MyFSD_SetFilePointer

This function moves the file pointer of an open file in an installable file system. The application does not call this function directly. Instead, use the corresponding standard Win32 function SetFilePointer. The file system driver (FSD) Manager determines the file system type and calls the MyFSD_SetFilePointer implementation of the function.

At a Glance

Syntax

DWORD MyFSD_SetFilePointer( PFILE pFile, LONG lDistanceToMove,
PLONG pDistanceToMoveHigh, DWORD dwMoveMethod );

Parameters

pFile

Pointer to the value that the file system driver (FSD) passes to the FSDMGR_CreateFileHandle function when creating the file handle.

lDistanceToMove

Low-order 32 bits of a signed value that specifies the number of bytes to move the file pointer. If lpDistanceToMoveHigh is not NULL, lpDistanceToMoveHigh and lDistanceToMove form a single 64-bit signed value that specifies the distance to move. If lpDistanceToMoveHigh is NULL, lDistanceToMove is a 32-bit signed value. A positive value for lDistanceToMove moves the file pointer forward in the file, and a negative value moves the file pointer backward.

pDistanceToMoveHigh

Pointer to the high-order 32 bits of the signed 64-bit distance to move. If you do not need the high-order 32 bits, this pointer may be NULL. When non-NULL, this parameter also receives the high-order DWORD of the new value of the file pointer. For more information, see the Remarks section later in this topic.

dwMoveMethod

Starting point for the file pointer move. It is one of the following values:

Return Values

If the SetFilePointer function succeeds and lpDistanceToMoveHigh is NULL, the return value is the low-order DWORD of the new file pointer. If lpDistanceToMoveHigh is not NULL, the function returns the low order DWORD of the new file pointer, and puts the high-order DWORD of the new file pointer into the LONG pointed to by that parameter.

If the function fails and lpDistanceToMoveHigh is NULL, the return value is 0xFFFFFFFF. To get extended error information, call GetLastError.

If the function fails, and lpDistanceToMoveHigh is non-NULL, the return value is 0xFFFFFFFF. However, because 0xFFFFFFFF is a valid value for the low-order DWORD of the new file pointer, you must check GetLastError to determine whether an error occurred. If an error occurred, GetLastError returns a value other than NO_ERROR. For a code example that illustrates this point, see the Remarks section later in this topic.

If the new file pointer would have been a negative value, the function fails, the file pointer is not moved, and the code returned by GetLastError is ERROR_NEGATIVE_SEEK.

Remarks

An FSD exports this function if it wants to support the SetFilePointer function. All FSD functions can be called reentryly, therefore, FSD developers must take this into account when developing an FSD.

The Fsdmgr component is a dynamic-link library (DLL) that manages all operating system interaction with installable files systems. Each installable file system requires an FSD, which is a DLL that exports an API needed to support an installable file system. The name of the DLL and the names of the functions it exports start with the name of the associated installable file system. For example, if the name of file system is MyFSD, then its DLL is MyFSD.dll and its exported functions are prefaced with MyFSD_*.

Fsdmgr provides services to FSDs. The FSDMGR_RegisterVolume, FSDMGR_CreateFileHandle, and FSDMGR_CreateSearchHandle functions record a DWORD of volume-specific data the FSD needs to keep associated with volume. This volume-specific data is passed as the first parameter of these three functions.

Applications that access an installable file system use standard Win32 functions. For example, when an application wants to create a folder on a device that contains an installable file system, it calls CreateDirectory. Fsdmgr recognizes that the path is to a device containing an installable file system and calls the appropriate function, which in the case of the FAT file system is FATFSD_CreateDirectoryW. That is, the application calls CreateDirectory, causing Fsdmgr to call FATFSD_CreateDirectoryW.

You cannot use the SetFilePointer function with a handle to a nonseeking device, such as a pipe or a communications device. To determine the file type for hFile, use the GetFileType function.

You should be careful when setting the file pointer in a multithreaded application. For example, an application whose threads share a file handle, update the file pointer, and read from the file must protect this sequence by using a critical section object or mutex object.

If the hFile file handle was opened with the FILE_FLAG_NO_BUFFERING flag set, an application can move the file pointer only to sector-aligned positions. A sector-aligned position is a position that is a whole number multiple of the volume’s sector size. An application can obtain a volume’s sector size by calling the GetDiskFreeSpace function. If an application calls SetFilePointer with distance-to-move values that result in a position that is not sector-aligned and a handle that was opened with FILE_FLAG_NO_BUFFERING, the function fails, and GetLastError returns ERROR_INVALID_PARAMETER.

Note that it is not an error to set the file pointer to a position beyond the end of the file. The size of the file does not increase until you call the SetEndOfFile, WriteFile, or WriteFileEx function. A write operation increases the size of the file to the file pointer position plus the size of the buffer written, leaving the intervening bytes uninitialized.

If the return value is 0xFFFFFFFF and if lpDistanceToMoveHigh is non-NULL, an application must call GetLastError to determine whether the function has succeeded or failed. The following sample code illustrates this point:

// 
// Case One: calling the function with lpDistanceToMoveHigh == NULL 
 
// Try to move hFile's file pointer some distance 
dwPtr = SetFilePointer (hFile, lDistance, NULL, FILE_BEGIN) ; 
 
if (dwPtr == 0xFFFFFFFF) // Test for failure
{ 
    // Obtain the error code 
    dwError = GetLastError() ; 
 
    // Deal with failure 
    // . . . 
 
} // End of error handler 


// 
// Case Two: calling the function with lpDistanceToMoveHigh != NULL 
 
// Try to move hFile's file pointer some huge distance 
dwPtrLow = SetFilePointer (hFile, lDistLow, & lDistHigh, FILE_BEGIN) ; 
 
// Test for failure
if (dwPtrLow == 0xFFFFFFFF && (dwError = GetLastError()) != NO_ERROR )
{ 
    // Deal with failure 
    // . . . 

} // End of error handler 
 

The parameter lpDistanceToMoveHigh is used to manipulate huge files. If it is set to NULL, then lDistanceToMove has a maximum value of 2^31–2, or 2 gigabytes less two. This is because all file pointer values are signed values. Therefore if there is even a small change that the file will grow to that size, you should treat the file as a huge file and work with 64-bit file pointers. With file compression on NTFS, and sparse files, it is possible to have files that large even if the underlying volume is not that large.

If lpDistanceToMoveHigh is not NULL, then lpDistanceToMoveHigh and lDistanceToMove form a single 64-bit signed value. The lDistanceToMove parameter is treated as the low-order 32 bits of the value, and lpDistanceToMoveHigh as the upper 32 bits. Thus, lpDistanceToMoveHigh is a sign extension of lDistanceToMove.

To move the file pointer from zero to 2 gigabytes, lpDistanceToMoveHigh can be either NULL or a sign extension of lDistanceToMove. To move the pointer more than 2 gigabytes, use lpDistanceToMoveHigh and lDistanceToMove as a single 64-bit quantity. For example, to move in the range from 2 gigabytes to 4 gigabytes set the contents of lpDistanceToMoveHigh to zero, or to –1 for a negative sign extension of lDistanceToMove.

To work with 64-bit file pointers, you can declare a LONG, treat it as the upper half of the 64-bit file pointer, and pass its address in lpDistanceToMoveHigh. This means you have to treat two different variables as a logical unit, which is error-prone. The problems can be ameliorated by using the LARGE_INTEGER structure to create a 64-bit value and passing the two 32-bit values by means of the appropriate elements of the union.

It is conceptually simpler and better design to use a function to hide the interface to SetFilePointer. To do so, use something like this:

__int64 myFileSeek (HANDLE hf, __int64 distance, DWORD MoveMethod)
{
   LARGE_INTEGER li;

   li.QuadPart = distance;

   li.LowPart = SetFilePointer (hf, li.LowPart, &li.HighPart, MoveMethod);

   if (li.LowPart == 0xFFFFFFFF && GetLastError() != NO_ERROR)
   {
      li.QuadPart = –1;
   }

   return li.QuadPart;
}

Note

You can use SetFilePointer to determine the length of a file. To do this, use FILE_END for dwMoveMethod and seek to location zero. The file offset returned is the length of the file. However, this practice can have unintended side effects, such as failure to save the current file pointer so that the program can return to that location. It is simpler and safer to use GetFileSize instead.

You can also use the SetFilePointer function to query the current file pointer position. To do this, specify a move method of FILE_CURRENT and a distance of zero.

See Also

MyFSD_ReadFile