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// Copyright 2011 Evan Shaw. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package mmap
import (
"errors"
"os"
"sync"
"golang.org/x/sys/windows"
)
// mmap on Windows is a two-step process.
// First, we call CreateFileMapping to get a handle.
// Then, we call MapviewToFile to get an actual pointer into memory.
// Because we want to emulate a POSIX-style mmap, we don't want to expose
// the handle -- only the pointer. We also want to return only a byte slice,
// not a struct, so it's convenient to manipulate.
// We keep this map so that we can get back the original handle from the memory address.
type addrinfo struct {
file windows.Handle
mapview windows.Handle
writable bool
}
var handleLock sync.Mutex
var handleMap = map[uintptr]*addrinfo{}
func mmap(len int, prot, flags, hfile uintptr, off int64) ([]byte, error) {
flProtect := uint32(windows.PAGE_READONLY)
dwDesiredAccess := uint32(windows.FILE_MAP_READ)
writable := false
switch {
case prot© != 0:
flProtect = windows.PAGE_WRITECOPY
dwDesiredAccess = windows.FILE_MAP_COPY
writable = true
case prot&RDWR != 0:
flProtect = windows.PAGE_READWRITE
dwDesiredAccess = windows.FILE_MAP_WRITE
writable = true
}
if prot&EXEC != 0 {
flProtect <<= 4
dwDesiredAccess |= windows.FILE_MAP_EXECUTE
}
// The maximum size is the area of the file, starting from 0,
// that we wish to allow to be mappable. It is the sum of
// the length the user requested, plus the offset where that length
// is starting from. This does not map the data into memory.
maxSizeHigh := uint32((off + int64(len)) >> 32)
maxSizeLow := uint32((off + int64(len)) & 0xFFFFFFFF)
// TODO: Do we need to set some security attributes? It might help portability.
h, errno := windows.CreateFileMapping(windows.Handle(hfile), nil, flProtect, maxSizeHigh, maxSizeLow, nil)
if h == 0 {
return nil, os.NewSyscallError("CreateFileMapping", errno)
}
// Actually map a view of the data into memory. The view's size
// is the length the user requested.
fileOffsetHigh := uint32(off >> 32)
fileOffsetLow := uint32(off & 0xFFFFFFFF)
addr, errno := windows.MapViewOfFile(h, dwDesiredAccess, fileOffsetHigh, fileOffsetLow, uintptr(len))
if addr == 0 {
windows.CloseHandle(windows.Handle(h))
return nil, os.NewSyscallError("MapViewOfFile", errno)
}
handleLock.Lock()
handleMap[addr] = &addrinfo{
file: windows.Handle(hfile),
mapview: h,
writable: writable,
}
handleLock.Unlock()
m := MMap{}
dh := m.header()
dh.Data = addr
dh.Len = len
dh.Cap = dh.Len
return m, nil
}
func (m MMap) flush() error {
addr, len := m.addrLen()
errno := windows.FlushViewOfFile(addr, len)
if errno != nil {
return os.NewSyscallError("FlushViewOfFile", errno)
}
handleLock.Lock()
defer handleLock.Unlock()
handle, ok := handleMap[addr]
if !ok {
// should be impossible; we would've errored above
return errors.New("unknown base address")
}
if handle.writable && handle.file != windows.Handle(^uintptr(0)) {
if err := windows.FlushFileBuffers(handle.file); err != nil {
return os.NewSyscallError("FlushFileBuffers", err)
}
}
return nil
}
func (m MMap) lock() error {
addr, len := m.addrLen()
errno := windows.VirtualLock(addr, len)
return os.NewSyscallError("VirtualLock", errno)
}
func (m MMap) unlock() error {
addr, len := m.addrLen()
errno := windows.VirtualUnlock(addr, len)
return os.NewSyscallError("VirtualUnlock", errno)
}
func (m MMap) unmap() error {
err := m.flush()
if err != nil {
return err
}
addr := m.header().Data
// Lock the UnmapViewOfFile along with the handleMap deletion.
// As soon as we unmap the view, the OS is free to give the
// same addr to another new map. We don't want another goroutine
// to insert and remove the same addr into handleMap while
// we're trying to remove our old addr/handle pair.
handleLock.Lock()
defer handleLock.Unlock()
err = windows.UnmapViewOfFile(addr)
if err != nil {
return err
}
handle, ok := handleMap[addr]
if !ok {
// should be impossible; we would've errored above
return errors.New("unknown base address")
}
delete(handleMap, addr)
e := windows.CloseHandle(windows.Handle(handle.mapview))
return os.NewSyscallError("CloseHandle", e)
}
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