go mapfile_mmap 源码
golang mapfile_mmap 代码
文件路径：/src/cmd/compile/internal/base/mapfile_mmap.go
// Copyright 2018 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 darwin || dragonfly || freebsd || linux || netbsd || openbsd
// +build darwin dragonfly freebsd linux netbsd openbsd
package base
import (
"os"
"reflect"
"syscall"
"unsafe"
)
// TODO(mdempsky): Is there a higher-level abstraction that still
// works well for iimport?
// mapFile returns length bytes from the file starting at the
// specified offset as a string.
func MapFile(f *os.File, offset, length int64) (string, error) {
// POSIX mmap: "The implementation may require that off is a
// multiple of the page size."
x := offset & int64(os.Getpagesize()-1)
offset -= x
length += x
buf, err := syscall.Mmap(int(f.Fd()), offset, int(length), syscall.PROT_READ, syscall.MAP_SHARED)
keepAlive(f)
if err != nil {
return "", err
}
buf = buf[x:]
pSlice := (*reflect.SliceHeader)(unsafe.Pointer(&buf))
var res string
pString := (*reflect.StringHeader)(unsafe.Pointer(&res))
pString.Data = pSlice.Data
pString.Len = pSlice.Len
return res, nil
}
// keepAlive is a reimplementation of runtime.KeepAlive, which wasn't
// added until Go 1.7, whereas we need to compile with Go 1.4.
var keepAlive = func(interface{}) {}
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