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// Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
//go:build linux || darwin
// Package mmap provides a way to memory-map a file.
package mmap
import (
"errors"
"fmt"
"io"
"os"
"runtime"
"syscall"
)
// debug is whether to print debugging messages for manual testing.
//
// The runtime.SetFinalizer documentation says that, "The finalizer for x is
// scheduled to run at some arbitrary time after x becomes unreachable. There
// is no guarantee that finalizers will run before a program exits", so we
// cannot automatically test that the finalizer runs. Instead, set this to true
// when running the manual test.
const debug = false
// ReaderAt reads a memory-mapped file.
//
// Like any io.ReaderAt, clients can execute parallel ReadAt calls, but it is
// not safe to call Close and reading methods concurrently.
type ReaderAt struct {
data []byte
}
// Close closes the reader.
func (r *ReaderAt) Close() error {
if r.data == nil {
return nil
} else if len(r.data) == 0 {
r.data = nil
return nil
}
data := r.data
r.data = nil
if debug {
var p *byte
if len(data) != 0 {
p = &data[0]
}
println("munmap", r, p)
}
runtime.SetFinalizer(r, nil)
return syscall.Munmap(data)
}
// Len returns the length of the underlying memory-mapped file.
func (r *ReaderAt) Len() int {
return len(r.data)
}
// At returns the byte at index i.
func (r *ReaderAt) At(i int) byte {
return r.data[i]
}
// ReadAt implements the io.ReaderAt interface.
func (r *ReaderAt) ReadAt(p []byte, off int64) (int, error) {
if r.data == nil {
return 0, errors.New("mmap: closed")
}
if off < 0 || int64(len(r.data)) < off {
return 0, fmt.Errorf("mmap: invalid ReadAt offset %d", off)
}
n := copy(p, r.data[off:])
if n < len(p) {
return n, io.EOF
}
return n, nil
}
// Open memory-maps the named file for reading.
func Open(filename string) (*ReaderAt, error) {
f, err := os.Open(filename)
if err != nil {
return nil, err
}
defer f.Close()
fi, err := f.Stat()
if err != nil {
return nil, err
}
size := fi.Size()
if size == 0 {
// Treat (size == 0) as a special case, avoiding the syscall, since
// "man 2 mmap" says "the length... must be greater than 0".
//
// As we do not call syscall.Mmap, there is no need to call
// runtime.SetFinalizer to enforce a balancing syscall.Munmap.
return &ReaderAt{
data: make([]byte, 0),
}, nil
}
if size < 0 {
return nil, fmt.Errorf("mmap: file %q has negative size", filename)
}
if size != int64(int(size)) {
return nil, fmt.Errorf("mmap: file %q is too large", filename)
}
data, err := syscall.Mmap(int(f.Fd()), 0, int(size), syscall.PROT_READ, syscall.MAP_SHARED)
if err != nil {
return nil, err
}
r := &ReaderAt{data}
if debug {
var p *byte
if len(data) != 0 {
p = &data[0]
}
println("mmap", r, p)
}
runtime.SetFinalizer(r, (*ReaderAt).Close)
return r, nil
}
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