windows 核心编程中关于MapViewOfFile不太明白的地方，求指教。

[mister_exia]

HANDLE hFile = CreateFile(...);
HANDLE hFileMapping = CreateFileMapping(hFile, ...);
PVOID pvFile = MapViewOfFile(hFileMapping, ...);
// Use the memory-mapped file.
UnmapViewOfFile(pvFile);
CloseHandle(hFileMapping);
CloseHandle(hFile);

The preceding code shows the "expected" method for manipulating memory-mapped files. However, what it does not show is that the system increments the usage counts of the file object and the file-mapping object when you call MapViewOfFile. This side effect is significant because it means that we could rewrite the preceding code fragment as follows:

HANDLE hFile = CreateFile(...);
HANDLE hFileMapping = CreateFileMapping(hFile, ...);
CloseHandle(hFile);
PVOID pvFile = MapViewOfFile(hFileMapping, ...);
CloseHandle(hFileMapping);
// Use the memory-mapped file.
UnmapViewOfFile(pvFile);

第一段代码应该是标准写法了，第二段代码有什么问题吗？标红的那句实在看不懂是啥意思。

[Eleven]

上面那段话简单的说就是说调用MapViewOfFile会增加文件对象和文件映射对象的引用计数。

[Lactoferrin]

事情没这么简单，不要管这句话，如果你不是非常清楚，总是使用像第一种的代码

[fly4free]

copy on write 和 5楼的讲述有没有什么关联？
（都看不懂啊……）

[Lactoferrin]

和copyonwrite无关

[mister_exia]

[Quote=引用 5 楼 的回复:]

先看这个代码
C/C++ code

HANDLE hFile = CreateFileW(L"D:\\Downloads\\fg.ini",GENERIC_READ,FILE_SHARE_READ,0,OPEN_EXISTING,0,0);
HANDLE hFileMapping = CreateFileMappingW(hFile,0,PAGE_READONLY,0,0,0);
……
[/Quote]

pointerCount 和 HandleCount 哪一个才是引用计数呢？不是说MapViewOfFile会增加引用计数吗？那么为什么这两个值都为1呢？

[Lactoferrin]

引用不一定是句柄，所以pointerCount>=HandleCount
只有pointercount=0才可能释放掉对象

[Lactoferrin]

system increments the usage counts of the file object and the file-mapping object when you call MapViewOfFile
这句话可以忘掉

[向立天]

如果不关心内核对象的统计信息就直接关掉其句柄
不只是这个位置
《windows核心编程》很多地方都有提醒读者CloseHandle并没有关闭对象
只是影响引用计数

[fly4free]

个人猜测

由于调用 UnmapViewOfFile 会导致系统 increments the usage counts of the file object and the file-mapping object

所以，这段代码涉及到的三个句柄，先关闭哪一个都可以。
(嗯……，至少第二段代码所示的调用顺序是没问题的)

[mister_exia]

[Quote=引用 10 楼 的回复:]

system increments the usage counts of the file object and the file-mapping object when you call MapViewOfFile
这句话可以忘掉
[/Quote]

为什么呢？能讲一下原因吗？

[Lactoferrin]

这一下说不完，具体的看wrk或windows internals

[Lactoferrin]

先看这个代码

HANDLE hFile = CreateFileW(L"D:\\Downloads\\fg.ini",GENERIC_READ,FILE_SHARE_READ,0,OPEN_EXISTING,0,0);
    HANDLE hFileMapping = CreateFileMappingW(hFile,0,PAGE_READONLY,0,0,0);
    PVOID pvFile = MapViewOfFile(hFileMapping,FILE_MAP_READ,0,0,0);

printf("%x,%x,%x,%p\n",GetCurrentProcessId(),hFile,hFileMapping,pvFile);
getchar();
CloseHandle(hFileMapping);
CloseHandle(hFile);
puts("closed");
getchar();
getchar();
UnmapViewOfFile(pvFile);

执行closehandle之前的对象状况
lkd> .process 8997a5d0
Implicit process is now 8997a5d0
lkd> !handle 30

PROCESS 8997a5d0 SessionId: 0 Cid: 05a4 Peb: 7ffdd000 ParentCid: 055c
DirBase: 0a3c0280 ObjectTable: e187cd40 HandleCount: 13.
Image: cbtest.exe

Handle table at e1f85000 with 13 entries in use

0030: Object: 89937540 GrantedAccess: 00120089 Entry: e1f85060
Object: 89937540 Type: (89a16040) File
ObjectHeader: 89937528 (old version)
HandleCount: 1 PointerCount: 1
Directory Object: 00000000 Name: \Downloads\fg.ini {HarddiskVolume2}


lkd> !object 89937540
Object: 89937540 Type: (89a16040) File
ObjectHeader: 89937528 (old version)
HandleCount: 1 PointerCount: 1
Directory Object: 00000000 Name: \Downloads\fg.ini {HarddiskVolume2}
lkd> !handle 34

PROCESS 8997a5d0 SessionId: 0 Cid: 05a4 Peb: 7ffdd000 ParentCid: 055c
DirBase: 0a3c0280 ObjectTable: e187cd40 HandleCount: 13.
Image: cbtest.exe

Handle table at e1f85000 with 13 entries in use

0034: Object: e196e240 GrantedAccess: 000f0005 Entry: e1f85068
Object: e196e240 Type: (89a25040) Section
ObjectHeader: e196e228 (old version)
HandleCount: 1 PointerCount: 1


lkd> !object e196e240
Object: e196e240 Type: (89a25040) Section
ObjectHeader: e196e228 (old version)
HandleCount: 1 PointerCount: 1
执行close之后的对象状况
lkd> !object 89937540
Object: 89937540 Type: (bad0b0b0)
ObjectHeader: 89937528 (old version)
HandleCount: 0 PointerCount: 0
lkd> !object e196e240
Object: e196e240 Type: (bad0b0b0)
ObjectHeader: e196e228 (old version)
HandleCount: 0 PointerCount: 0
说明view并不需要文件对象和section对象的存在
由于其他程序的影响，文件对象和节对象可能存在，但不能保证
它们的关系见windows internals 5

如果仅仅是同一个进程用这些，当然没问题，但如果涉及到进程间通信，则必须予以考虑
下面的代码的OpenFileMapping不会成功，第二个CreateFileMapping得到的和第一个内容独立，如果这样的代码用于IPC则会与预料的不同

HANDLE sec=CreateFileMappingW((HANDLE)-1,0,PAGE_READWRITE,0,64,L"sec");
    printf("%x\n",sec);
    PVOID view = MapViewOfFile(sec,FILE_MAP_READ,0,0,0);
    printf("%p\n",view);
    CloseHandle(sec);
    HANDLE sec2=OpenFileMappingW(FILE_MAP_WRITE,0,L"sec");
    HANDLE sec3=CreateFileMappingW((HANDLE)-1,0,PAGE_READWRITE,0,64,L"sec");
    printf("%x",sec2);

[fly4free]

那岂不是说，微软很坑爹吗？

[Lactoferrin]

这只能说是写书的人太靠直观