github.com/ie310mu/ie310go/forks/golang.org/x/sys@v0.0.0-20190821095322-9a46783d4de5/unix/syscall_netbsd.go (about)

     1  // Copyright 2009,2010 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // NetBSD system calls.
     6  // This file is compiled as ordinary Go code,
     7  // but it is also input to mksyscall,
     8  // which parses the //sys lines and generates system call stubs.
     9  // Note that sometimes we use a lowercase //sys name and wrap
    10  // it in our own nicer implementation, either here or in
    11  // syscall_bsd.go or syscall_unix.go.
    12  
    13  package unix
    14  
    15  import (
    16  	"runtime"
    17  	"syscall"
    18  	"unsafe"
    19  )
    20  
    21  // SockaddrDatalink implements the Sockaddr interface for AF_LINK type sockets.
    22  type SockaddrDatalink struct {
    23  	Len    uint8
    24  	Family uint8
    25  	Index  uint16
    26  	Type   uint8
    27  	Nlen   uint8
    28  	Alen   uint8
    29  	Slen   uint8
    30  	Data   [12]int8
    31  	raw    RawSockaddrDatalink
    32  }
    33  
    34  func Syscall9(trap, a1, a2, a3, a4, a5, a6, a7, a8, a9 uintptr) (r1, r2 uintptr, err syscall.Errno)
    35  
    36  func sysctlNodes(mib []_C_int) (nodes []Sysctlnode, err error) {
    37  	var olen uintptr
    38  
    39  	// Get a list of all sysctl nodes below the given MIB by performing
    40  	// a sysctl for the given MIB with CTL_QUERY appended.
    41  	mib = append(mib, CTL_QUERY)
    42  	qnode := Sysctlnode{Flags: SYSCTL_VERS_1}
    43  	qp := (*byte)(unsafe.Pointer(&qnode))
    44  	sz := unsafe.Sizeof(qnode)
    45  	if err = sysctl(mib, nil, &olen, qp, sz); err != nil {
    46  		return nil, err
    47  	}
    48  
    49  	// Now that we know the size, get the actual nodes.
    50  	nodes = make([]Sysctlnode, olen/sz)
    51  	np := (*byte)(unsafe.Pointer(&nodes[0]))
    52  	if err = sysctl(mib, np, &olen, qp, sz); err != nil {
    53  		return nil, err
    54  	}
    55  
    56  	return nodes, nil
    57  }
    58  
    59  func nametomib(name string) (mib []_C_int, err error) {
    60  	// Split name into components.
    61  	var parts []string
    62  	last := 0
    63  	for i := 0; i < len(name); i++ {
    64  		if name[i] == '.' {
    65  			parts = append(parts, name[last:i])
    66  			last = i + 1
    67  		}
    68  	}
    69  	parts = append(parts, name[last:])
    70  
    71  	// Discover the nodes and construct the MIB OID.
    72  	for partno, part := range parts {
    73  		nodes, err := sysctlNodes(mib)
    74  		if err != nil {
    75  			return nil, err
    76  		}
    77  		for _, node := range nodes {
    78  			n := make([]byte, 0)
    79  			for i := range node.Name {
    80  				if node.Name[i] != 0 {
    81  					n = append(n, byte(node.Name[i]))
    82  				}
    83  			}
    84  			if string(n) == part {
    85  				mib = append(mib, _C_int(node.Num))
    86  				break
    87  			}
    88  		}
    89  		if len(mib) != partno+1 {
    90  			return nil, EINVAL
    91  		}
    92  	}
    93  
    94  	return mib, nil
    95  }
    96  
    97  func SysctlClockinfo(name string) (*Clockinfo, error) {
    98  	mib, err := sysctlmib(name)
    99  	if err != nil {
   100  		return nil, err
   101  	}
   102  
   103  	n := uintptr(SizeofClockinfo)
   104  	var ci Clockinfo
   105  	if err := sysctl(mib, (*byte)(unsafe.Pointer(&ci)), &n, nil, 0); err != nil {
   106  		return nil, err
   107  	}
   108  	if n != SizeofClockinfo {
   109  		return nil, EIO
   110  	}
   111  	return &ci, nil
   112  }
   113  
   114  //sysnb pipe() (fd1 int, fd2 int, err error)
   115  func Pipe(p []int) (err error) {
   116  	if len(p) != 2 {
   117  		return EINVAL
   118  	}
   119  	p[0], p[1], err = pipe()
   120  	return
   121  }
   122  
   123  //sys getdents(fd int, buf []byte) (n int, err error)
   124  func Getdirentries(fd int, buf []byte, basep *uintptr) (n int, err error) {
   125  	return getdents(fd, buf)
   126  }
   127  
   128  const ImplementsGetwd = true
   129  
   130  //sys	Getcwd(buf []byte) (n int, err error) = SYS___GETCWD
   131  
   132  func Getwd() (string, error) {
   133  	var buf [PathMax]byte
   134  	_, err := Getcwd(buf[0:])
   135  	if err != nil {
   136  		return "", err
   137  	}
   138  	n := clen(buf[:])
   139  	if n < 1 {
   140  		return "", EINVAL
   141  	}
   142  	return string(buf[:n]), nil
   143  }
   144  
   145  // TODO
   146  func sendfile(outfd int, infd int, offset *int64, count int) (written int, err error) {
   147  	return -1, ENOSYS
   148  }
   149  
   150  func setattrlistTimes(path string, times []Timespec, flags int) error {
   151  	// used on Darwin for UtimesNano
   152  	return ENOSYS
   153  }
   154  
   155  //sys	ioctl(fd int, req uint, arg uintptr) (err error)
   156  
   157  // ioctl itself should not be exposed directly, but additional get/set
   158  // functions for specific types are permissible.
   159  
   160  // IoctlSetInt performs an ioctl operation which sets an integer value
   161  // on fd, using the specified request number.
   162  func IoctlSetInt(fd int, req uint, value int) error {
   163  	return ioctl(fd, req, uintptr(value))
   164  }
   165  
   166  func ioctlSetWinsize(fd int, req uint, value *Winsize) error {
   167  	return ioctl(fd, req, uintptr(unsafe.Pointer(value)))
   168  }
   169  
   170  func ioctlSetTermios(fd int, req uint, value *Termios) error {
   171  	return ioctl(fd, req, uintptr(unsafe.Pointer(value)))
   172  }
   173  
   174  // IoctlGetInt performs an ioctl operation which gets an integer value
   175  // from fd, using the specified request number.
   176  func IoctlGetInt(fd int, req uint) (int, error) {
   177  	var value int
   178  	err := ioctl(fd, req, uintptr(unsafe.Pointer(&value)))
   179  	return value, err
   180  }
   181  
   182  func IoctlGetWinsize(fd int, req uint) (*Winsize, error) {
   183  	var value Winsize
   184  	err := ioctl(fd, req, uintptr(unsafe.Pointer(&value)))
   185  	return &value, err
   186  }
   187  
   188  func IoctlGetTermios(fd int, req uint) (*Termios, error) {
   189  	var value Termios
   190  	err := ioctl(fd, req, uintptr(unsafe.Pointer(&value)))
   191  	return &value, err
   192  }
   193  
   194  func IoctlGetPtmget(fd int, req uint) (*Ptmget, error) {
   195  	var value Ptmget
   196  	err := ioctl(fd, req, uintptr(unsafe.Pointer(&value)))
   197  	runtime.KeepAlive(value)
   198  	return &value, err
   199  }
   200  
   201  func Uname(uname *Utsname) error {
   202  	mib := []_C_int{CTL_KERN, KERN_OSTYPE}
   203  	n := unsafe.Sizeof(uname.Sysname)
   204  	if err := sysctl(mib, &uname.Sysname[0], &n, nil, 0); err != nil {
   205  		return err
   206  	}
   207  
   208  	mib = []_C_int{CTL_KERN, KERN_HOSTNAME}
   209  	n = unsafe.Sizeof(uname.Nodename)
   210  	if err := sysctl(mib, &uname.Nodename[0], &n, nil, 0); err != nil {
   211  		return err
   212  	}
   213  
   214  	mib = []_C_int{CTL_KERN, KERN_OSRELEASE}
   215  	n = unsafe.Sizeof(uname.Release)
   216  	if err := sysctl(mib, &uname.Release[0], &n, nil, 0); err != nil {
   217  		return err
   218  	}
   219  
   220  	mib = []_C_int{CTL_KERN, KERN_VERSION}
   221  	n = unsafe.Sizeof(uname.Version)
   222  	if err := sysctl(mib, &uname.Version[0], &n, nil, 0); err != nil {
   223  		return err
   224  	}
   225  
   226  	// The version might have newlines or tabs in it, convert them to
   227  	// spaces.
   228  	for i, b := range uname.Version {
   229  		if b == '\n' || b == '\t' {
   230  			if i == len(uname.Version)-1 {
   231  				uname.Version[i] = 0
   232  			} else {
   233  				uname.Version[i] = ' '
   234  			}
   235  		}
   236  	}
   237  
   238  	mib = []_C_int{CTL_HW, HW_MACHINE}
   239  	n = unsafe.Sizeof(uname.Machine)
   240  	if err := sysctl(mib, &uname.Machine[0], &n, nil, 0); err != nil {
   241  		return err
   242  	}
   243  
   244  	return nil
   245  }
   246  
   247  func Sendfile(outfd int, infd int, offset *int64, count int) (written int, err error) {
   248  	if raceenabled {
   249  		raceReleaseMerge(unsafe.Pointer(&ioSync))
   250  	}
   251  	return sendfile(outfd, infd, offset, count)
   252  }
   253  
   254  /*
   255   * Exposed directly
   256   */
   257  //sys	Access(path string, mode uint32) (err error)
   258  //sys	Adjtime(delta *Timeval, olddelta *Timeval) (err error)
   259  //sys	Chdir(path string) (err error)
   260  //sys	Chflags(path string, flags int) (err error)
   261  //sys	Chmod(path string, mode uint32) (err error)
   262  //sys	Chown(path string, uid int, gid int) (err error)
   263  //sys	Chroot(path string) (err error)
   264  //sys	Close(fd int) (err error)
   265  //sys	Dup(fd int) (nfd int, err error)
   266  //sys	Dup2(from int, to int) (err error)
   267  //sys	Exit(code int)
   268  //sys	ExtattrGetFd(fd int, attrnamespace int, attrname string, data uintptr, nbytes int) (ret int, err error)
   269  //sys	ExtattrSetFd(fd int, attrnamespace int, attrname string, data uintptr, nbytes int) (ret int, err error)
   270  //sys	ExtattrDeleteFd(fd int, attrnamespace int, attrname string) (err error)
   271  //sys	ExtattrListFd(fd int, attrnamespace int, data uintptr, nbytes int) (ret int, err error)
   272  //sys	ExtattrGetFile(file string, attrnamespace int, attrname string, data uintptr, nbytes int) (ret int, err error)
   273  //sys	ExtattrSetFile(file string, attrnamespace int, attrname string, data uintptr, nbytes int) (ret int, err error)
   274  //sys	ExtattrDeleteFile(file string, attrnamespace int, attrname string) (err error)
   275  //sys	ExtattrListFile(file string, attrnamespace int, data uintptr, nbytes int) (ret int, err error)
   276  //sys	ExtattrGetLink(link string, attrnamespace int, attrname string, data uintptr, nbytes int) (ret int, err error)
   277  //sys	ExtattrSetLink(link string, attrnamespace int, attrname string, data uintptr, nbytes int) (ret int, err error)
   278  //sys	ExtattrDeleteLink(link string, attrnamespace int, attrname string) (err error)
   279  //sys	ExtattrListLink(link string, attrnamespace int, data uintptr, nbytes int) (ret int, err error)
   280  //sys	Faccessat(dirfd int, path string, mode uint32, flags int) (err error)
   281  //sys	Fadvise(fd int, offset int64, length int64, advice int) (err error) = SYS_POSIX_FADVISE
   282  //sys	Fchdir(fd int) (err error)
   283  //sys	Fchflags(fd int, flags int) (err error)
   284  //sys	Fchmod(fd int, mode uint32) (err error)
   285  //sys	Fchmodat(dirfd int, path string, mode uint32, flags int) (err error)
   286  //sys	Fchown(fd int, uid int, gid int) (err error)
   287  //sys	Fchownat(dirfd int, path string, uid int, gid int, flags int) (err error)
   288  //sys	Flock(fd int, how int) (err error)
   289  //sys	Fpathconf(fd int, name int) (val int, err error)
   290  //sys	Fstat(fd int, stat *Stat_t) (err error)
   291  //sys	Fstatat(fd int, path string, stat *Stat_t, flags int) (err error)
   292  //sys	Fsync(fd int) (err error)
   293  //sys	Ftruncate(fd int, length int64) (err error)
   294  //sysnb	Getegid() (egid int)
   295  //sysnb	Geteuid() (uid int)
   296  //sysnb	Getgid() (gid int)
   297  //sysnb	Getpgid(pid int) (pgid int, err error)
   298  //sysnb	Getpgrp() (pgrp int)
   299  //sysnb	Getpid() (pid int)
   300  //sysnb	Getppid() (ppid int)
   301  //sys	Getpriority(which int, who int) (prio int, err error)
   302  //sysnb	Getrlimit(which int, lim *Rlimit) (err error)
   303  //sysnb	Getrusage(who int, rusage *Rusage) (err error)
   304  //sysnb	Getsid(pid int) (sid int, err error)
   305  //sysnb	Gettimeofday(tv *Timeval) (err error)
   306  //sysnb	Getuid() (uid int)
   307  //sys	Issetugid() (tainted bool)
   308  //sys	Kill(pid int, signum syscall.Signal) (err error)
   309  //sys	Kqueue() (fd int, err error)
   310  //sys	Lchown(path string, uid int, gid int) (err error)
   311  //sys	Link(path string, link string) (err error)
   312  //sys	Linkat(pathfd int, path string, linkfd int, link string, flags int) (err error)
   313  //sys	Listen(s int, backlog int) (err error)
   314  //sys	Lstat(path string, stat *Stat_t) (err error)
   315  //sys	Mkdir(path string, mode uint32) (err error)
   316  //sys	Mkdirat(dirfd int, path string, mode uint32) (err error)
   317  //sys	Mkfifo(path string, mode uint32) (err error)
   318  //sys	Mkfifoat(dirfd int, path string, mode uint32) (err error)
   319  //sys	Mknod(path string, mode uint32, dev int) (err error)
   320  //sys	Mknodat(dirfd int, path string, mode uint32, dev int) (err error)
   321  //sys	Nanosleep(time *Timespec, leftover *Timespec) (err error)
   322  //sys	Open(path string, mode int, perm uint32) (fd int, err error)
   323  //sys	Openat(dirfd int, path string, mode int, perm uint32) (fd int, err error)
   324  //sys	Pathconf(path string, name int) (val int, err error)
   325  //sys	Pread(fd int, p []byte, offset int64) (n int, err error)
   326  //sys	Pwrite(fd int, p []byte, offset int64) (n int, err error)
   327  //sys	read(fd int, p []byte) (n int, err error)
   328  //sys	Readlink(path string, buf []byte) (n int, err error)
   329  //sys	Readlinkat(dirfd int, path string, buf []byte) (n int, err error)
   330  //sys	Rename(from string, to string) (err error)
   331  //sys	Renameat(fromfd int, from string, tofd int, to string) (err error)
   332  //sys	Revoke(path string) (err error)
   333  //sys	Rmdir(path string) (err error)
   334  //sys	Seek(fd int, offset int64, whence int) (newoffset int64, err error) = SYS_LSEEK
   335  //sys	Select(n int, r *FdSet, w *FdSet, e *FdSet, timeout *Timeval) (err error)
   336  //sysnb	Setegid(egid int) (err error)
   337  //sysnb	Seteuid(euid int) (err error)
   338  //sysnb	Setgid(gid int) (err error)
   339  //sysnb	Setpgid(pid int, pgid int) (err error)
   340  //sys	Setpriority(which int, who int, prio int) (err error)
   341  //sysnb	Setregid(rgid int, egid int) (err error)
   342  //sysnb	Setreuid(ruid int, euid int) (err error)
   343  //sysnb	Setrlimit(which int, lim *Rlimit) (err error)
   344  //sysnb	Setsid() (pid int, err error)
   345  //sysnb	Settimeofday(tp *Timeval) (err error)
   346  //sysnb	Setuid(uid int) (err error)
   347  //sys	Stat(path string, stat *Stat_t) (err error)
   348  //sys	Symlink(path string, link string) (err error)
   349  //sys	Symlinkat(oldpath string, newdirfd int, newpath string) (err error)
   350  //sys	Sync() (err error)
   351  //sys	Truncate(path string, length int64) (err error)
   352  //sys	Umask(newmask int) (oldmask int)
   353  //sys	Unlink(path string) (err error)
   354  //sys	Unlinkat(dirfd int, path string, flags int) (err error)
   355  //sys	Unmount(path string, flags int) (err error)
   356  //sys	write(fd int, p []byte) (n int, err error)
   357  //sys	mmap(addr uintptr, length uintptr, prot int, flag int, fd int, pos int64) (ret uintptr, err error)
   358  //sys	munmap(addr uintptr, length uintptr) (err error)
   359  //sys	readlen(fd int, buf *byte, nbuf int) (n int, err error) = SYS_READ
   360  //sys	writelen(fd int, buf *byte, nbuf int) (n int, err error) = SYS_WRITE
   361  //sys	utimensat(dirfd int, path string, times *[2]Timespec, flags int) (err error)
   362  
   363  /*
   364   * Unimplemented
   365   */
   366  // ____semctl13
   367  // __clone
   368  // __fhopen40
   369  // __fhstat40
   370  // __fhstatvfs140
   371  // __fstat30
   372  // __getcwd
   373  // __getfh30
   374  // __getlogin
   375  // __lstat30
   376  // __mount50
   377  // __msgctl13
   378  // __msync13
   379  // __ntp_gettime30
   380  // __posix_chown
   381  // __posix_fchown
   382  // __posix_lchown
   383  // __posix_rename
   384  // __setlogin
   385  // __shmctl13
   386  // __sigaction_sigtramp
   387  // __sigaltstack14
   388  // __sigpending14
   389  // __sigprocmask14
   390  // __sigsuspend14
   391  // __sigtimedwait
   392  // __stat30
   393  // __syscall
   394  // __vfork14
   395  // _ksem_close
   396  // _ksem_destroy
   397  // _ksem_getvalue
   398  // _ksem_init
   399  // _ksem_open
   400  // _ksem_post
   401  // _ksem_trywait
   402  // _ksem_unlink
   403  // _ksem_wait
   404  // _lwp_continue
   405  // _lwp_create
   406  // _lwp_ctl
   407  // _lwp_detach
   408  // _lwp_exit
   409  // _lwp_getname
   410  // _lwp_getprivate
   411  // _lwp_kill
   412  // _lwp_park
   413  // _lwp_self
   414  // _lwp_setname
   415  // _lwp_setprivate
   416  // _lwp_suspend
   417  // _lwp_unpark
   418  // _lwp_unpark_all
   419  // _lwp_wait
   420  // _lwp_wakeup
   421  // _pset_bind
   422  // _sched_getaffinity
   423  // _sched_getparam
   424  // _sched_setaffinity
   425  // _sched_setparam
   426  // acct
   427  // aio_cancel
   428  // aio_error
   429  // aio_fsync
   430  // aio_read
   431  // aio_return
   432  // aio_suspend
   433  // aio_write
   434  // break
   435  // clock_getres
   436  // clock_gettime
   437  // clock_settime
   438  // compat_09_ogetdomainname
   439  // compat_09_osetdomainname
   440  // compat_09_ouname
   441  // compat_10_omsgsys
   442  // compat_10_osemsys
   443  // compat_10_oshmsys
   444  // compat_12_fstat12
   445  // compat_12_getdirentries
   446  // compat_12_lstat12
   447  // compat_12_msync
   448  // compat_12_oreboot
   449  // compat_12_oswapon
   450  // compat_12_stat12
   451  // compat_13_sigaction13
   452  // compat_13_sigaltstack13
   453  // compat_13_sigpending13
   454  // compat_13_sigprocmask13
   455  // compat_13_sigreturn13
   456  // compat_13_sigsuspend13
   457  // compat_14___semctl
   458  // compat_14_msgctl
   459  // compat_14_shmctl
   460  // compat_16___sigaction14
   461  // compat_16___sigreturn14
   462  // compat_20_fhstatfs
   463  // compat_20_fstatfs
   464  // compat_20_getfsstat
   465  // compat_20_statfs
   466  // compat_30___fhstat30
   467  // compat_30___fstat13
   468  // compat_30___lstat13
   469  // compat_30___stat13
   470  // compat_30_fhopen
   471  // compat_30_fhstat
   472  // compat_30_fhstatvfs1
   473  // compat_30_getdents
   474  // compat_30_getfh
   475  // compat_30_ntp_gettime
   476  // compat_30_socket
   477  // compat_40_mount
   478  // compat_43_fstat43
   479  // compat_43_lstat43
   480  // compat_43_oaccept
   481  // compat_43_ocreat
   482  // compat_43_oftruncate
   483  // compat_43_ogetdirentries
   484  // compat_43_ogetdtablesize
   485  // compat_43_ogethostid
   486  // compat_43_ogethostname
   487  // compat_43_ogetkerninfo
   488  // compat_43_ogetpagesize
   489  // compat_43_ogetpeername
   490  // compat_43_ogetrlimit
   491  // compat_43_ogetsockname
   492  // compat_43_okillpg
   493  // compat_43_olseek
   494  // compat_43_ommap
   495  // compat_43_oquota
   496  // compat_43_orecv
   497  // compat_43_orecvfrom
   498  // compat_43_orecvmsg
   499  // compat_43_osend
   500  // compat_43_osendmsg
   501  // compat_43_osethostid
   502  // compat_43_osethostname
   503  // compat_43_osetrlimit
   504  // compat_43_osigblock
   505  // compat_43_osigsetmask
   506  // compat_43_osigstack
   507  // compat_43_osigvec
   508  // compat_43_otruncate
   509  // compat_43_owait
   510  // compat_43_stat43
   511  // execve
   512  // extattr_delete_fd
   513  // extattr_delete_file
   514  // extattr_delete_link
   515  // extattr_get_fd
   516  // extattr_get_file
   517  // extattr_get_link
   518  // extattr_list_fd
   519  // extattr_list_file
   520  // extattr_list_link
   521  // extattr_set_fd
   522  // extattr_set_file
   523  // extattr_set_link
   524  // extattrctl
   525  // fchroot
   526  // fdatasync
   527  // fgetxattr
   528  // fktrace
   529  // flistxattr
   530  // fork
   531  // fremovexattr
   532  // fsetxattr
   533  // fstatvfs1
   534  // fsync_range
   535  // getcontext
   536  // getitimer
   537  // getvfsstat
   538  // getxattr
   539  // ktrace
   540  // lchflags
   541  // lchmod
   542  // lfs_bmapv
   543  // lfs_markv
   544  // lfs_segclean
   545  // lfs_segwait
   546  // lgetxattr
   547  // lio_listio
   548  // listxattr
   549  // llistxattr
   550  // lremovexattr
   551  // lseek
   552  // lsetxattr
   553  // lutimes
   554  // madvise
   555  // mincore
   556  // minherit
   557  // modctl
   558  // mq_close
   559  // mq_getattr
   560  // mq_notify
   561  // mq_open
   562  // mq_receive
   563  // mq_send
   564  // mq_setattr
   565  // mq_timedreceive
   566  // mq_timedsend
   567  // mq_unlink
   568  // mremap
   569  // msgget
   570  // msgrcv
   571  // msgsnd
   572  // nfssvc
   573  // ntp_adjtime
   574  // pmc_control
   575  // pmc_get_info
   576  // pollts
   577  // preadv
   578  // profil
   579  // pselect
   580  // pset_assign
   581  // pset_create
   582  // pset_destroy
   583  // ptrace
   584  // pwritev
   585  // quotactl
   586  // rasctl
   587  // readv
   588  // reboot
   589  // removexattr
   590  // sa_enable
   591  // sa_preempt
   592  // sa_register
   593  // sa_setconcurrency
   594  // sa_stacks
   595  // sa_yield
   596  // sbrk
   597  // sched_yield
   598  // semconfig
   599  // semget
   600  // semop
   601  // setcontext
   602  // setitimer
   603  // setxattr
   604  // shmat
   605  // shmdt
   606  // shmget
   607  // sstk
   608  // statvfs1
   609  // swapctl
   610  // sysarch
   611  // syscall
   612  // timer_create
   613  // timer_delete
   614  // timer_getoverrun
   615  // timer_gettime
   616  // timer_settime
   617  // undelete
   618  // utrace
   619  // uuidgen
   620  // vadvise
   621  // vfork
   622  // writev