Clear (zero) the child thread ID at the location pointed to by child_tid (clone()) or cl_args.child_tid (clone3()) 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 set_tid_address(2) system call. This is used by threading libraries.

Store the child thread ID at the location pointed to by child_tid (clone()) or cl_args.child_tid (clone3()) in the child_zsingle_quotesz_s memory. The store operation completes before the clone call returns control to user space in the child process. (Note that the store operation may not have completed before the clone call returns in the parent process, which will be relevant if the CLONE_VM flag is also employed.)

For a while (during the Linux 2.5 development series) there was a CLONE_DETACHED flag, which caused the parent not to receive a signal when the child terminated. Ultimately, the effect of this flag was subsumed under the CLONE_THREAD flag and by the time Linux 2.6.0 was released, this flag had no effect. Starting in Linux 2.6.2, the need to give this flag together with CLONE_THREAD disappeared.

This flag is still defined, but it is usually ignored when calling clone(). However, see the description of CLONE_PIDFD for some exceptions.

If CLONE_FILES is set, the calling process and the child process share the same file descriptor table. 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 (using the fcntl(2) F_SETFD operation), the other process is also affected. If a process sharing a file descriptor table calls execve(2), its file descriptor table is duplicated (unshared).

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 the clone call. 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. Note, however, that the duplicated file descriptors in the child refer to the same open file descriptions as the corresponding file descriptors in the calling process, and thus share file offsets and file status flags (see open(2)).

If CLONE_FS is set, the caller and the child process share the same filesystem information. This includes the root of the filesystem, the current working directory, and the umask. Any call to chroot(2), chdir(2), or umask(2) performed by the calling process or the child process also affects the other process.

If CLONE_FS is not set, the child process works on a copy of the filesystem information of the calling process at the time of the clone call. Calls to chroot(2), chdir(2), or umask(2) performed later by one of the processes do not affect the other process.

If CLONE_IO is set, then the new process shares an I/O context with the calling process. If this flag is not set, then (as with fork(2)) the new process has its own I/O context.

The I/O context is the I/O scope of the disk scheduler (i.e., what the I/O scheduler uses to model scheduling of a process_zsingle_quotesz_s I/O). If processes share the same I/O context, they are treated as one by the I/O scheduler. As a consequence, they get to share disk time. For some I/O schedulers, if two processes share an I/O context, they will be allowed to interleave their disk access. If several threads are doing I/O on behalf of the same process (aio_read(3), for instance), they should employ CLONE_IO to get better I/O performance.

If the kernel is not configured with the CONFIG_BLOCK option, this flag is a no-op.

Create the process in a new cgroup namespace. If this flag is not set, then (as with fork(2)) the process is created in the same cgroup namespaces as the calling process.

For further information on cgroup namespaces, see cgroup_namespaces(7).

Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWCGROUP.

If CLONE_NEWIPC is set, then create the process in a new IPC namespace. If this flag is not set, then (as with fork(2)), the process is created in the same IPC namespace as the calling process.

For further information on IPC namespaces, see ipc_namespaces(7).

Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWIPC. This flag can_zsingle_quotesz_t be specified in conjunction with CLONE_SYSVSEM.

(The implementation of this flag was completed only by about kernel version 2.6.29.)

If CLONE_NEWNET is set, then create the process in a new network namespace. If this flag is not set, then (as with fork(2)) the process is created in the same network namespace as the calling process.

For further information on network namespaces, see network_namespaces(7).

Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWNET.

If CLONE_NEWNS is set, the cloned child is started in a new mount namespace, initialized with a copy of the namespace of the parent. If CLONE_NEWNS is not set, the child lives in the same mount namespace as the parent.

For further information on mount namespaces, see namespaces(7) and mount_namespaces(7).

Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWNS. It is not permitted to specify both CLONE_NEWNS and CLONE_FS in the same clone call.

If CLONE_NEWPID is set, then create the process in a new PID namespace. If this flag is not set, then (as with fork(2)) the process is created in the same PID namespace as the calling process.

For further information on PID namespaces, see namespaces(7) and pid_namespaces(7).

Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWPID. This flag can_zsingle_quotesz_t be specified in conjunction with CLONE_THREAD or CLONE_PARENT.

(This flag first became meaningful for clone() in Linux 2.6.23, the current clone() semantics were merged in Linux 3.5, and the final pieces to make the user namespaces completely usable were merged in Linux 3.8.)

If CLONE_NEWUSER is set, then create the process in a new user namespace. If this flag is not set, then (as with fork(2)) the process is created in the same user namespace as the calling process.

For further information on user namespaces, see namespaces(7) and user_namespaces(7).

Before Linux 3.8, use of CLONE_NEWUSER required that the caller have three capabilities: CAP_SYS_ADMIN, CAP_SETUID, and CAP_SETGID. Starting with Linux 3.8, no privileges are needed to create a user namespace.

This flag can_zsingle_quotesz_t be specified in conjunction with CLONE_THREAD or CLONE_PARENT. For security reasons, CLONE_NEWUSER cannot be specified in conjunction with CLONE_FS.

If CLONE_NEWUTS is set, then create the process in a new UTS namespace, whose identifiers are initialized by duplicating the identifiers from the UTS namespace of the calling process. If this flag is not set, then (as with fork(2)) the process is created in the same UTS namespace as the calling process.

For further information on UTS namespaces, see uts_namespaces(7).

Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWUTS.

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_zsingle_quotesz_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.

Store the child thread ID at the location pointed to by parent_tid (clone()) or cl_args.child_tid (clone3()) in the parent_zsingle_quotesz_s memory. (In Linux 2.5.32-2.5.48 there was a flag CLONE_SETTID that did this.) The store operation completes before the clone call returns control to user space.

If 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. From Linux 2.3.21 onward, this flag could be specified only by the system boot process (PID 0). The flag disappeared completely from the kernel sources in Linux 2.5.16. Subsequently, the kernel silently ignored this bit if it was specified in the flags mask. Much later, the same bit was recycled for use as the CLONE_PIDFD flag.

If this flag is specified, a PID file descriptor referring to the child process is allocated and placed at a specified location in the parent_zsingle_quotesz_s memory. The close-on-exec flag is set on this new file descriptor. PID file descriptors can be used for the purposes described in pidfd_open(2).

It is currently not possible to use this flag together with CLONE_THREAD. This means that the process identified by the PID file descriptor will always be a thread group leader.

If the obsolete CLONE_DETACHED flag is specified alongside CLONE_PIDFD when calling clone(), an error is returned. An error also results if CLONE_DETACHED is specified when calling clone3(). This error behavior ensures that the bit corresponding to CLONE_DETACHED can be reused for further PID file descriptor features in the future.

If CLONE_PTRACE is specified, and the calling process is being traced, then trace the child also (see ptrace(2)).

The TLS (Thread Local Storage) descriptor is set to tls.

The interpretation of tls and the resulting effect is architecture dependent. On x86, tls is interpreted as a struct user_desc * (see set_thread_area(2)). On x86-64 it is the new value to be set for the %fs base register (see the ARCH_SET_FS argument to arch_prctl(2)). On architectures with a dedicated TLS register, it is the new value of that register.

Use of this flag requires detailed knowledge and generally it should not be used except in libraries implementing threading.

If CLONE_SIGHAND is set, the calling process and the child process 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 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 of the clone call. Calls to sigaction(2) performed later by one of the processes have no effect on the other process.

Since Linux 2.6.0, the flags mask must also include CLONE_VM if CLONE_SIGHAND is specified

If CLONE_STOPPED is set, then the child is initially stopped (as though it was sent a SIGSTOP signal), and must be resumed by sending it a SIGCONT signal.

This flag was deprecated from Linux 2.6.25 onward, and was removed altogether in Linux 2.6.38. Since then, the kernel silently ignores it without error. Starting with Linux 4.6, the same bit was reused for the CLONE_NEWCGROUP flag.

If CLONE_SYSVSEM is set, then the child and the calling process share a single list of System V semaphore adjustment (semadj) values (see semop(2)). In this case, the shared list accumulates semadj values across all processes sharing the list, and semaphore adjustments are performed only when the last process that is sharing the list terminates (or ceases sharing the list using unshare(2)). If this flag is not set, then the child has a separate semadj list that is initially empty.

If CLONE_THREAD is set, the child is placed in the same thread group as the calling process. To make the remainder of the discussion of CLONE_THREAD more readable, the term thread is used to refer to the processes within a thread group.

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_zsingle_quotesz_s TID is available as the function result returned to the caller, and a thread can obtain its own TID using gettid(2).

When a clone call is made without specifying CLONE_THREAD, then the resulting thread is placed in a new thread group whose TGID is the same as the thread_zsingle_quotesz_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 process that made the clone call (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 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, the flags mask must also include CLONE_SIGHAND if CLONE_THREAD is specified (and note that, since Linux 2.6.0, CLONE_SIGHAND also requires CLONE_VM to be included).

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).

A signal may be process-directed or thread-directed. A process-directed signal is targeted at a thread group (i.e., a TGID), and is delivered to an arbitrarily selected thread from among those that are not blocking the signal. A signal may be process-directed because it was generated by the kernel for reasons other than a hardware exception, or because it was sent using kill(2) or sigqueue(3). A thread-directed signal is targeted at (i.e., delivered to) a specific thread. A signal may be thread directed because it was sent using tgkill(2) or pthread_sigqueue(3), or because the thread executed a machine language instruction that triggered a hardware exception (e.g., invalid memory access triggering SIGSEGV or a floating-point exception triggering SIGFPE).

A call to sigpending(2) returns a signal set that is the union of the pending process-directed signals and the signals that are pending for the calling thread.

If a process-directed signal is delivered 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 the signal.

If CLONE_UNTRACED is specified, then a tracing process cannot force CLONE_PTRACE on this child process.

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.

If CLONE_VM is set, the calling process and the child process 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 the clone call. Memory writes or file mappings/unmappings performed by one of the processes do not affect the other, as with fork(2).