clone() - Unix, Linux System Call

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Copyright © 2014 by tutorialspoint
clone, __clone2 - create a child process
#include <sched.h> 

int clone(int (*fn)(void *), void *child_stack, 
          int flags, void *arg, ...   
          /* pid_t *pid, struct user_desc *tls 
", pid_t *" ctid " */ );"


int __clone2(int (*fn)(void *),  void *child_stack_base, 
             size_t stack_size, int flags, void *arg, ... 
             /* pid_t *pid, struct user_desc *tls 
", pid_t *" ctid " */ );"


int clone(int (*fn)(void *), void *child_stack, 
          int flags, void *arg, ...   
          /* pid_t *pid, struct user_desc *tls 
", pid_t *" ctid " */ );"



int __clone2(int (*fn)(void *),  void *child_stack_base, 
             size_t stack_size, int flags, void *arg, ... 
             /* pid_t *pid, struct user_desc *tls 
", pid_t *" ctid " */ );"

clone() creates a new process, in a manner similar to fork(2). It is actually 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 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 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 termination 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 zero or more of the following constants, in order to specify what is shared between the calling process and the child 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.


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 not set, the child process inherits a copy of all file descriptors
opened in the calling process at the time of
clone(). (The duplicated file descriptors in the child refer to the
same open file descriptions (see
open(2))
as the corresponding file descriptors in the calling process.)
Subsequent operations that open or close file descriptors,
or change file descriptor flags,
performed by either the calling
process or the child process do not affect the other process.


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 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.


Since Linux 2.6.0-test6,
flags must also include
CLONE_VM if
CLONE_SIGHAND is specified


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 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).


Thread groups were a feature added in Linux 2.4 to support the
POSIX threads notion of a set of threads that share a single PID.
Internally, this shared PID is the so-called
thread group identifier (TGID) for the thread group.
Since Linux 2.4, calls to
getpid(2)
return the TGID of the caller.


The threads within a group can be distinguished by their (system-wide)
unique thread IDs (TID).
A new thread’s TID is available as the function result
returned to the caller of
clone(), and a thread can obtain
its own TID using
gettid(2).


When a call is made to
clone() without specifying
CLONE_THREAD, then the resulting thread is placed in a new thread group
whose TGID is the same as the thread’s TID.
This thread is the
leader of the new thread group.


A new thread created with
CLONE_THREAD has the same parent process as the caller of
clone() (i.e., like
CLONE_PARENT), so that calls to
getppid(2)
return the same value for all of the threads in a thread group.
When a
CLONE_THREAD thread terminates, the thread that created it using
clone() is not sent a
SIGCHLD (or other termination) signal;
nor can the status of such a thread be obtained
using
wait(2).
(The thread is said to be
detached.) 

After all of the threads in a thread group terminate
the parent process of the thread group is sent a
SIGCHLD (or other termination) signal.


If any of the threads in a thread group performs an
execve(2),
then all threads other than the thread group leader are terminated,
and the new program is executed in the thread group leader.


If one of the threads in a thread group creates a child using
fork(2),
then any thread in the group can
wait(2)
for that child.


Since Linux 2.5.35,
flags must also include
CLONE_SIGHAND if
CLONE_THREAD is specified.


Signals may be sent to a thread group as a whole (i.e., a TGID) using
kill(2),
or to a specific thread (i.e., TID) using
tgkill(2).


Signal dispositions and actions are process-wide:
if an unhandled signal is delivered to a thread, then
it will affect (terminate, stop, continue, be ignored in)
all members of the thread group.


Each thread has its own signal mask, as set by
sigprocmask(2),
but signals can be pending either: for the whole process
(i.e., deliverable to any member of the thread group),
when sent with
kill(2);
or for an individual thread, when sent with
tgkill(2).
A call to
sigpending(2)
returns a signal set that is the union of the signals pending for the
whole process and the signals that are pending for the calling thread.


If
kill(2)
is used to send a signal to a thread group,
and the thread group has installed a handler for the signal, then
the handler will be invoked in exactly one, arbitrarily selected
member of the thread group that has not blocked the signal.
If multiple threads in a group are waiting to accept the same signal using
sigwaitinfo(2),
the kernel will arbitrarily select one of these threads
to receive a signal sent using
kill(2).


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.


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 child 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 was specified.

There is no entry for clone() in libc5. glibc2 provides clone() as described in this manual page.
The clone() and sys_clone calls are Linux specific and should not be used in programs intended to be portable.
In the kernel 2.4.x series, CLONE_THREAD generally does not make the parent of the new thread the same as the parent of the calling process. However, for kernel versions 2.4.7 to 2.4.18 the CLONE_THREAD flag implied the
CLONE_PARENT flag (as in kernel 2.6). 
For a while there was CLONE_DETACHED (introduced in 2.5.32): parent wants no child-exit signal. In 2.6.2 the need to give this together with CLONE_THREAD disappeared. This flag is still defined, but has no effect. On x86, clone() should not be called through vsyscall, but directly through int $0x80. On IA-64, a different system call is used:

int __clone2(int (*fn)(void *),  void *child_stack_base, 
             size_t stack_size, int flags, void *arg, ... 
             /* pid_t *pid, struct user_desc *tls 
", pid_t *" ctid " */ );"


int __clone2(int (*fn)(void *),  void *child_stack_base, 
             size_t stack_size, int flags, void *arg, ... 
             /* pid_t *pid, struct user_desc *tls 
", pid_t *" ctid " */ );"

The __clone2() system call operates in the same way as clone(), except that  child_stack_base points to the lowest address of the child’s stack area, and
stack_size specifies the size of the stack pointed to by child_stack_base.
Versions of the GNU C library that include the NPTL threading library contain a wrapper function for getpid(2) that performs caching of PIDs. In programs linked against such libraries, calls to
getpid(2) may return the same value, even when the threads were not created using CLONE_THREAD (and thus are not in the same thread group). To get the truth, it may be necessary to use code such as the following
#include <syscall.h>

pid_t mypid;

mypid = syscall(SYS_getpid);

fork (2)
fork (2)
futex (2)
futex (2)
getpid (2)
getpid (2)
gettid (2)
gettid (2)
set_thread_area (2)
set_thread_area (2)
set_tid_address (2)
set_tid_address (2)
tkill (2)
tkill (2)
unshare (2)
unshare (2)
wait (2)
wait (2)

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