Chapter10-“I/O设备的同步和异步”之打开和关闭设备
打开设备：CreateFile函数
函数原型：
HANDLE WINAPI CreateFile(
__in LPCTSTR lpFileName,
__in DWORD dwDesiredAccess,
__in DWORD dwShareMode,
__in_opt LPSECURITY_ATTRIBUTES lpSecurityAttributes,
__in DWORD dwCreationDisposition,
__in DWORD dwFlagsAndAttributes,
__in_opt HANDLE hTemplateFile);
首先不要被它的名字所迷惑，这个函数不仅可以打开文件(Files)，它还可以打开不少的I/O设备（如串口等）。下面再介绍参数用法：
1) lpFileName参数：指定需要打开或创建的文件名或设备名。
2) dwDesiredAccess参数：指定请求的权限，综合来说就是read/write/both/neither.详情参考：GenericAccess Rights,FileSecurity and Access Rights, FileAccess Rights Constants, and ACCESS_MASK.
3) dwShareMode参数：指定打开设备的共享模式（read, write, both, delete, all of these, or none），下面的表格列出了其值域：
| Value |
Meaning |
| 0 0x00000000 |
Prevents other processes from opening a file or device if they request delete, read, or write access. |
| FILE_SHARE_DELETE 0x00000004 |
Enables subsequent open operations on a file or device to request delete access. Note Delete access allows both delete and rename operations. |
| FILE_SHARE_READ 0x00000001 |
Enables subsequent open operations on a file or device to request read access. |
| FILE_SHARE_WRITE 0x00000002 |
Enables subsequent open operations on a file or device to request write access. |
4) lpSecurityAttributes参数：指定一些相关的安全属性。
5) dwCreationDisposition参数：指定一些创建时处理操作，除了打开文件(files)外，对于其他设备一般使用OPEN_EXISTING作为参数值。其值必须是一下某个值，且不能结合使用。
|
|
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| CREATE_ALWAYS 2 |
Creates a new file, always. If the specified file exists and is writable, the function overwrites the file, the function succeeds, and last-error code is set toERROR_ALREADY_EXISTS (183). If the specified file does not exist and is a valid path, a new file is created, the function succeeds, and the last-error code is set to zero.
|
| CREATE_NEW 1 |
Creates a new file, only if it does not already exist. If the specified file exists, the function fails and the last-error code is set toERROR_FILE_EXISTS (80). If the specified file does not exist and is a valid path to a writable location, a new file is created.
|
| OPEN_ALWAYS 4
|
Opens a file, always. If the specified file exists, the function succeeds and the last-error code is set toERROR_ALREADY_EXISTS (183). If the specified file does not exist and is a valid path to a writable location, the function creates a file and the last-error code is set to zero.
|
| OPEN_EXISTING 3
|
Opens a file or device, only if it exists. If the specified file or device does not exist, the function fails and the last-error code is set toERROR_FILE_NOT_FOUND (2).
|
| TRUNCATE_EXISTING 5
|
Opens a file and truncates it so that its size is zero bytes, only if it exists. If the specified file does not exist, the function fails and the last-error code is set toERROR_FILE_NOT_FOUND (2). The calling process must open the file with the GENERIC_WRITE bit set as part of the dwDesiredAccess parameter.
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6) dwFlagsAndAttributes参数：最常用的值就是FILE_ATTRIBUTE_NORMAL，其他可用值请参考上面函数的MSDN链接。
7) hTemplateFile参数：指定拥有GENERIC_READ访问权限的模板文件的可用句柄，这个模板文件提供了要创建的文件属性和扩展属性。该参数可以为NULL，如果是调用CreateFile去打开一个已经存在的文件，这个参数被忽略。
关闭设备——CloseHandle函数
BOOL WINAPI CloseHandle( __in HANDLEhObject );
这个函数很简单，将CreateFile函数返回的句柄作为参数传入，就可关闭相应的设备了。
一个在文件尾添加数据的简单示例：
int AppendData()
{
HANDLE h = CreateFile(TEXT("test.txt"), GENERIC_WRITE, 0, NULL, OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL);
if (INVALID_HANDLE_VALUE == h)
{
cout<<"打开文件失败！"<<endl;
return -1;
}
else
{
TCHAR str[]=TEXT("fuck you");
DWORD lenWritten;
SetFilePointer(h, 0, NULL, FILE_END);
WriteFile(h, str, sizeof(str), &lenWritten, NULL);
CloseHandle(h);
}
return 0;
}
注意的是：需要通过SetFilePointer函数来将写入数据的指针定位于文件尾部，然后在尾部添加数据。
WriteFile是不可以直接插入数据的，通过SetFilePoint函数定位指针再利用WriteFile写入的数据，会覆盖掉原本的数据而不是插入数据。