117 lines
2.6 KiB
Go
117 lines
2.6 KiB
Go
// Copyright 2015 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// +build linux darwin
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// Package mmap provides a way to memory-map a file.
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package mmap
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import (
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"errors"
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"fmt"
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"io"
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"os"
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"runtime"
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"syscall"
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)
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// debug is whether to print debugging messages for manual testing.
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//
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// The runtime.SetFinalizer documentation says that, "The finalizer for x is
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// scheduled to run at some arbitrary time after x becomes unreachable. There
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// is no guarantee that finalizers will run before a program exits", so we
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// cannot automatically test that the finalizer runs. Instead, set this to true
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// when running the manual test.
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const debug = false
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// ReaderAt reads a memory-mapped file.
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//
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// Like any io.ReaderAt, clients can execute parallel ReadAt calls, but it is
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// not safe to call Close and reading methods concurrently.
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type ReaderAt struct {
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data []byte
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}
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// Close closes the reader.
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func (r *ReaderAt) Close() error {
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if r.data == nil {
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return nil
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}
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data := r.data
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r.data = nil
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if debug {
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var p *byte
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if len(data) != 0 {
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p = &data[0]
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}
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println("munmap", r, p)
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}
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runtime.SetFinalizer(r, nil)
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return syscall.Munmap(data)
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}
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// Len returns the length of the underlying memory-mapped file.
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func (r *ReaderAt) Len() int {
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return len(r.data)
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}
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// At returns the byte at index i.
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func (r *ReaderAt) At(i int) byte {
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return r.data[i]
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}
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// ReadAt implements the io.ReaderAt interface.
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func (r *ReaderAt) ReadAt(p []byte, off int64) (int, error) {
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if r.data == nil {
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return 0, errors.New("mmap: closed")
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}
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if off < 0 || int64(len(r.data)) < off {
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return 0, fmt.Errorf("mmap: invalid ReadAt offset %d", off)
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}
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n := copy(p, r.data[off:])
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if n < len(p) {
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return n, io.EOF
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}
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return n, nil
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}
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// Open memory-maps the named file for reading.
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func Open(filename string) (*ReaderAt, error) {
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f, err := os.Open(filename)
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if err != nil {
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return nil, err
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}
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defer f.Close()
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fi, err := f.Stat()
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if err != nil {
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return nil, err
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}
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size := fi.Size()
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if size == 0 {
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return &ReaderAt{}, nil
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}
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if size < 0 {
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return nil, fmt.Errorf("mmap: file %q has negative size", filename)
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}
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if size != int64(int(size)) {
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return nil, fmt.Errorf("mmap: file %q is too large", filename)
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}
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data, err := syscall.Mmap(int(f.Fd()), 0, int(size), syscall.PROT_READ, syscall.MAP_SHARED)
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if err != nil {
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return nil, err
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}
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r := &ReaderAt{data}
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if debug {
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var p *byte
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if len(data) != 0 {
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p = &data[0]
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}
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println("mmap", r, p)
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}
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runtime.SetFinalizer(r, (*ReaderAt).Close)
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return r, nil
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}
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