#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 */ );
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.
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 */ );
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.
#include <syscall.h> pid_t mypid; mypid = syscall(SYS_getpid);