NAME
__clone - create a child process
SYNOPSIS
#include <sched.h>
int __clone(int (*fn)) (void *arg)), void *child_stack
DESCRIPTION
__clone creates a new process like fork(2) does. Unlike
fork(2), __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 __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 fn(arg). The fn argument is a pointer to a
function that is called by the child process at the begin-
ning of its execution. The arg argument is passed back 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 ter-
minate explicitely by calling exit(1) 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 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 __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. flags may also be
bitwise-or'ed with one or several of the following con-
stants, in order to specify what is shared between the
parent and child processes:
CLONE_VM
If 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. More-
over, any memory mapping or unmapping performed with
mmap(2) or munmap(2) by the child or parent 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 parent at the
time of __clone. Memory writes or file
mapping/unmapping performed by one of the processes
does not affect the other, as in the case of fork(2).
CLONE_FS
If 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 direc-
tory, and the umask. Any call to chroot(2), chdir(2),
or umask(2) performed by the parent or 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 parent at
the time of __clone. Calls to
chroot(2),chdir(2),umask(2) performed later by one of
the processes does not affect the other.
CLONE_FILES
If 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 CLONE_FILES is not set, the child process inherits a
copy of all file descriptors opened in the parent pro-
cess at the time of __clone. Operations on file
descriptors performed later by one of the parent or
child processes do not affect the other.
CLONE_SIGHAND
If CLONE_SIGHAND is set, the parent and the child
processes share the same table of signal handlers. If
the parent or child process calls 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
sigprocmask(2) without affecting the other process.
If CLONE_SIGHAND is not set, the child process inherits
a copy of the signal handlers of its parent 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_PID
If CLONE_PID is set, the child process is created with
the same process ID as its parent process.
If CLONE_PID is not set, the child process possesses a
unique process ID, distinct from that of its parent.
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 errno will be set appropriately.
ERRORS
EAGAIN
Too many processes are already running.
ENOMEM
__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.
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 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. See
pthread_create(3thr).
This manual page corresponds to kernels 2.0.x and 2.1.x, and
to glibc 2.0.x.
SEE ALSO
fork(2), pthread_create(3thr).