clone

CLONE(2)                    Linux Programmer's Manual                   CLONE(2)



NAME
       clone, __clone2 - create a child process

SYNOPSIS
       /* 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 */

DESCRIPTION
       clone() creates a new process, in a manner similar to fork(2).

       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:

       CLONE_CHILD_CLEARTID (since Linux 2.5.49)
              Clear (zero) the child thread ID at the location ctid in child
              memory when the child exits, and do a wakeup on the futex at that
              address.  The address involved may be changed by the
              set_tid_address(2) system call.  This is used by threading
              libraries.

       CLONE_CHILD_SETTID (since Linux 2.5.49)
              Store the child thread ID at the location ctid in the child's
              memory.  The store operation completes before clone() returns
              control to user space.

       CLONE_FILES (since Linux 2.0)
              If CLONE_FILES is set, the calling process and the child process
              share the same file descriptor table.  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 (using the
              fcntl(2) F_SETFD operation), the other process is also affected.
              If a process sharing a file descriptor table calls execve(2), its
              file descriptor table is duplicated (unshared).

              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)).

       CLONE_FS (since Linux 2.0)
              If CLONE_FS is set, the caller and the child process share the
              same filesystem information.  This includes the root of the
              filesystem, 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 affects the other process.

              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.

       CLONE_IO (since Linux 2.6.25)
              If CLONE_IO is set, then the new process shares an I/O context
              with the calling process.  If this flag is not set, then (as with
              fork(2)) the new process has its own I/O context.

              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.

       CLONE_NEWIPC (since Linux 2.6.19)
              If CLONE_NEWIPC is set, then create the process in a new IPC
              namespace.  If this flag is not set, then (as with fork(2)), the
              process is created in the same IPC namespace as the calling
              process.  This flag is intended for the implementation of
              containers.

              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).

       CLONE_NEWNET (since Linux 2.6.24)
              (The implementation of this flag was completed only by about
              kernel version 2.6.29.)

              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.

       CLONE_NEWNS (since Linux 2.4.19)
              If CLONE_NEWNS is set, the cloned child is started in a new mount
              namespace, initialized with a copy of the namespace of the parent.
              If CLONE_NEWNS is not set, the child lives in the same mount
              namespace as the parent.

              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).

       CLONE_NEWPID (since Linux 2.6.24)
              If CLONE_NEWPID is set, then create the process in a new PID
              namespace.  If this flag is not set, then (as with fork(2)) the
              process is created in the same PID namespace as the calling
              process.  This flag is intended for the implementation of
              containers.

              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.

       CLONE_NEWUSER
              (This flag first became meaningful for clone() in Linux 2.6.23,
              the current clone() semantics were merged in Linux 3.5, and the
              final pieces to make the user namespaces completely usable were
              merged in Linux 3.8.)

              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).

       CLONE_NEWUTS (since Linux 2.6.19)
              If CLONE_NEWUTS is set, then create the process in a new UTS
              namespace, whose identifiers are initialized by duplicating the
              identifiers from the UTS namespace of the calling process.  If
              this flag is not set, then (as with fork(2)) the process is
              created in the same UTS namespace as the calling process.  This
              flag is intended for the implementation of containers.

              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).

       CLONE_PARENT (since Linux 2.3.12)
              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_PARENT_SETTID (since Linux 2.5.49)
              Store the child thread ID at the location ptid in the parent's
              memory.  (In Linux 2.5.32-2.5.48 there was a flag CLONE_SETTID
              that did this.)  The store operation completes before clone()
              returns control to user space.

       CLONE_PID (obsolete)
              If CLONE_PID is set, the child process is created with the same
              process ID as the calling process.  This is good for hacking the
              system, but otherwise of not much use.  Since 2.3.21 this flag can
              be specified only by the system boot process (PID 0).  It
              disappeared in Linux 2.5.16.  Since then, the kernel silently
              ignores it without error.

       CLONE_PTRACE (since Linux 2.2)
              If CLONE_PTRACE is specified, and the calling process is being
              traced, then trace the child also (see ptrace(2)).

       CLONE_SETTLS (since Linux 2.5.32)
              The TLS (Thread Local Storage) descriptor is set to newtls.

              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.

       CLONE_SIGHAND (since Linux 2.0)
              If CLONE_SIGHAND is set, the calling process and the child process
              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.

              Since Linux 2.6.0-test6, flags must also include CLONE_VM if
              CLONE_SIGHAND is specified

       CLONE_STOPPED (since Linux 2.6.0-test2)
              If CLONE_STOPPED is set, then the child is initially stopped (as
              though it was sent a SIGSTOP signal), and must be resumed by
              sending it a SIGCONT signal.

              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.

       CLONE_SYSVSEM (since Linux 2.5.10)
              If CLONE_SYSVSEM is set, then the child and the calling process
              share a single list of System V semaphore adjustment (semadj)
              values (see semop(2)).  In this case, the shared list accumulates
              semadj values across all processes sharing the list, and semaphore
              adjustments are performed only when the last process that is
              sharing the list terminates (or ceases sharing the list using
              unshare(2)).  If this flag is not set, then the child has a
              separate semadj list that is initially empty.

       CLONE_THREAD (since Linux 2.4.0-test8)
              If CLONE_THREAD is set, the child is placed in the same thread
              group as the calling process.  To make the remainder of the
              discussion of CLONE_THREAD more readable, the term "thread" is
              used to refer to the processes within a thread group.

              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).

       CLONE_UNTRACED (since Linux 2.5.46)
              If CLONE_UNTRACED is specified, then a tracing process cannot
              force CLONE_PTRACE on this child process.

       CLONE_VFORK (since Linux 2.2)
              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 (since Linux 2.0)
              If CLONE_VM is set, the calling process and the child process 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).

   C library/kernel differences
       The raw clone() system call corresponds more closely to fork(2) in that
       execution in the child continues from the point of the call.  As such,
       the fn and arg arguments of the clone() wrapper function are omitted.
       Furthermore, the argument order changes.  In addition, there are
       variations across architectures.

       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.

   blackfin, m68k, and sparc
       The argument-passing conventions on blackfin, m68k, and sparc are
       different from the descriptions above.  For details, see the kernel (and
       glibc) source.

   ia64
       On ia64, a different interface is used:

       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.

   Linux 2.4 and earlier
       In Linux 2.4 and earlier, clone() does not take arguments ptid, tls, and
       ctid.

RETURN VALUE
       On success, the thread ID of the child process is returned in the
       caller's thread of execution.  On failure, -1 is 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; see fork(2).

       EINVAL CLONE_SIGHAND was specified, but CLONE_VM was not.  (Since Linux
              2.6.0-test6.)

       EINVAL CLONE_THREAD was specified, but CLONE_SIGHAND was not.  (Since
              Linux 2.5.35.)

       EINVAL Both CLONE_FS and CLONE_NEWNS were specified in flags.

       EINVAL (since Linux 3.9)
              Both CLONE_NEWUSER and CLONE_FS were specified in flags.

       EINVAL Both CLONE_NEWIPC and CLONE_SYSVSEM were specified in flags.

       EINVAL One (or both) of CLONE_NEWPID or CLONE_NEWUSER and one (or both)
              of CLONE_THREAD or CLONE_PARENT were specified in flags.

       EINVAL Returned by the glibc clone() wrapper function when fn or
              child_stack is specified as NULL.

       EINVAL CLONE_NEWIPC was specified in flags, but the kernel was not
              configured with the CONFIG_SYSVIPC and CONFIG_IPC_NS options.

       EINVAL CLONE_NEWNET was specified in flags, but the kernel was not
              configured with the CONFIG_NET_NS option.

       EINVAL CLONE_NEWPID was specified in flags, but the kernel was not
              configured with the CONFIG_PID_NS option.

       EINVAL CLONE_NEWUTS was specified in flags, but the kernel was not
              configured with the CONFIG_UTS option.

       EINVAL child_stack is not aligned to a suitable boundary for this
              architecture.  For example, on aarch64, child_stack must be a
              multiple of 16.

       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.

       ENOSPC (since Linux 3.7)
              CLONE_NEWPID was specified in flags, but the limit on the nesting
              depth of PID namespaces would have been exceeded; see
              pid_namespaces(7).

       ENOSPC (since Linux 4.9; beforehand EUSERS)
              CLONE_NEWUSER was specified in flags, and the call would cause the
              limit on the number of nested user namespaces to be exceeded.  See
              user_namespaces(7).

              From Linux 3.11 to Linux 4.8, the error diagnosed in this case was
              EUSERS.

       ENOSPC (since Linux 4.9)
              One of the values in flags specified the creation of a new user
              namespace, but doing so would have caused the limit defined by the
              corresponding file in /proc/sys/user to be exceeded.  For further
              details, see namespaces(7).

       EPERM  CLONE_NEWIPC, CLONE_NEWNET, CLONE_NEWNS, CLONE_NEWPID, or
              CLONE_NEWUTS was specified by an unprivileged process (process
              without CAP_SYS_ADMIN).

       EPERM  CLONE_PID was specified by a process other than process 0.

       EPERM  CLONE_NEWUSER was specified in flags, but either the effective
              user ID or the effective group ID of the caller does not have a
              mapping in the parent namespace (see user_namespaces(7)).

       EPERM (since Linux 3.9)
              CLONE_NEWUSER was specified in flags and the caller is in a chroot
              environment (i.e., the caller's root directory does not match the
              root directory of the mount namespace in which it resides).

       ERESTARTNOINTR (since Linux 2.6.17)
              System call was interrupted by a signal and will be restarted.
              (This can be seen only during a trace.)

       EUSERS (Linux 3.11 to Linux 4.8)
              CLONE_NEWUSER was specified in flags, and the limit on the number
              of nested user namespaces would be exceeded.  See the discussion
              of the ENOSPC error above.

CONFORMING TO
       clone() is Linux-specific and should not be used in programs intended to
       be portable.

NOTES
       The kcmp(2) system call can be used to test whether two processes share
       various resources such as a file descriptor table, System V semaphore
       undo operations, or a virtual address space.


       Handlers registered using pthread_atfork(3) are not executed during a
       call to clone().

       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.

BUGS
       Versions of the GNU C library that include the NPTL threading library
       contain a wrapper function for getpid(2) that performs caching of PIDs.
       This caching relies on support in the glibc wrapper for clone(), but as
       currently implemented, the cache may not be up to date in some
       circumstances.  In particular, if a signal is delivered to the child
       immediately after the clone() call, then a call to getpid(2) in a handler
       for the signal may return the PID of the calling process ("the parent"),
       if the clone wrapper has not yet had a chance to update the PID cache in
       the child.  (This discussion ignores the case where the child was created
       using CLONE_THREAD, when getpid(2) should return the same value in the
       child and in the process that called clone(), since the caller and the
       child are in the same thread group.  The stale-cache problem also does
       not occur if the flags argument includes CLONE_VM.)  To get the truth, it
       may be necessary to use code such as the following:

           #include <syscall.h>

           pid_t mypid;

           mypid = syscall(SYS_getpid);

EXAMPLE
       The following program demonstrates the use of clone() to create a child
       process that executes in a separate UTS namespace.  The child changes the
       hostname in its UTS namespace.  Both parent and child then display the
       system hostname, making it possible to see that the hostname differs in
       the UTS namespaces of the parent and child.  For an example of the use of
       this program, see setns(2).

   Program source
       #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);
       }

SEE ALSO
       fork(2), futex(2), getpid(2), gettid(2), kcmp(2), set_thread_area(2),
       set_tid_address(2), setns(2), tkill(2), unshare(2), wait(2),
       capabilities(7), namespaces(7), pthreads(7)

COLOPHON
       This page is part of release 3.53 of the Linux man-pages project.  A
       description of the project, and information about reporting bugs, can be
       found at http://www.kernel.org/doc/man-pages/.



Linux                              2016-12-12                           CLONE(2)