CLONE

__clone ? create a child process

#include <sched.h>

int __clone(int (*fn) (void *arg), void *child_stack, int flags, void *arg)

__clone creates a new process like fork(2) does. Unlike fork(2), __clone allows the child process to share parts of its execution context with its parent process, such as the memory space, the table of file descriptors, and the table of signal handlers. 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, it executes the function application fn(arg). 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 back 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(1) 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 parent processes may share memory, it is not possible in general for the child process to execute in the same stack as the parent process. The parent 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. flags may also be bitwise-or’ed with one or several of the following constants, in order to specify what is shared between the parent and child processes:

CLONE_VM

If CLONE_VM is set, the parent and the child processes run in the same memory space. In particular, memory writes performed by the parent 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 parent 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 parent at the time of __clone. Memory writes or file mapping/unmapping performed by one of the processes does not affect the other, as in the case of fork(2).

CLONE_FS

If CLONE_FS is set, the parent 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 parent or 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 parent at the time of __clone. Calls to chroot(2),chdir(2),umask(2) performed later by one of the processes does not affect the other.

CLONE_FILES

If CLONE_FILES is set, the parent and the child processes share the same file descriptor table. File descriptors always refer to the same files in the parent and in the child process. Any file descriptor created by the parent 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 parent process at the time of __clone. Operations on file descriptors performed later by one of the parent or child processes do not affect the other.

CLONE_SIGHAND

If CLONE_SIGHAND is set, the parent and the child processes share the same table of signal handlers. If the parent or child process calls sigaction(2) to change the behavior associated with a signal, the behavior is also changed in the other process as well. However, the parent 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 its parent 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_PID

If CLONE_PID is set, the child process is created with the same process ID as its parent process.

If CLONE_PID is not set, the child process possesses a unique process ID, distinct from that of its parent.

On success, the PID of the child process is returned in the parent’s thread of execution. On failure, a ?1 will be returned in the parent’s context, no child process will be created, and errno will be set appropriately.

EAGAIN

Too many processes are already running.

ENOMEM

__clone cannot allocate sufficient memory to allocate a task structure for the child, or to copy those parts of the parent’s context that need to be copied.

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.

The __clone call is 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. See pthread_create(3thr).

This manual page corresponds to kernels 2.0.x and 2.1.x, and to glibc 2.0.x.

fork(2), pthread_create(3thr)