Question

API高手进来下！关于ReadFile函数

对这个ReadFile函数不知道怎样理解了！想知道你们是怎样理解的？能说个例子吗

Answer 1

MSDN上说明 的够细了，你哪点儿不明白？可以百度HI详谈。

ReadFile Function

Reads data from a file, and starts at the position that the file pointer indicates. You can use this function for both synchronous and asynchronous operations.

The ReadFileEx function is for only asynchronous operation.

Syntax
BOOL WINAPI ReadFile(
__in HANDLE hFile,
__out LPVOID lpBuffer,
__in DWORD nNumberOfBytesToRead,
__out_opt LPDWORD lpNumberOfBytesRead,
__inout_opt LPOVERLAPPED lpOverlapped
);

Parameters
hFile
A handle to the file to be read.

The file handle must be created with the GENERIC_READ access right. For more information, see File Security and Access Rights.

For asynchronous read operations, hFile can be any handle that is opened with the FILE_FLAG_OVERLAPPED flag by the CreateFile function, or a socket handle returned by the socket or accept function.

lpBuffer
A pointer to the buffer that receives the data read from a file.

nNumberOfBytesToRead
The maximum number of bytes to be read.

lpNumberOfBytesRead
A pointer to the variable that receives the number of bytes read.

ReadFile sets this value to zero (0) before doing any work or error checking. If this parameter is zero (0) when ReadFile returns TRUE on a named pipe, the other end of the message mode pipe calls the WriteFile function with nNumberOfBytesToWrite set to zero (0).

If lpOverlapped is NULL, lpNumberOfBytesRead cannot be NULL.

If lpOverlapped is not NULL, lpNumberOfBytesRead can be NULL.

If this is an overlapped read operation, you can get the number of bytes read by calling GetOverlappedResult.

If hFile is associated with an I/O completion port, you can get the number of bytes read by calling GetQueuedCompletionStatus.

If I/O completion ports are used and you are using a callback routine to free the memory that is 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 causes an invalid number of bytes to be returned in this parameter.

lpOverlapped
A pointer to an OVERLAPPED structure.

This structure is required if hFile is created with FILE_FLAG_OVERLAPPED.

If hFile is opened with FILE_FLAG_OVERLAPPED, the lpOverlapped parameter must not be NULL. It must point to a valid OVERLAPPED structure.

If hFile is created with FILE_FLAG_OVERLAPPED and lpOverlappedis NULL, the function can report incorrectly that the read operation is complete.

If hFile is opened with FILE_FLAG_OVERLAPPED and lpOverlapped is not NULL, the read operation starts at the offset that is specified in the OVERLAPPED structure, and ReadFile may return before the read operation is complete. In this scenario, ReadFile returns FALSE and the GetLastError function returns ERROR_IO_PENDING, which allows the calling process to continue while the read operation completes. The event specified in the OVERLAPPED structure is set to the signaled state when the read operation is complete, and then the caller must adjust the position of the file pointer.

ReadFile resets the event specified by the hEvent member of the OVERLAPPED structure to a nonsignaled state when it begins the I/O operation. Therefore, the caller does not need to do that.

If hFile is not opened with FILE_FLAG_OVERLAPPED and lpOverlapped is NULL, the read operation starts at the current file position and ReadFile does not return until the operation is complete, and then the system updates the file pointer.

If hFile is not opened with FILE_FLAG_OVERLAPPED and lpOverlapped is not NULL, the read operation starts at the offset that is specified in the OVERLAPPED structure. ReadFile does not return until the read operation is complete, and then the system updates the file pointer.

Return Value
The ReadFile function returns when one of the following conditions occur:

A write operation completes on the write end of the pipe.
The number of bytes requested is read.
An error occurs.

If the function succeeds, the return value is nonzero.

If the function fails, the return value is zero (0). To get extended error information, call GetLastError.

If the return value is nonzero and the number of bytes read is zero (0), the file pointer is beyond the current end of the file at the time of the read operation. However, if the file is opened with FILE_FLAG_OVERLAPPED and lpOverlapped is not NULL, the return value is zero (0) and GetLastError returns ERROR_HANDLE_EOF when the file pointer goes beyond the current end of file.

Remarks
If part of a file is locked by another process and the read operation overlaps the locked portion, this function fails.

An application must meet the following specific requirements when working with files that are opened with FILE_FLAG_NO_BUFFERING:

File access must begin at byte offsets within the file that are integer multiples of the volume sector size. To determine the sector size of a volume, call the GetDiskFreeSpace function.
File access must be for numbers of bytes that are integer multiples of the volume sector size. For example, if the sector size is 512 bytes, an application can request read and write operations of 512, 1024, 1536, or 2048 bytes, but not of 335, 981, or 7171 bytes.
Buffer addresses for read and write operations must be sector aligned (that is, aligned on addresses in memory that are integer multiples of the volume sector size).
One way to sector align buffers is to use the VirtualAlloc function to allocate the buffers. This function allocates memory that is aligned on addresses that are integer multiples of the system page size. Because both page and volume sector sizes are powers of 2 (two), memory aligned by multiples of the system page size is also aligned by multiples of the volume sector size.

Accessing the input buffer while a read operation is using the buffer may lead to corruption of the data read into that buffer. Applications must not read from, write to, reallocate, or free the input buffer that a read operation is using until the read operation completes.

Characters can be read from the console input buffer by using ReadFile with a handle to console input. The console mode determines the exact behavior of the ReadFile function. By default, the console mode is ENABLE_LINE_INPUT, which indicates that ReadFile should read until it reaches a carriage return. If you press Ctrl+C, the call succeeds, but GetLastError returns ERROR_OPERATION_ABORTED.

If a named pipe is being read in message mode and the next message is longer than the nNumberOfBytesToRead parameter specifies, ReadFile returns FALSE and GetLastError returns ERROR_MORE_DATA. The remainder of the message may be read by a subsequent call to the ReadFile or PeekNamedPipefunction.

When reading from a communications device, the behavior of ReadFile is governed by the current communication time-out as set and retrieved by using the SetCommTimeouts and GetCommTimeouts functions. Unpredictable results can occur if you fail to set the time-out values. For more information about communication time-outs, see COMMTIMEOUTS.

If ReadFile attempts to read from a mailslot that has a buffer which is too small, the function returns FALSE and GetLastError returns ERROR_INSUFFICIENT_BUFFER.

If the anonymous write pipe handle has been closed and ReadFile attempts to read using the corresponding anonymous read pipe handle, the function returns FALSE and GetLastError returns ERROR_BROKEN_PIPE.

The ReadFile function may fail and return ERROR_INVALID_USER_BUFFER or ERROR_NOT_ENOUGH_MEMORY when there are too many outstanding asynchronous I/O requests.

The ReadFile code to check for the end-of-file condition (EOF) differs for synchronous and asynchronous read operations.

When a synchronous read operation reaches the end of a file, ReadFile returns TRUE and sets *lpNumberOfBytesRead to zero (0).

An asynchronous read operation can get to the end of a file during the initiating call to ReadFile, or during subsequent asynchronous operation. If EOF is detected at ReadFile time for an asynchronous read operation, ReadFile returns FALSE and GetLastError returns ERROR_HANDLE_EOF. If EOF is detected during subsequent asynchronous operation, the call to GetOverlappedResult to obtain the results of that operation returns FALSE and GetLastError returns ERROR_HANDLE_EOF.

To cancel all pending asynchronous I/O operations, use either:

CancelIo—this function only cancels operations issued by the calling thread for the specified file handle.
CancelIoEx—this function cancels all operations issued by the threads for the specified file handle.

Use CancelSynchronousIo to cancel pending synchronous I/O operations.

I/O operations that are canceled complete with the error ERROR_OPERATION_ABORTED.

If you are attempting to read from a floppy drive that does not have a floppy disk, the system displays a message box that prompts the user to retry the operation. To prevent the system from displaying this message box, call the SetErrorMode function with SEM_NOOPENFILEERRORBOX.

Transacted Operations
If there is a transaction bound to the file handle, then the function returns data from the transacted view of the file. A transacted read handle is guaranteed to show the same view of a file for the duration of the handle.

追问

这堆英文全都不明白！当然！现在正在恶补英文！

追答

你在百度百科里搜下，那儿全是中文 的。