| 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 childs 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 calling 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_NEWNS |
| || (Linux 2.4.19 onwards) Start the child in a new namespace. |
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 may specify the CLONE_NEWNS flag. It is not permitted to specify both CLONE_NEWNS and CLONE_FS in the same clone call.
| 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.
On success, the PID of the child process is returned in the callers thread of execution. On failure, a -1 will be returned in the callers context, no child process will be created, and errno will be set appropriately.