View Source: clone(2) - Waikato Linux Users Group

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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, 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, __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 explicitely 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 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 process 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 callng 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) performed 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 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_SIGHAND__: 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 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 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).

;__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.)

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
appropriately.

!!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 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 glibc2 (libc6) ). See
pthread_create(3thr).

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(3thr)
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Last edited on Thursday, October 31, 2002 8:47:44 am by PerryLorier
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