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.\" Hey Emacs! This file is -*- nroff -*- source.
.\"
.\" Copyright (c) 1992 Drew Eckhardt <drew@cs.colorado.edu>, March 28, 1992
.\" May be distributed under the GNU General Public License.
.\" Modified by Michael Haardt <michael@moria.de>
.\" Modified Sat Jul 24 13:22:07 1993 by Rik Faith <faith@cs.unc.edu>
.\" Modified 21 Aug 1994 by Michael Chastain <mec@shell.portal.com>:
.\" New man page (copied from 'fork.2').
.\" Modified 10 June 1995 by Andries Brouwer <aeb@cwi.nl>
.\" Modified 25 april 1998 by Xavier Leroy <Xavier.Leroy@inria.fr>
.\"
.TH CLONE 2 "25 april 1998" "Linux 2.0.33" "Linux Programmer's Manual"
.SH NAME
__clone \- create a child process
.SH SYNOPSIS
.B #include <sched.h>
.sp
.BI "int __clone(int (*" "fn" ") (void *" "arg" "), void *" "child_stack" ", int " "flags" ", void *" "arg" ")"
.SH DESCRIPTION
.B __clone
creates a new process like
.BR fork (2)
does. Unlike
.BR fork (2),
.B __clone
allows the child process to share parts of its execution context with
its parent process, such as the memory space, the table of file
descriptors, and the table of signal handlers. The main use of
.B __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, it executes the function
application
.IR fn ( arg ).
The
.I fn
argument is a pointer to a function that is called by the child
process at the beginning of its execution.
The
.I arg
argument is passed back to the
.I fn
function.
When the
.IR fn ( arg )
function application returns, the child process terminates. The
integer returned by
.I fn
is the exit code for the child process. The child process may also
terminate explicitely by calling
.BR exit (1)
or after receiving a fatal signal.
The
.I child_stack
argument specifies the location of the stack used by the child
process. Since the child and parent processes may share memory,
it is not possible in general for the child process to execute in the
same stack as the parent process. The parent process must therefore
set up memory space for the child stack and pass a pointer to this
space to
.BR __clone .
Stacks grow downwards on all processors that run Linux
(except the HP PA processors), so
.I child_stack
usually points to the topmost address of the memory space set up for
the child stack.
The low byte of
.I flags
contains the number of the signal sent to the parent when the child
dies.
.I flags
may also be bitwise-or'ed with one or several of the following
constants, in order to specify what is shared between the parent and
child processes:
.TP
.B CLONE_VM
If
.B CLONE_VM
is set, the parent and the child processes run in the same memory
space. In particular, memory writes performed by the parent process
or by the child process are also visible in the other process.
Moreover, any memory mapping or unmapping performed with
.BR mmap (2)
or
.BR munmap (2)
by the child or parent process also affects the other process.
If
.B CLONE_VM
is not set, the child process runs in a separate copy of the memory
space of the parent at the time of
.BR __clone .
Memory writes or file mapping/unmapping performed by one of the
processes does not affect the other, as in the case of
.BR fork (2).
.TP
.B CLONE_FS
If
.B CLONE_FS
is set, the parent 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
.BR chroot (2),
.BR chdir (2),
or
.BR umask (2)
performed by the parent or child process also takes effect in the
other process.
If
.B CLONE_FS
is not set, the child process works on a copy of the file system
information of the parent at the time of
.BR __clone .
Calls to
.BR chroot (2),
.BR chdir (2),
.BR umask (2)
performed later by one of the processes does not affect the other.
.TP
.B CLONE_FILES
If
.B CLONE_FILES
is set, the parent and the child processes share the same file
descriptor table. File descriptors always refer to the same files in
the parent and in the child process. Any file descriptor created by
the parent 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
.B CLONE_FILES
is not set, the child process inherits a copy of all file descriptors
opened in the parent process at the time of
.BR __clone .
Operations on file descriptors performed later by one of the parent or
child processes do not affect the other.
.TP
.B CLONE_SIGHAND
If
.B CLONE_SIGHAND
is set, the parent and the child processes share the same table of
signal handlers. If the parent or child process calls
.BR sigaction (2)
to change the behavior associated with a signal, the behavior is also
changed in the other process as well. However, the parent and child
processes still have distinct signal masks and sets of pending
signals. So, one of them may block or unblock some signals using
.BR sigprocmask (2)
without affecting the other process.
If
.B CLONE_SIGHAND
is not set, the child process inherits a copy of the signal handlers
of its parent at the time
.B __clone
is called. Calls to
.BR sigaction (2)
performed later by one of the processes have no effect on the other
process.
.TP
.B CLONE_PID
If
.B CLONE_PID
is set, the child process is created with the same process ID as its
parent process.
If
.B CLONE_PID
is not set, the child process possesses a unique process ID, distinct
from that of its parent.
.SH "RETURN VALUE"
On success, the PID of the child process is returned in the parent's thread
of execution. On failure, a \-1 will be returned in the parent's
context, no child process will be created, and
.I errno
will be set appropriately.
.SH ERRORS
.TP
.B EAGAIN
Too many processes are already running.
.TP
.B ENOMEM
.B __clone
cannot allocate sufficient memory to allocate a task structure for the
child, or to copy those parts of the parent's context that need to be
copied.
.SH BUGS
As of version 2.1.97 of the kernel,
the
.B 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
.B __clone
in libc version 5. libc 6 (a.k.a. glibc 2) provides
.B __clone
as described in this manual page.
.SH CONFORMING TO
The
.B __clone
call is 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 the libc6 (glibc2) library). See
.BR pthread_create (3thr).
This manual page corresponds to kernels 2.0.x and 2.1.x, and to glibc
2.0.x.
.SH "SEE ALSO"
.BR fork (2),
.BR pthread_create (3thr)
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