/* Prototype for the glibc wrapper function */ #define _GNU_SOURCE #include <sched.h> int clone(int (*fn)(void *), void *child_stack, int flags, void *arg, ... /* pid_t *ptid, void *newtls, pid_t *ctid */ ); /* For the prototype of the raw system call, see NOTES */
This page describes both the glibc clone() wrapper function and the underlying system call on which it is based. The main text describes the wrapper function; the differences for the raw system call are described toward the end of this page.
Unlike fork(2), clone() allows 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.)
One 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 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 downward 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_FILES is not set, the child process inherits a copy of all file descriptors opened in the calling process at the time of clone(). 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 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), umask(2) performed later by one of the processes do not affect the other process.
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'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.
For further information on cgroup namespaces, see cgroup_namespaces(7).
Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWCGROUP.
An IPC namespace provides an isolated view of System V IPC objects (see svipc(7)) and (since Linux 2.6.30) POSIX message queues (see mq_overview(7)). The common characteristic of these IPC mechanisms is that IPC objects are identified by mechanisms other than filesystem pathnames.
Objects created in an IPC namespace are visible to all other processes that are members of that namespace, but are not visible to processes in other IPC namespaces.
When an IPC namespace is destroyed (i.e., when the last process that is a member of the namespace terminates), all IPC objects in the namespace are automatically destroyed.
Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWIPC. This flag can't be specified in conjunction with CLONE_SYSVSEM.
For further information on IPC namespaces, see namespaces(7).
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. This flag is intended for the implementation of containers.
A network namespace provides an isolated view of the networking stack (network device interfaces, IPv4 and IPv6 protocol stacks, IP routing tables, firewall rules, the /proc/net and /sys/class/net directory trees, sockets, etc.). A physical network device can live in exactly one network namespace. A virtual network device ("veth") pair provides a pipe-like abstraction that can be used to create tunnels between network namespaces, and can be used to create a bridge to a physical network device in another namespace.
When a network namespace is freed (i.e., when the last process in the namespace terminates), its physical network devices are moved back to the initial network namespace (not to the parent of the process). For further information on network namespaces, see namespaces(7).
Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWNET.
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.
For further information on mount namespaces, see namespaces(7) and mount_namespaces(7).
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't be specified in conjunction with CLONE_THREAD or CLONE_PARENT.
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't be specified in conjunction with CLONE_THREAD or CLONE_PARENT. For security reasons, CLONE_NEWUSER cannot be specified in conjunction with CLONE_FS.
For further information on user namespaces, see user_namespaces(7).
A UTS namespace is the set of identifiers returned by uname(2); among these, the domain name and the hostname can be modified by setdomainname(2) and sethostname(2), respectively. Changes made to the identifiers in a UTS namespace are visible to all other processes in the same namespace, but are not visible to processes in other UTS namespaces.
Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWUTS.
For further information on UTS namespaces, see namespaces(7).
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.
The interpretation of newtls and the resulting effect is architecture dependent. On x86, newtls 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.
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
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.
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 (and note that, since Linux 2.6.0-test6, CLONE_SIGHAND also requires CLONE_VM to be included).
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).
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).
The raw system call interface on x86-64 and some other architectures (including sh, tile, and alpha) is roughly:
long clone(unsigned long flags, void *child_stack,
int *ptid, int *ctid,
unsigned long newtls);
On x86-32, and several other common architectures (including score, ARM, ARM 64, PA-RISC, arc, Power PC, xtensa, and MIPS), the order of the last two arguments is reversed:
long clone(unsigned long flags, void *child_stack,
int *ptid, unsigned long newtls,
int *ctid);
On the cris and s390 architectures, the order of the first two arguments is reversed:
long clone(void *child_stack, unsigned long flags,
int *ptid, int *ctid,
unsigned long newtls);
On the microblaze architecture, an additional argument is supplied:
long clone(unsigned long flags, void *child_stack,
int stack_size, /* Size of stack */
int *ptid, int *ctid,
unsigned long newtls);
Another difference for the raw system call 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.
int __clone2(int (*fn)(void *),
void *child_stack_base, size_t stack_size,
int flags, void *arg, ...
/* pid_t *ptid, struct user_desc *tls, pid_t *ctid */ );
The prototype shown above is for the glibc wrapper function; the raw system call interface has no fn or arg argument, and changes the order of the arguments so that flags is the first argument, and tls is the last argument.
__clone2() 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.
In the Linux 2.4.x series, CLONE_THREAD generally does not make the parent of the new thread the same as the parent of the calling process. However, for kernel versions 2.4.7 to 2.4.18 the CLONE_THREAD flag implied the CLONE_PARENT flag (as in Linux 2.6.0 and later).
For a while there was CLONE_DETACHED (introduced in 2.5.32): parent wants no child-exit signal. In Linux 2.6.2, the need to give this flag together with CLONE_THREAD disappeared. This flag is still defined, but has no effect.
On i386, clone() should not be called through vsyscall, but directly through int $0x80.
#include <syscall.h>
pid_t mypid;
mypid = syscall(SYS_getpid);
#define _GNU_SOURCE #include <sys/wait.h> #include <sys/utsname.h> #include <sched.h> #include <string.h> #include <stdio.h> #include <stdlib.h> #include <unistd.h> #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \ } while (0) static int /* Start function for cloned child */ childFunc(void *arg) { struct utsname uts; /* Change hostname in UTS namespace of child */ if (sethostname(arg, strlen(arg)) == -1) errExit("sethostname"); /* Retrieve and display hostname */ if (uname(&uts) == -1) errExit("uname"); printf("uts.nodename in child: %s\n", uts.nodename); /* Keep the namespace open for a while, by sleeping. This allows some experimentation--for example, another process might join the namespace. */ sleep(200); return 0; /* Child terminates now */ } #define STACK_SIZE (1024 * 1024) /* Stack size for cloned child */ int main(int argc, char *argv[]) { char *stack; /* Start of stack buffer */ char *stackTop; /* End of stack buffer */ pid_t pid; struct utsname uts; if (argc < 2) { fprintf(stderr, "Usage: %s <child-hostname>\n", argv[0]); exit(EXIT_SUCCESS); } /* Allocate stack for child */ stack = malloc(STACK_SIZE); if (stack == NULL) errExit("malloc"); stackTop = stack + STACK_SIZE; /* Assume stack grows downward */ /* Create child that has its own UTS namespace; child commences execution in childFunc() */ pid = clone(childFunc, stackTop, CLONE_NEWUTS | SIGCHLD, argv[1]); if (pid == -1) errExit("clone"); printf("clone() returned %ld\n", (long) pid); /* Parent falls through to here */ sleep(1); /* Give child time to change its hostname */ /* Display hostname in parent's UTS namespace. This will be different from hostname in child's UTS namespace. */ if (uname(&uts) == -1) errExit("uname"); printf("uts.nodename in parent: %s\n", uts.nodename); if (waitpid(pid, NULL, 0) == -1) /* Wait for child */ errExit("waitpid"); printf("child has terminated\n"); exit(EXIT_SUCCESS); }