1. Technology
Linux / Unix Command: clone
Command Library

NAME

clone - create a child process  

SYNOPSIS

#include <sched.h>

int clone(int (*fn)(void *), void *child_stack, int flags, void *arg);

_syscall2(int, clone, int, flags, void *, child_stack);

 

DESCRIPTION

clone creates a new process, just like fork(2). clone is a library function layered on top of the underlying clone system call, hereinafter referred to as sys_clone. A description of sys_clone is given towards the end of this page.

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 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 one or several of the following constants, in order to specify what is shared between the calling process and the child process:

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 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.

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_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.)

If this flag is specified in Linux 2.4.7 onwards, the parent of the child is made the same as the caller's parent (i.e., this flag includes the effect of the CLONE_PARENT flag).

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.

 

RETURN VALUE

On success, the PID 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 errno will be set appropriately.

 

ERRORS

EAGAIN
Too many processes are already running.
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.
EINVAL
Returned by clone when a zero value is specified for child_stack.
EPERM
CLONE_PID was specified by a process with a non-zero PID.
 

SEE ALSO

fork(2), wait(2), pthread_create(3)


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