syscall.go

// Copyright 2019 shimingyah.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// ee the License for the specific language governing permissions and
// limitations under the License.

package ioengine

import (
	"os"
	"syscall"
)

// Single-word zero for use when we need a valid pointer to 0 bytes.
var zero uintptr

// simulate writeatv by calling writeat serially and dose not change the file offset.
func genericWriteAtv(fd File, bs [][]byte, off int64) (n int, err error) {
	nOffset := off
	nw := 0

	for _, b := range bs {
		nw, err = fd.WriteAt(b, nOffset)
		n += nw
		nOffset += int64(nw)
		if err != nil {
			break
		}
	}

	return n, err
}

// simulate writev by calling write serially and it will change the file offset.
func genericWritev(fd File, bs [][]byte) (n int, err error) {
	nw := 0

	for _, b := range bs {
		nw, err = fd.Write(b)
		n += nw
		if err != nil {
			break
		}
	}

	return n, err
}

func genericAppend(fd File, bs [][]byte) (int, error) {
	opt := fd.Option()

	// open file with O_APPEND not need to seek
	if (opt.Flag & os.O_APPEND) > 0 {
		return genericWritev(fd, bs)
	}

	// acquire file size to append write
	size, err := fd.Seek(0, os.SEEK_END)
	if err != nil {
		return 0, err
	}
	// Because use writeAt to simulate an append write
	// it doesn't change the file offset, to keep append semantic
	// so that make sure file offset is the file end.
	defer fd.Seek(0, os.SEEK_END)

	return fd.WriteAtv(bs, size)
}

func bytes2Iovec(bs [][]byte) []syscall.Iovec {
	var iovecs []syscall.Iovec
	for _, chunk := range bs {
		if len(chunk) == 0 {
			continue
		}
		iovecs = append(iovecs, syscall.Iovec{Base: &chunk[0]})
		iovecs[len(iovecs)-1].SetLen(len(chunk))
	}
	return iovecs
}