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       syscalls - Linux system calls


       Linux system calls.


       The system call is the fundamental interface between an application and the Linux kernel.

   System calls and library wrapper functions
       System calls are generally not invoked directly, but rather via wrapper functions in glibc
       (or perhaps some other library).  For details of direct invocation of a system  call,  see
       intro(2).  Often, but not always, the name of the wrapper function is the same as the name
       of the system call that it invokes.  For example, glibc  contains  a  function  truncate()
       which invokes the underlying "truncate" system call.

       Often  the  glibc  wrapper  function  is  quite thin, doing little work other than copying
       arguments to the right registers before invoking the system call, and then  setting  errno
       appropriately  after  the  system  call  has returned.  (These are the same steps that are
       performed by syscall(2), which can be used to invoke system calls  for  which  no  wrapper
       function  is  provided.)   Note:  system  calls indicate a failure by returning a negative
       error number to the caller; when this happens, the wrapper function negates  the  returned
       error  number  (to  make it positive), copies it to errno, and returns -1 to the caller of
       the wrapper.

       Sometimes, however, the wrapper function does some extra work before invoking  the  system
       call.   For  example,  nowadays there are (for reasons described below) two related system
       calls, truncate(2) and truncate64(2), and the glibc  truncate()  wrapper  function  checks
       which  of  those  system  calls  are provided by the kernel and determines which should be

   System call list
       Below is a list of the Linux system calls.  In the list, the Kernel column  indicates  the
       kernel  version  for those system calls that were new in Linux 2.2, or have appeared since
       that kernel version.  Note the following points:

       *  Where no kernel version is indicated,  the  system  call  appeared  in  kernel  1.0  or

       *  Where  a  system call is marked "1.2" this means the system call probably appeared in a
          1.1.x kernel version, and first appeared in a stable kernel with 1.2.  (Development  of
          the  1.2  kernel  was  initiated  from  a branch of kernel 1.0.6 via the 1.1.x unstable
          kernel series.)

       *  Where a system call is marked "2.0" this means the system call probably appeared  in  a
          1.3.x  kernel version, and first appeared in a stable kernel with 2.0.  (Development of
          the 2.0 kernel was initiated from a branch of kernel 1.2.x,  somewhere  around  1.2.10,
          via the 1.3.x unstable kernel series.)

       *  Where  a  system call is marked "2.2" this means the system call probably appeared in a
          2.1.x kernel version, and first appeared in a stable kernel with  2.2.0.   (Development
          of  the  2.2 kernel was initiated from a branch of kernel 2.0.21 via the 2.1.x unstable
          kernel series.)

       *  Where a system call is marked "2.4" this means the system call probably appeared  in  a
          2.3.x  kernel  version, and first appeared in a stable kernel with 2.4.0.  (Development
          of the 2.4 kernel was initiated from a branch of kernel 2.2.8 via  the  2.3.x  unstable
          kernel series.)

       *  Where  a  system call is marked "2.6" this means the system call probably appeared in a
          2.5.x kernel version, and first appeared in a stable kernel with  2.6.0.   (Development
          of  kernel  2.6  was  initiated  from  a branch of kernel 2.4.15 via the 2.5.x unstable
          kernel series.)

       *  Starting with kernel 2.6.0, the development model changed, and  new  system  calls  may
          appear  in each 2.6.x release.  In this case, the exact version number where the system
          call appeared is shown.  This convention continues with the 3.x  kernel  series,  which
          followed  on  from  kernel  2.6.39,  and  the 4.x kernel series, which followed on from
          kernel 3.19.

       *  In some cases, a system call was added to a stable kernel series after it branched from
          the  previous  stable kernel series, and then backported into the earlier stable kernel
          series.  For example some system calls that appeared in 2.6.x were also backported into
          a  2.4.x  release  after  2.4.15.   When  this is so, the version where the system call
          appeared in both of the major kernel series is listed.

       The list of system calls that are available as at kernel 4.4 (or in a few  cases  only  on
       older kernels) is as follows:

       System call                Kernel        Notes

       _llseek(2)                 1.2
       _newselect(2)              2.0
       _sysctl(2)                 2.0
       accept(2)                  2.0           See notes on socketcall(2)
       accept4(2)                 2.6.28
       access(2)                  1.0
       acct(2)                    1.0
       add_key(2)                 2.6.11
       adjtimex(2)                1.0
       alarm(2)                   1.0
       alloc_hugepages(2)         2.5.36        Removed in 2.5.44
       bdflush(2)                 1.2           Deprecated (does nothing)
                                                since 2.6
       bind(2)                    2.0           See notes on socketcall(2)
       bpf(2)                     3.18
       brk(2)                     1.0
       cacheflush(2)              1.2           Not on x86
       capget(2)                  2.2
       capset(2)                  2.2
       chdir(2)                   1.0
       chmod(2)                   1.0
       chown(2)                   2.2           See chown(2) for
                                                version details
       chown32(2)                 2.4
       chroot(2)                  1.0
       clock_adjtime(2)           2.6.39
       clock_getres(2)            2.6
       clock_gettime(2)           2.6
       clock_nanosleep(2)         2.6
       clock_settime(2)           2.6
       clone(2)                   1.0
       close(2)                   1.0
       connect(2)                 2.0           See notes on socketcall(2)
       creat(2)                   1.0
       create_module(2)           1.0           Removed in 2.6
       delete_module(2)           1.0
       dup(2)                     1.0
       dup2(2)                    1.0
       dup3(2)                    2.6.27
       epoll_create(2)            2.6
       epoll_create1(2)           2.6.27
       epoll_ctl(2)               2.6
       epoll_pwait(2)             2.6.19
       epoll_wait(2)              2.6
       eventfd(2)                 2.6.22

       eventfd2(2)                2.6.27
       execve(2)                  1.0
       execveat(2)                3.19
       exit(2)                    1.0
       exit_group(2)              2.6
       faccessat(2)               2.6.16
       fadvise64(2)               2.6
       fadvise64_64(2)            2.6
       fallocate(2)               2.6.23
       fanotify_init(2)           2.6.37
       fanotify_mark(2)           2.6.37
       fchdir(2)                  1.0
       fchmod(2)                  1.0
       fchmodat(2)                2.6.16
       fchown(2)                  1.0
       fchown32(2)                2.4
       fchownat(2)                2.6.16
       fcntl(2)                   1.0
       fcntl64(2)                 2.4
       fdatasync(2)               2.0
       fgetxattr(2)               2.6; 2.4.18
       finit_module(2)            3.8
       flistxattr(2)              2.6; 2.4.18
       flock(2)                   2.0
       fork(2)                    1.0
       free_hugepages(2)          2.5.36        Removed in 2.5.44
       fremovexattr(2)            2.6; 2.4.18
       fsetxattr(2)               2.6; 2.4.18
       fstat(2)                   1.0
       fstat64(2)                 2.4
       fstatat64(2)               2.6.16
       fstatfs(2)                 1.0
       fstatfs64(2)               2.6
       fsync(2)                   1.0
       ftruncate(2)               1.0
       ftruncate64(2)             2.4
       futex(2)                   2.6
       futimesat(2)               2.6.16
       get_kernel_syms(2)         1.0           Removed in 2.6
       get_mempolicy(2)           2.6.6
       get_robust_list(2)         2.6.17
       get_thread_area(2)         2.6
       getcpu(2)                  2.6.19
       getcwd(2)                  2.2
       getdents(2)                2.0
       getdents64(2)              2.4
       getegid(2)                 1.0
       getegid32(2)               2.4
       geteuid(2)                 1.0
       geteuid32(2)               2.4
       getgid(2)                  1.0
       getgid32(2)                2.4
       getgroups(2)               1.0
       getgroups32(2)             2.4
       getitimer(2)               1.0
       getpeername(2)             2.0           See notes on socketcall(2)
       getpagesize(2)             2.0           Not on x86
       getpgid(2)                 1.0
       getpgrp(2)                 1.0
       getpid(2)                  1.0
       getppid(2)                 1.0
       getpriority(2)             1.0
       getrandom(2)               3.17
       getresgid(2)               2.2
       getresgid32(2)             2.4

       getresuid(2)               2.2
       getresuid32(2)             2.4
       getrlimit(2)               1.0
       getrusage(2)               1.0
       getsid(2)                  2.0
       getsockname(2)             2.0           See notes on socketcall(2)
       getsockopt(2)              2.0           See notes on socketcall(2)
       gettid(2)                  2.4.11
       gettimeofday(2)            1.0
       getuid(2)                  1.0
       getuid32(2)                2.4
       getxattr(2)                2.6; 2.4.18
       init_module(2)             1.0
       inotify_add_watch(2)       2.6.13
       inotify_init(2)            2.6.13
       inotify_init1(2)           2.6.27
       inotify_rm_watch(2)        2.6.13
       io_cancel(2)               2.6
       io_destroy(2)              2.6
       io_getevents(2)            2.6
       io_setup(2)                2.6
       io_submit(2)               2.6
       ioctl(2)                   1.0
       ioperm(2)                  1.0
       iopl(2)                    1.0
       ioprio_get(2)              2.6.13
       ioprio_set(2)              2.6.13
       ipc(2)                     1.0
       kcmp(2)                    3.5
       kern_features(2)           3.7           Sparc64
       kexec_file_load(2)         3.17
       kexec_load(2)              2.6.13
       keyctl(2)                  2.6.11
       kill(2)                    1.0
       lchown(2)                  1.0           See chown(2) for
                                                version details
       lchown32(2)                2.4
       lgetxattr(2)               2.6; 2.4.18
       link(2)                    1.0
       linkat(2)                  2.6.16
       listen(2)                  2.0           See notes on socketcall(2)
       listxattr(2)               2.6; 2.4.18
       llistxattr(2)              2.6; 2.4.18
       lookup_dcookie(2)          2.6
       lremovexattr(2)            2.6; 2.4.18
       lseek(2)                   1.0
       lsetxattr(2)               2.6; 2.4.18
       lstat(2)                   1.0
       lstat64(2)                 2.4
       madvise(2)                 2.4
       mbind(2)                   2.6.6
       memfd_create(2)            3.17
       migrate_pages(2)           2.6.16
       mincore(2)                 2.4
       mkdir(2)                   1.0
       mkdirat(2)                 2.6.16
       mknod(2)                   1.0
       mknodat(2)                 2.6.16
       mlock(2)                   2.0
       mlock2(2)                  4.4
       mlockall(2)                2.0
       mmap(2)                    1.0
       mmap2(2)                   2.4
       modify_ldt(2)              1.0
       mount(2)                   1.0

       move_pages(2)              2.6.18
       mprotect(2)                1.0
       mq_getsetattr(2)           2.6.6
       mq_notify(2)               2.6.6
       mq_open(2)                 2.6.6
       mq_timedreceive(2)         2.6.6
       mq_timedsend(2)            2.6.6
       mq_unlink(2)               2.6.6
       mremap(2)                  2.0
       msgctl(2)                  2.0           See notes on ipc(2)
       msgget(2)                  2.0           See notes on ipc(2)
       msgrcv(2)                  2.0           See notes on ipc(2)
       msgsnd(2)                  2.0           See notes on ipc(2)
       msync(2)                   2.0
       munlock(2)                 2.0
       munlockall(2)              2.0
       munmap(2)                  1.0
       name_to_handle_at(2)       2.6.39
       nanosleep(2)               2.0
       nfsservctl(2)              2.2           Removed in 3.1
       nice(2)                    1.0
       oldfstat(2)                1.0
       oldlstat(2)                1.0
       oldolduname(2)             1.0
       oldstat(2)                 1.0
       olduname(2)                1.0
       open(2)                    1.0
       open_by_handle_at(2)       2.6.39
       openat(2)                  2.6.16
       pause(2)                   1.0
       pciconfig_iobase(2)        2.2.15; 2.4   Not on x86
       pciconfig_read(2)          2.0.26; 2.2   Not on x86
       pciconfig_write(2)         2.0.26; 2.2   Not on x86
       perf_event_open(2)         2.6.31        Was perf_counter_open() in
                                                2.6.31; renamed in 2.6.32
       personality(2)             1.2
       perfctr(2)                 2.2           Sparc; removed in 2.6.34
       perfmonctl(2)              2.4           ia64
       pipe(2)                    1.0
       pipe2(2)                   2.6.27
       pivot_root(2)              2.4
       poll(2)                    2.0.36; 2.2
       ppc_rtas(2)                2.6.2         PowerPC only
       ppoll(2)                   2.6.16
       prctl(2)                   2.2
       pread64(2)                               Added as "pread" in 2.2;
                                                renamed "pread64" in 2.6
       preadv(2)                  2.6.30
       prlimit64(2)               2.6.36
       process_vm_readv(2)        3.2
       process_vm_writev(2)       3.2
       pselect6(2)                2.6.16
       ptrace(2)                  1.0
       pwrite64(2)                              Added as "pwrite" in 2.2;
                                                renamed "pwrite64" in 2.6
       pwritev(2)                 2.6.30
       query_module(2)            2.2           Removed in 2.6
       quotactl(2)                1.0
       read(2)                    1.0
       readahead(2)               2.4.13
       readdir(2)                 1.0
       readlink(2)                1.0
       readlinkat(2)              2.6.16
       readv(2)                   2.0
       reboot(2)                  1.0

       recv(2)                    2.0           See notes on socketcall(2)
       recvfrom(2)                2.0           See notes on socketcall(2)
       recvmsg(2)                 2.0           See notes on socketcall(2)
       recvmmsg(2)                2.6.33
       remap_file_pages(2)        2.6           Deprecated since 3.16
       removexattr(2)             2.6; 2.4.18
       rename(2)                  1.0
       renameat(2)                2.6.16
       renameat2(2)               3.15
       request_key(2)             2.6.11
       restart_syscall(2)         2.6
       rmdir(2)                   1.0
       rt_sigaction(2)            2.2
       rt_sigpending(2)           2.2
       rt_sigprocmask(2)          2.2
       rt_sigqueueinfo(2)         2.2
       rt_sigreturn(2)            2.2
       rt_sigsuspend(2)           2.2
       rt_sigtimedwait(2)         2.2
       rt_tgsigqueueinfo(2)       2.6.31
       s390_runtime_instr(2)      3.7           s390 only
       s390_pci_mmio_read(2)      3.19          s390 only
       s390_pci_mmio_write(2)     3.19          s390 only
       sched_get_priority_max(2)  2.0
       sched_get_priority_min(2)  2.0
       sched_getaffinity(2)       2.6
       sched_getattr(2)           3.14
       sched_getparam(2)          2.0
       sched_getscheduler(2)      2.0
       sched_rr_get_interval(2)   2.0
       sched_setaffinity(2)       2.6
       sched_setattr(2)           3.14
       sched_setparam(2)          2.0
       sched_setscheduler(2)      2.0
       sched_yield(2)             2.0
       seccomp(2)                 3.17
       select(2)                  1.0
       semctl(2)                  2.0           See notes on ipc(2)
       semget(2)                  2.0           See notes on ipc(2)
       semop(2)                   2.0           See notes on ipc(2)
       semtimedop(2)              2.6; 2.4.22
       send(2)                    2.0           See notes on socketcall(2)
       sendfile(2)                2.2
       sendfile64(2)              2.6; 2.4.19
       sendmmsg(2)                3.0
       sendmsg(2)                 2.0           See notes on socketcall(2)
       sendto(2)                  2.0           See notes on socketcall(2)
       set_mempolicy(2)           2.6.6
       set_robust_list(2)         2.6.17
       set_thread_area(2)         2.6
       set_tid_address(2)         2.6
       setdomainname(2)           1.0
       setfsgid(2)                1.2
       setfsgid32(2)              2.4
       setfsuid(2)                1.2
       setfsuid32(2)              2.4
       setgid(2)                  1.0
       setgid32(2)                2.4
       setgroups(2)               1.0
       setgroups32(2)             2.4
       sethostname(2)             1.0
       setitimer(2)               1.0
       setns(2)                   3.0
       setpgid(2)                 1.0
       setpriority(2)             1.0

       setregid(2)                1.0
       setregid32(2)              2.4
       setresgid(2)               2.2
       setresgid32(2)             2.4
       setresuid(2)               2.2
       setresuid32(2)             2.4
       setreuid(2)                1.0
       setreuid32(2)              2.4
       setrlimit(2)               1.0
       setsid(2)                  1.0
       setsockopt(2)              2.0           See notes on socketcall(2)
       settimeofday(2)            1.0
       setuid(2)                  1.0
       setuid32(2)                2.4
       setup(2)                   1.0           Removed in 2.2
       setxattr(2)                2.6; 2.4.18
       sgetmask(2)                1.0
       shmat(2)                   2.0           See notes on ipc(2)
       shmctl(2)                  2.0           See notes on ipc(2)
       shmdt(2)                   2.0           See notes on ipc(2)
       shmget(2)                  2.0           See notes on ipc(2)
       shutdown(2)                2.0           See notes on socketcall(2)
       sigaction(2)               1.0
       sigaltstack(2)             2.2
       signal(2)                  1.0
       signalfd(2)                2.6.22
       signalfd4(2)               2.6.27
       sigpending(2)              1.0
       sigprocmask(2)             1.0
       sigreturn(2)               1.0
       sigsuspend(2)              1.0
       socket(2)                  2.0           See notes on socketcall(2)
       socketcall(2)              1.0
       socketpair(2)              2.0           See notes on socketcall(2)
       splice(2)                  2.6.17
       spu_create(2)              2.6.16        PowerPC only
       spu_run(2)                 2.6.16        PowerPC only
       ssetmask(2)                1.0
       stat(2)                    1.0
       stat64(2)                  2.4
       statfs(2)                  1.0
       statfs64(2)                2.6
       stime(2)                   1.0
       subpage_prot(2)            2.6.25        PowerPC only
       swapoff(2)                 1.0
       swapon(2)                  1.0
       symlink(2)                 1.0
       symlinkat(2)               2.6.16
       sync(2)                    1.0
       sync_file_range(2)         2.6.17
       sync_file_range2(2)        2.6.22
       syncfs(2)                  2.6.39
       sysfs(2)                   1.2
       sysinfo(2)                 1.0
       syslog(2)                  1.0
       tee(2)                     2.6.17
       tgkill(2)                  2.6
       time(2)                    1.0
       timer_create(2)            2.6
       timer_delete(2)            2.6
       timer_getoverrun(2)        2.6
       timer_gettime(2)           2.6
       timer_settime(2)           2.6
       timerfd_create(2)          2.6.25
       timerfd_gettime(2)         2.6.25

       timerfd_settime(2)         2.6.25
       times(2)                   1.0
       tkill(2)                   2.6; 2.4.22
       truncate(2)                1.0
       truncate64(2)              2.4
       ugetrlimit(2)              2.4
       umask(2)                   1.0
       umount(2)                  1.0
       umount2(2)                 2.2
       uname(2)                   1.0
       unlink(2)                  1.0
       unlinkat(2)                2.6.16
       unshare(2)                 2.6.16
       uselib(2)                  1.0
       ustat(2)                   1.0
       userfaultfd(2)             4.2
       utime(2)                   1.0
       utimensat(2)               2.6.22
       utimes(2)                  2.2
       utrap_install(2)           2.2           Sparc only
       vfork(2)                   2.2
       vhangup(2)                 1.0
       vm86old(2)                 1.0           Was "vm86"; renamed in
       vm86(2)                    2.0.28; 2.2
       vmsplice(2)                2.6.17
       wait4(2)                   1.0
       waitid(2)                  2.6.10
       waitpid(2)                 1.0
       write(2)                   1.0
       writev(2)                  2.0

       On many platforms, including x86-32, socket calls are all multiplexed (via  glibc  wrapper
       functions)  through socketcall(2) and similarly System V IPC calls are multiplexed through

       Although slots are reserved for them in the system call table, the following system  calls
       are   not   implemented  in  the  standard  kernel:  afs_syscall(2),  break(2),  ftime(2),
       getpmsg(2), gtty(2), idle(2), lock(2), madvise1(2), mpx(2), phys(2),  prof(2),  profil(2),
       putpmsg(2),   security(2),  stty(2),  tuxcall(2),  ulimit(2),  and  vserver(2)  (see  also
       unimplemented(2)).  However, ftime(3), profil(3), and ulimit(3) exist as library routines.
       The  slot  for phys(2) is in use since kernel 2.1.116 for umount(2); phys(2) will never be
       implemented.  The getpmsg(2) and putpmsg(2) calls  are  for  kernels  patched  to  support
       STREAMS, and may never be in the standard kernel.

       There  was briefly set_zone_reclaim(2), added in Linux 2.6.13, and removed in 2.6.16; this
       system call was never available to user space.


       Roughly speaking, the code belonging to the system call with number  __NR_xxx  defined  in
       /usr/include/asm/unistd.h  can  be  found  in  the  Linux  kernel  source  in  the routine
       sys_xxx().     (The    dispatch     table     for     i386     can     be     found     in
       /usr/src/linux/arch/i386/kernel/entry.S.)   There  are  many  exceptions,  however, mostly
       because older system calls were superseded by  newer  ones,  and  this  has  been  treated
       somewhat unsystematically.  On platforms with proprietary operating-system emulation, such
       as parisc, sparc, sparc64, and alpha, there are many additional system calls; mips64  also
       contains a full set of 32-bit system calls.

       Over time, changes to the interfaces of some system calls have been necessary.  One reason
       for such changes was the need to increase the size of structures or scalar  values  passed
       to   the   system  call.   Because  of  these  changes,  certain  architectures  (notably,
       longstanding 32-bit architectures such as i386) now have various groups of related  system
       calls (e.g., truncate(2) and truncate64(2)) which perform similar tasks, but which vary in
       details such as the  size  of  their  arguments.   (As  noted  earlier,  applications  are
       generally  unaware  of  this:  the glibc wrapper functions do some work to ensure that the
       right system call is invoked, and that ABI compatibility is preserved for  old  binaries.)
       Examples of systems calls that exist in multiple versions are the following:

       *  By  now  there are three different versions of stat(2): sys_stat() (slot __NR_oldstat),
          sys_newstat() (slot __NR_stat), and sys_stat64()  (slot  __NR_stat64),  with  the  last
          being the most current.  A similar story applies for lstat(2) and fstat(2).

       *  Similarly,  the  defines  __NR_oldolduname,  __NR_olduname, and __NR_uname refer to the
          routines sys_olduname(), sys_uname() and sys_newuname().

       *  In Linux 2.0, a new version of vm86(2) appeared,  with  the  old  and  the  new  kernel
          routines being named sys_vm86old() and sys_vm86().

       *  In  Linux  2.4, a new version of getrlimit(2) appeared, with the old and the new kernel
          routines being named  sys_old_getrlimit()  (slot  __NR_getrlimit)  and  sys_getrlimit()
          (slot __NR_ugetrlimit).

       *  Linux 2.4 increased the size of user and group IDs from 16 to 32 bits.  To support this
          change,  a  range  of  system  calls  were  added   (e.g.,   chown32(2),   getuid32(2),
          getgroups32(2), setresuid32(2)), superseding earlier calls of the same name without the
          "32" suffix.

       *  Linux 2.4 added support for applications on 32-bit architectures to access large  files
          (i.e., files for which the sizes and file offsets can't be represented in 32 bits.)  To
          support this change, replacements were required for system calls that  deal  with  file
          offsets   and   sizes.   Thus  the  following  system  calls  were  added:  fcntl64(2),
          getdents64(2), stat64(2), statfs64(2), truncate64(2), and their analogs that work  with
          file  descriptors  or  symbolic  links.   These system calls supersede the older system
          calls which, except in the case of the "stat" calls, have the  same  name  without  the
          "64" suffix.

          On newer platforms that only have 64-bit file access and 32-bit UIDs/GIDs (e.g., alpha,
          ia64, s390x, x86-64), there is just a single version of the  UID/GID  and  file  access
          system  calls.   On platforms (typically, 32-bit platforms) where the *64 and *32 calls
          exist, the other versions are obsolete.

       *  The rt_sig* calls were added in kernel 2.2 to support the addition of real-time signals
          (see  signal(7)).  These system calls supersede the older system calls of the same name
          without the "rt_" prefix.

       *  The select(2) and mmap(2) system  calls  use  five  or  more  arguments,  which  caused
          problems  in the way argument passing on the i386 used to be set up.  Thus, while other
          architectures  have  sys_select()  and  sys_mmap()  corresponding  to  __NR_select  and
          __NR_mmap,  on  i386 one finds old_select() and old_mmap() (routines that use a pointer
          to a argument block) instead.  These days passing five arguments is not a  problem  any
          more,  and  there  is  a  __NR__newselect that corresponds directly to sys_select() and
          similarly __NR_mmap2.


       intro(2), syscall(2), unimplemented(2), errno(3), libc(7), vdso(7)


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