bionic (2) clone.2.gz

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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 virtual address 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 flows of control in a program that run  concurrently
       in a shared address space.

       When the child process is created with clone(), it commences execution by calling the function pointed to
       by the argument fn.  (This differs from fork(2), where execution continues in the child from the point of
       the fork(2) call.)  The arg argument is passed as the argument of the function fn.

       When  the fn(arg) function returns, the child process terminates.  The integer returned by fn is the exit
       status 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-ORed 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), or 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_NEWCGROUP (since Linux 4.6)
              Create the process in a new cgroup namespace.  If this flag is not set, then (as with fork(2)) the
              process  is  created  in the same cgroup namespaces as the calling process.  This flag is intended
              for the implementation of containers.

              For further information on cgroup namespaces, see cgroup_namespaces(7).

              Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWCGROUP.

       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 (veth(4)) device 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.

              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 namespaces(7) and 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 (Linux 2.0 to 2.5.15)
              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.  From Linux 2.3.21 onward,
              this flag could be specified only by the system  boot  process  (PID  0).   The  flag  disappeared
              completely  from the kernel sources in Linux 2.5.16.  Since then, the kernel silently ignores this
              bit if it is specified in flags.

       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 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.  Starting with Linux 4.6, the same bit
              was reused for the CLONE_NEWCGROUP flag.

       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).

NOTES

       Note that the glibc clone() wrapper function makes some changes in the memory pointed to  by  child_stack
       (changes  required  to set the stack up correctly for the child) before invoking the clone() system call.
       So, in cases where clone() is used to recursively create children, do not use the buffer employed for the
       parent's stack as the stack of the child.

   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.

       Another difference for the raw clone() system call is that the child_stack argument may be zero, in which
       case the child uses a duplicate of the parent's stack.  (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.  (If the child shares the parent's memory because
       of the use of the CLONE_VM flag, then no copy-on-write duplication occurs and chaos is likely to result.)

       The order of the arguments also differs in the raw system call, and there are variations in the arguments
       across architectures, as detailed in the following paragraphs.

       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);

   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_NEWCGROUP,  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.  (This error occurs only on Linux
              2.5.15 and earlier.)

       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

       GNU  C  library  versions  2.3.4 up to and including 2.24 contained a wrapper function for getpid(2) that
       performed caching of PIDs.  This caching relied  on  support  in  the  glibc  wrapper  for  clone(),  but
       limitations  in  the  implementation  meant  that the cache was not up to date in some circumstances.  In
       particular, if a signal was delivered to the child immediately after the clone() call,  then  a  call  to
       getpid(2)  in a handler for the signal could return the PID of the calling process ("the parent"), if the
       clone wrapper had 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 was sometimes necessary to use code such as the following:

           #include <syscall.h>

           pid_t mypid;

           mypid = syscall(SYS_getpid);

       Because of the stale-cache problem, as well as other problems noted in getpid(2), the PID caching feature
       was removed in glibc 2.25.

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  4.15  of  the  Linux man-pages project.  A description of the project,
       information  about  reporting  bugs,  and  the  latest  version  of  this   page,   can   be   found   at
       https://www.kernel.org/doc/man-pages/.