串口之ReadFile、WriteFile函数详解

BOOL ReadFile(

　　HANDLE hFile, //文件的句柄

　　LPVOID lpBuffer, //用于保存读入数据的一个缓冲区

　　DWORD nNumberOfBytesToRead, //要读入的字符数

　　LPDWORD lpNumberOfBytesRead, //指向实际读取字节数的指针

　　LPOVERLAPPED lpOverlapped //如文件打开时指定了FILE_FLAG_OVERLAPPED，那么必须，用这个参数引用一个特殊的结构。该结构定义了一次异步读取操作。否则，应将这个参数设为NULL

　　);

功能说明

　　从文件指针指向的位置开始将数据读出到一个文件中, 且支持同步和异步操作,

　　如果文件打开方式没有指明FILE_FLAG_OVERLAPPED的话,当程序调用成功时,它将实际读出文件的字节数保存到lpNumberOfBytesRead指明的地址空间中。

　　如果文件要交互使用的话,当函数调用完毕时要记得调整文件指针。

　　从文件中读出数据。与lread函数相比，这个函数要明显灵活的多。该函数能够操作通信设备、管道、套接字以及邮槽。

参数说明

　　HANDLE hFile, 需要写入数据的文件指针,这个指针指向的文件必须是GENERIC_READaccess 访问属性的文件。

　　LPOVERLAPPED lpOverlapped OVERLAPPED结构体指针,如果文件是以FILE_FLAG_OVERLAPPED方式打开的话,那么这个指针就不能为NULL。

返回值

　　调用成功,返回非0

　　调用不成功,返回为0

　　会设置GetLastError。如启动的是一次异步读操作，则函数会返回零值，并将ERROR_IO_PENDING设置成GetLastError的结果。如结果不是零值，但读入的字节数小于nNumberOfBytesToRead参数指定的值，表明早已抵达了文件的结尾。

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WriteFile

The WriteFile function writes data to a file and is designed for both synchronous and asynchronous operation. The function starts writing data to the file at the position indicated by the file pointer. After the write operation has been completed, the file pointer is adjusted by the number of bytes actually written, except when the file is opened with FILE_FLAG_OVERLAPPED. If the file handle was created for overlapped input and output (I/O), the application must adjust the position of the file pointer after the write operation is finished.

This function is designed for both synchronous and asynchronous operation. The WriteFileEx function is designed solely for asynchronous operation. It lets an application perform other processing during a file write operation.

BOOL WriteFile(
  HANDLE hFile,                    // handle to file
  LPCVOID lpBuffer,                // data buffer
  DWORD nNumberOfBytesToWrite,     // number of bytes to write
  LPDWORD lpNumberOfBytesWritten,  // number of bytes written
  LPOVERLAPPED lpOverlapped        // overlapped buffer
);

Parameters

hFile

[in] Handle to the file to be written to. The file handle must have been created with GENERIC_WRITE access to the file.

Windows NT/2000/XP: For asynchronous write operations, hFile can be any handle opened with the FILE_FLAG_OVERLAPPED flag by the CreateFile function, or a socket handle returned by the socket or accept function.

Windows 95/98/Me: For asynchronous write operations, hFile can be a communications resource opened with the FILE_FLAG_OVERLAPPED flag by CreateFile, or a socket handle returned by socket or accept. You cannot perform asynchronous write operations on mailslots, named pipes, or disk files.

lpBuffer

[in] Pointer to the buffer containing the data to be written to the file.

nNumberOfBytesToWrite

[in] Specifies the number of bytes to write to the file.

A value of zero specifies a null write operation. The behavior of a null write operation depends on the underlying file system. To truncate or extend a file, use the SetEndOfFile function.

Named pipe write operations across a network are limited to 65,535 bytes.

lpNumberOfBytesWritten

[out] Pointer to the variable that receives the number of bytes written. WriteFile sets this value to zero before doing any work or error checking.

Windows NT/2000/XP: If lpOverlapped is NULL, lpNumberOfBytesWritten cannot be NULL. If lpOverlapped is not NULL, lpNumberOfBytesWritten can be NULL. If this is an overlapped write operation, you can get the number of bytes written by calling GetOverlappedResult. If hFile is associated with an I/O completion port, you can get the number of bytes written by calling GetQueuedCompletionStatus.

If I/O completion ports are used and you are using a callback routine to free the memory allocated to the OVERLAPPED structure pointed to by the lpOverlapped parameter, specify NULL as the value of this parameter to avoid a memory corruption problem during the deallocation. This memory corruption problem will cause an invalid number of bytes to be returned in this parameter.

Windows 95/98/Me: This parameter cannot be NULL.

lpOverlapped

[in] Pointer to an OVERLAPPED structure. This structure is required if hFile was opened with FILE_FLAG_OVERLAPPED.