Make Partial Updates to FILESTREAM Data
An application uses FSCTL_SQL_FILESTREAM_FETCH_OLD_CONTENT to make partial updates to FILESTREAM BLOB data. The DeviceIoControl function passes this value and the handle that is returned from OpenSqlFilestream to the FILESTREAM driver. The driver then forces a server-side copy of the current FILESTREAM data into the file that is referenced by the handle. If the application issues the FSCTL_SQL_FILESTREAM_FETCH_OLD_CONTENT value after the handle has been written to, the last write operation persists and previous write operations that were made to the handle are lost.
Note
FILESTREAM relies on the SMB protocol for remote access.
Example
The following example shows you how to use the FSCTL_SQL_FILESTREAM_FETCH_OLD_CONTENT value to perform a partial update of an inserted FILESTREAM BLOB.
Note
This example requires the FILESTREAM-enabled database and table that are created in Create a FILESTREAM-Enabled Database and Create a Table for Storing FILESTREAM Data.
#include <windows.h>
#include<sqlext.h>
#include <stdio.h>
#include <sqlncli.h>
#include <tchar.h>
#include <strsafe.h>
/// <summary>
///This class iterates though the ODBC error queue and prints all of the
///accumulated error messages to the console.
/// </summary>
class ODBCErrors
{
private:
int m_iLine; //Source code line on which the error occurred
SQLSMALLINT m_type; //Type of handle on which the error occurred
SQLHANDLE m_handle; //ODBC handle on which the error occurred
public:
/// <summary>
///Default constructor for the ODBCErrors class
///</summary>
ODBCErrors()
{
m_iLine = -1;
m_type = 0;
m_handle = SQL_NULL_HANDLE;
}
/// <summary>
///Constructor for the ODBCErrors class
/// </summary>
/// <param name="iLine">
/// This parameter is the source code line
/// at which the error occurred.
///</param>
/// <param name="type">
/// This parameter is the type of ODBC handle passed in
/// the next parameter.
///</param>
/// <param name="handle">
/// This parameter is the handle on which the error occurred.
///</param>
ODBCErrors(int iLine, SQLSMALLINT type, SQLHANDLE handle)
{
m_iLine = iLine;
m_type = type;
m_handle = handle;
}
///<summary>
/// This method iterates though the error stack for the handle passed
/// into the constructor and displays those errors on the console.
///</summary>
void Print()
{
SQLSMALLINT i = 0, len = 0;
SQLINTEGER native;
SQLTCHAR state[9], text[256];
SQLRETURN sqlReturn = SQL_SUCCESS;
if ( m_handle == SQL_NULL_HANDLE )
{
wprintf_s(TEXT("The error handle is not a valid handle.\n"), m_iLine);
return;
}
wprintf_s(TEXT("Error Line(%d)\n"), m_iLine);
while( sqlReturn == SQL_SUCCESS )
{
len = 0;
sqlReturn = SQLGetDiagRec(
m_type,
m_handle,
++i,
state,
&native,
text,
sizeof(text)/sizeof(SQLTCHAR),
&len);
if ( SQL_SUCCEEDED(sqlReturn) )
wprintf_s(TEXT("Error(%d, %ld, %s) : %s\n"), i, native, state, text);
}
}
};
/// <summary>
///This function waits for the overlapped IO operation to complete; TRUE
///is returned if the function completes successfully and FALSE is
///returned if an error occurs.
///</summary>
BOOL WaitForIOToComplete(HANDLE srcHandle, OVERLAPPED *pOverLapped, LPDWORD pBytes)
{
DWORD bError = GetLastError();
if ( bError == ERROR_HANDLE_EOF )
return TRUE;
//If GetLastError() does not return EOF or IO PENDING then
//an error has occured.
if ( bError != ERROR_IO_PENDING )
return FALSE;
do
{
//In a real application you would perform non-blocking
//processing here.
SleepEx(0, TRUE);
if ( GetOverlappedResult(srcHandle, pOverLapped, pBytes, FALSE) == 0 )
return FALSE;
} while( GetLastError() == ERROR_IO_PENDING );
return TRUE;
}
/// <summary>
///This function performs a partial update of the inserted
///FILESTREAM BLOB.
///</summary>
BOOL UpdateBLOB(LPTSTR srcFilePath, LPBYTE transactionToken, SQLINTEGER cbTransactionToken)
{
BOOL bRetCode = FALSE;
HANDLE srcHandle = INVALID_HANDLE_VALUE;
TCHAR *szErrMsgSrc = TEXT("Error opening FILESTREAM BLOB.");
TCHAR *szErrMsgRead = TEXT("Error reading source file.");
TCHAR *szErrMsgWrite = TEXT("Error writing SQL file.");
TCHAR *szErrMsgIOCtl = TEXT("Error calling DeviceIoControl.");
try
{
//Obtain a handle to the FILESTREAM BLOB data.
srcHandle = OpenSqlFilestream(srcFilePath,
ReadWrite,
0,
transactionToken,
cbTransactionToken,
0);
if ( srcHandle == INVALID_HANDLE_VALUE )
throw szErrMsgSrc;
//The DeviceIoControl API will fill in an return this
//structure. This structure can be used to implement
//overlapped IO.
OVERLAPPED overlapped;
ZeroMemory(&overlapped, sizeof (overlapped));
DWORD bytesRead = 0;
DWORD bytesWritten = 0;
//If ReadFile is called it will return EOF because the
//FILESTREAM BLOB data has not been copied to the open
//File handle. Issuing This IOCTL will cause the server
//to copy the BLOB data to the file referenced by the
//srcHandle.
if ( !DeviceIoControl(srcHandle,
FSCTL_SQL_FILESTREAM_FETCH_OLD_CONTENT,
NULL,
0,
NULL,
0,
NULL,
&overlapped))
{
throw szErrMsgIOCtl;
}
//In a real application you would read more information than a
//few bytes. This small sample is provided to illustrate how to
//perform partial reads and writes starting at an offset within
//the FILESTREAM BLOB.
BYTE buffer[8];
overlapped.Offset = 2;
if ( ReadFile(srcHandle, buffer, 4, &bytesRead, &overlapped) == 0 )
if ( WaitForIOToComplete(srcHandle, &overlapped, &bytesRead) == 0 )
throw szErrMsgRead;
//Change the word Temp currently stored in the BLOB to the
//word Done. Write the new word back to the FILESTREAM BLOB.
buffer[0] = 'D';
buffer[1] = 'o';
buffer[2] = 'n';
buffer[3] = 'e';
if ( WriteFile(srcHandle, buffer, bytesRead, &bytesWritten, &overlapped) == 0 )
if ( WaitForIOToComplete(srcHandle, &overlapped, &bytesWritten) == 0 )
throw szErrMsgWrite;
bRetCode = TRUE;
}
catch( TCHAR *szErrMsg )
{
LPTSTR lpMsgBuf = NULL;
FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER |
FORMAT_MESSAGE_FROM_SYSTEM |
FORMAT_MESSAGE_IGNORE_INSERTS,
NULL,
GetLastError(),
MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
(LPTSTR) &lpMsgBuf,
0, NULL );
wprintf_s(szErrMsg );
wprintf_s(lpMsgBuf);
bRetCode = FALSE;
LocalFree(lpMsgBuf);
}
if ( srcHandle != INVALID_HANDLE_VALUE )
CloseHandle(srcHandle);
return bRetCode;
}
void main()
{
//This query inserts a new record, reads the FILESTREAM path of the
//inserted record and returns the current transaction context.
TCHAR *sqlDBQuery =
TEXT("INSERT INTO Archive.dbo.Records(Id, SerialNumber, Chart)")
TEXT(" OUTPUT GET_FILESTREAM_TRANSACTION_CONTEXT(), inserted.Chart.PathName()")
TEXT("VALUES (newid (), 35, CONVERT(VARBINARY, '**Temp**'))");
SQLCHAR transactionToken[32];
SQLHANDLE henv = SQL_NULL_HANDLE;
SQLHANDLE hdbc = SQL_NULL_HANDLE;
SQLHANDLE hstmt = SQL_NULL_HANDLE;
try
{
//These statements Initialize ODBC for the client application and
//connect to the database.
if ( SQLAllocHandle(SQL_HANDLE_ENV, SQL_NULL_HANDLE, &henv) != SQL_SUCCESS )
throw new ODBCErrors(__LINE__, SQL_HANDLE_ENV, henv);
if ( SQLSetEnvAttr(henv, SQL_ATTR_ODBC_VERSION,(void*)SQL_OV_ODBC3, NULL) != SQL_SUCCESS )
throw new ODBCErrors(__LINE__, SQL_HANDLE_ENV, henv);
if ( SQLAllocHandle(SQL_HANDLE_DBC, henv, &hdbc) != SQL_SUCCESS )
throw new ODBCErrors(__LINE__, SQL_HANDLE_ENV, henv);
//This code assumes that an ODBC User DSN has been created
//with the following settings:
//
//Driver: Sql Server Native Client 10.0
//Name: Sql Server FILESTREAM
//Description: Sql Server FILESTREAM
//Server: .
//Change Default Database to: Archive
if ( SQLConnect(hdbc, TEXT("Sql Server FILESTREAM"),
SQL_NTS, NULL, 0, NULL, 0) <= 0 )
throw new ODBCErrors(__LINE__, SQL_HANDLE_DBC, hdbc);
//FILESTREAM requires that all read and write operations occur
//within a transaction. The ODBC driver will not automatically
//COMMIT the transaction it BEGIN if SQL_AUTOCOMMIT_OFF is set.
if ( SQLSetConnectAttr(hdbc,
SQL_ATTR_AUTOCOMMIT,
(SQLPOINTER)SQL_AUTOCOMMIT_OFF,
SQL_IS_UINTEGER) != SQL_SUCCESS )
throw new ODBCErrors(__LINE__, SQL_HANDLE_DBC, hdbc);
if ( SQLAllocHandle(SQL_HANDLE_STMT, hdbc, &hstmt) != SQL_SUCCESS )
throw new ODBCErrors(__LINE__, SQL_HANDLE_DBC, hdbc);
if ( SQLExecDirect(hstmt, sqlDBQuery, SQL_NTS) != SQL_SUCCESS )
throw new ODBCErrors(__LINE__, SQL_HANDLE_STMT, hstmt);
//Retrieve the transaction token.
if ( SQLFetch(hstmt) != SQL_SUCCESS )
throw new ODBCErrors(__LINE__, SQL_HANDLE_STMT, hstmt);
SQLINTEGER cbTransactionToken = sizeof(transactionToken);
if ( SQLGetData(hstmt, 1,
SQL_C_BINARY,
transactionToken,
sizeof(transactionToken),
&cbTransactionToken) != SQL_SUCCESS )
throw new ODBCErrors(__LINE__, SQL_HANDLE_STMT, hstmt);
//Retrieve the file path for the inserted record.
TCHAR srcFilePath[1024];
SQLINTEGER cbsrcFilePath;
if ( SQLGetData(hstmt, 2, SQL_C_TCHAR, srcFilePath, sizeof(srcFilePath), &cbsrcFilePath) != SQL_SUCCESS )
throw new ODBCErrors(__LINE__, SQL_HANDLE_STMT, hstmt);
if ( SQLCloseCursor(hstmt) != SQL_SUCCESS )
throw new ODBCErrors(__LINE__, SQL_HANDLE_STMT, hstmt);
SQLUSMALLINT mode = SQL_ROLLBACK;
//Update the FILESTREAM BLOB data from **Temp** to **Done**.
if ( UpdateBLOB(srcFilePath,
transactionToken,
cbTransactionToken) == TRUE )
mode = SQL_COMMIT;
//Commit the TRANSACTION if the BLOB was successfully udpated;
//ROLLBACK the TRANSACTION if it was not.
SQLTransact(henv, hdbc, mode);
}
catch(ODBCErrors *pErrors)
{
pErrors->Print();
delete pErrors;
}
//Free and clean up the ODBC data handles that were initially created.
if ( hstmt != SQL_NULL_HANDLE )
SQLFreeHandle(SQL_HANDLE_STMT, hstmt);
if ( hdbc != SQL_NULL_HANDLE )
SQLDisconnect(hdbc);
if ( hdbc != SQL_NULL_HANDLE )
SQLFreeHandle(SQL_HANDLE_DBC, hdbc);
if ( henv != SQL_NULL_HANDLE )
SQLFreeHandle(SQL_HANDLE_ENV, henv);
}
See Also
Access FILESTREAM Data with OpenSqlFilestream
Create Client Applications for FILESTREAM Data