of the kernel and most system software still
assume that process IDs are unique.
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.
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 applica-
tions (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_cre-
ate(3).
This manual page corresponds to kernels 2.0.x, 2.1.x, 2.2.x, 2.4.x, and
to glibc 2.0.x and 2.1.x.
SEE ALSO
fork(2), wait(2), pthread_create(3)
Linux 2.4 2001-06-26 CLONE(2)
