.\" Hey Emacs! This file is -*- nroff -*- source. .\" .\" Copyright (c) 1992 Drew Eckhardt , March 28, 1992 .\" and Michael Kerrisk, 2001, 2002 .\" May be distributed under the GNU General Public License. .\" Modified by Michael Haardt .\" Modified 24 Jul 1993 by Rik Faith .\" Modified 21 Aug 1994 by Michael Chastain : .\" New man page (copied from 'fork.2'). .\" Modified 10 June 1995 by Andries Brouwer .\" Modified 25 April 1998 by Xavier Leroy .\" Modified 26 Jun 2001 by Michael Kerrisk .\" Mostly upgraded to 2.4.x .\" Added prototype for sys_clone() plus description .\" Added CLONE_THREAD with a brief description of thread groups .\" Added CLONE_PARENT and revised entire page remove ambiguity .\" between "calling process" and "parent process" .\" Added CLONE_PTRACE and CLONE_VFORK .\" Added EPERM and EINVAL error codes .\" Renamed "__clone" to "clone" (which is the protype in ) .\" various other minor tidy ups and clarifications. .\" Modified 26 Jun 2001 by Michael Kerrisk .\" Updated notes for 2.4.7+ behaviour of CLONE_THREAD .\" Modified 15 Oct 2002 by Michael Kerrisk .\" Added description for CLONE_NEWNS, which was added in 2.4.19 .\" Slightly rephrased, aeb. .\" Modified 1 Feb 2003 - added CLONE_SIGHAND restriction, aeb. .\" Modified 1 Jan 2004 - various updates, aeb .\" Modified 2004-09-10 - added CLONE_PARENT_SETTID etc - aeb. .\" .TH CLONE 2 2004-09-10 "Linux 2.6" "Linux Programmer's Manual" .SH NAME clone \- create a child process .SH SYNOPSIS .B #include .sp .BI "int clone(int (*" "fn" ")(void *), void *" "child_stack" ", int " "flags" ", void *" "arg" ); .sp .BI "_syscall2(int, " clone ", int, " flags ", void *, " child_stack ) .sp .BI "_syscall5(int, " clone ", int, " flags ", void *, " child_stack , .br .BI " int *, " parent_tidptr ", struct user_desc *, " newtls , .br .BI " int *, " child_tidptr ) .SH DESCRIPTION .B clone creates a new process, just like .BR fork (2). .B clone is a library function layered on top of the underlying .BR clone system call, hereinafter referred to as .BR sys_clone . A description of .BR sys_clone is given towards the end of this page. Unlike .BR 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 .B CLONE_PARENT below.) 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 with .BR clone , it executes the function application .IR fn ( arg ). (This differs from .BR fork (2), where execution continues in the child from the point of the .BR fork (2) call.) 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 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 explicitly by calling .BR exit (2) 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 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 .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. If this signal is specified as anything other than .BR SIGCHLD , then the parent process must specify the .B __WALL or .B __WCLONE options when waiting for the child with .BR wait (2). If no signal is specified, then the parent process is not signaled when the child terminates. .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 calling process and the child process: .TP .BR CLONE_PARENT " (since Linux 2.3.12)" If .B CLONE_PARENT is set, then the parent of the new child (as returned by .BR getppid (2)) will be the same as that of the calling process. If .B CLONE_PARENT is not set, then (as with .BR fork (2)) the child's parent is the calling process. Note that it is the parent process, as returned by .BR getppid (2), which is signaled when the child terminates, so that if .B CLONE_PARENT is set, then the parent of the calling process, rather than the calling process itself, will be signaled. .TP .B CLONE_FS If .B 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 .BR chroot (2), .BR chdir (2), or .BR umask (2) performed by the calling process or the 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 calling process at the time of the .BR clone call. Calls to .BR chroot (2), .BR chdir (2), .BR umask (2) performed later by one of the processes do not affect the other process. .TP .B CLONE_FILES If .B 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 .B CLONE_FILES is not set, the child process inherits a copy of all file descriptors opened in the calling process at the time of .BR clone . Operations on file descriptors performed later by either the calling process or the child process do not affect the other process. .TP .BR CLONE_NEWNS " (since Linux 2.4.19) Start the child in a new namespace. Every process lives in a namespace. The .I namespace of a process is the data (the set of mounts) describing the file hierarchy as seen by that process. After a .BR fork (2) or .BR clone (2) where the .B CLONE_NEWNS flag is not set, the child lives in the same namespace as the parent. The system calls .BR mount (2) and .BR 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 .BR clone (2) where the .B 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 .B CLONE_NEWNS flag. It is not permitted to specify both .B CLONE_NEWNS and .B CLONE_FS in the same .BR clone call. .TP .B CLONE_SIGHAND If .B 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 .BR 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 .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 the calling process 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_PTRACE If .B CLONE_PTRACE is specified, and the calling process is being traced, then trace the child also (see .BR ptrace (2)). .TP .B CLONE_VFORK If .B CLONE_VFORK is set, the execution of the calling process is suspended until the child releases its virtual memory resources via a call to .BR execve (2) or .BR _exit (2) (as with .BR vfork (2)). If .B 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. .TP .B CLONE_VM If .B 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 .BR mmap (2) or .BR munmap (2) by the child or calling 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 calling process at the time of .BR clone . Memory writes or file mappings/unmappings performed by one of the processes do not affect the other, as with .BR fork (2). .TP .BR CLONE_PID " (obsolete)" If .B CLONE_PID is set, the child process is created with the same process ID as the calling process. This is good for hacking the system, but otherwise of not much use. Since 2.3.21 this flag can be specified only by the system boot process (PID 0). It disappeared in Linux 2.5.16. .TP .BR CLONE_THREAD " (since Linux 2.4.0-test8)" If .B CLONE_THREAD is set, the child is placed in the same thread group as the calling process. .\" 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. If .B 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 since 2.4, calls to .BR getpid (2) return the thread group ID of the caller.) .TP .BR CLONE_SETTLS " (since Linux 2.5.32)" The .I newtls parameter is the new TLS (Thread Local Storage) descriptor. (See .BR set_thread_area (2).) .TP .BR CLONE_PARENT_SETTID " (since Linux 2.5.49)" Store child thread ID at location .I parent_tidptr in parent and child memory. (In Linux 2.5.32-2.5.48 there was a flag CLONE_SETTID that did this.) .TP .BR CLONE_CHILD_SETTID " (since Linux 2.5.49)" Store child thread ID at location .I child_tidptr in child memory. .TP .BR CLONE_CHILD_CLEARTID " (since Linux 2.5.49)" Erase child thread ID at location .I child_tidptr in child memory when the child exits, and do a wakeup on the futex at that address. The address involved may be changed by the .BR set_tid_address (2) system call. This is used by threading libraries. .SS "sys_clone" The .B sys_clone system call corresponds more closely to .BR fork (2) in that execution in the child continues from the point of the call. Thus, .B sys_clone only requires the .I flags and .I child_stack arguments, which have the same meaning as for .BR clone . (Note that the order of these arguments differs from .BR clone .) Another difference for .B sys_clone is that the .I 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 .B CLONE_VM option should not be specified. Since Linux 2.5.49 the system call has five parameters. The two new parameters are .I parent_tidptr which points to the location (in parent and child memory) where the parent thread ID will be written in case CLONE_PARENT_SETTID was specified, and .I 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. .SH "RETURN VALUE" .\" gettid() returns current->pid; .\" getpid() returns current->tgid; On success, the thread ID 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 .I errno will be set appropriately. .SH ERRORS .TP .B EAGAIN Too many processes are already running. .TP .B EINVAL .B CLONE_SIGHAND was specified, but .B CLONE_VM was not. (Since Linux 2.6.0-test6.) .TP .B EINVAL .B CLONE_THREAD was specified, but .B CLONE_SIGHAND was not. (Since Linux 2.5.35.) .TP .B EINVAL Precisely one of .B CLONE_DETACHED and .B CLONE_THREAD was specified. (Since Linux 2.6.0-test6.) .TP .B EINVAL Both .B CLONE_FS and .B CLONE_NEWNS were specified in .IR flags . .TP .B EINVAL Returned by .B clone when a zero value is specified for .IR child_stack . .TP .B 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. .TP .B EPERM .B CLONE_NEWNS was specified by a non-root process (process without CAP_SYS_ADMIN). .TP .B EPERM .B CLONE_PID was specified by a process other than process 0. .SH AVAILABILITY There is no entry for .B clone in libc5. glibc2 provides .B clone as described in this manual page. .SH NOTES For kernel versions 2.4.7-2.4.18 the CLONE_THREAD flag implied the CLONE_PARENT flag. .SH "CONFORMING TO" The .B clone and .B sys_clone calls are Linux-specific and should not be used in programs intended to be portable. .SH "SEE ALSO" .BR fork (2), .BR getpid (2), .BR gettid (2), .BR wait (2), .BR capabilities (7)