Insert raw data into a binary data field with ADO

With ADO it’s easy to append some common data type in to a field, such as char, int, short, string, double and so on. Just use the following sentences:

rs->Fields->Item[2]->Value = "value"; rs->Fields->Item["au_fname"]->Value = "value"; // or rs->Fields->GetItem(2)->PutValue("value"); rs->Fields->GetItem("au_fname")->PutValue("value");

 

 

But when it comes to the binary raw data, or large text, you need to do a little more job. One way to accomplish this is using SAFEARRAY provided by COM.

 

1.1.    Some introduction of SAFEARRAY

The data structure of SAFEARRAY:

 

typedef struct FARSTRUCT tagSAFEARRAY { unsigned short cDims; // Count of dimensions in this array. unsigned short fFeatures; // Flags used by the SafeArray // routines documented below. unsigned long cbElements; // Size of an element of the array. // Does not include size of // pointed-to data. unsigned long cLocks; // Number of times the array has been // locked without corresponding unlock. void HUGEP* pvData; // Pointer to the data. SAFEARRAYBOUND rgsabound[1]; // One bound for each dimension. } SAFEARRAY;

 

SAFEARRAY * psa; SafeArrayCreateVector(VT_UI1, // data type 0, // lLbound 5); // cElements if(psa == NULL) return E_OUTOFMEMORY;

The array rgsabound is stored with the left-most dimension in rgsabound[0] and the right-most dimension in rgsabound[cDims - 1]. If an array was specified in a C-like syntax as a [2][5], it would have two elements in the rgsabound vector. Element 0 has an lLbound of 0 and a cElements of 2. Element 1 has an lLbound of 0 and a cElements of 5.

The fFeatures flags describe attributes of an array that can affect how the array is released. The fFeatures field describes what type of data is stored in the SAFEARRAY and how the array is allocated. This allows freeing the array without referencing its containing variant. The bits are accessed using the following constants:

fFeatures Flags	Description
FADF_AUTO 0x0001	An array that is allocated on the stack.
FADF_STATIC 0x0002	An array that is statically allocated.
FADF_EMBEDDED 0x0004	An array that is embedded in a structure.
FADF_FIXEDSIZE 0x0010	An array that may not be resized or reallocated.
FADF_RECORD 0x0020	An array that contains records. When set, there will be a pointer to the IRecordinfo interface at negative offset 4 in the array descriptor.

 

The SAFEARRAYBOUND Structure Represents the bounds of one dimension of the array. The lower bound of the dimension is represented by lLbound, and cElements represents the number of elements in the dimension. The structure is defined as follows:

 

typedef struct tagSAFEARRAYBOUND { unsigned long cElements; long lLbound; } SAFEARRAYBOUND;

 

SAFEARRAY * psa; SAFEARRAYBOUND rgsabound[1]; rgsabound[0].lLbound = 0; rgsabound[0].cElements = 5; psa = SafeArrayCreate(VT_VARIANT, // data type 1, // dimension rgsabound); // length of each dimension if(psa == NULL) return E_OUTOFMEMORY; // Use the array return NOERROR;

lLbound indicates the beginning of the array.

From http://msdn.microsoft.com/en-us/library/ms221482.aspx

 

1.1.    How to insert:

Step1: create a SafeArray.

We call SafeArrayCreate to create a VT_VARIANT[5] array as follow:

 

 

 

if we only need a one-dimensional array, just call SafeArrayCreateVector as follow:

 

 

step 2: fill the array with data

long lngOffset = 0;		//rgIndices

while(lngOffset < iDataLen)   
{   
	chData  = ((UCHAR*)pValue)[lngOffset];   
	hr = SafeArrayPutElement(psa, 
							&lngOffset,	// Pointer to a vector of indexes for each dimension of the array
							&chData);	//data to be filled
	if(FAILED(hr))   
		return FALSE;;   

	lngOffset++;
}  

Step 3: put the array into record field

_variant_t varChunk;
// Assign the Safe array to a variant. 
varChunk.vt = VT_ARRAY|VT_UI1; 
varChunk.parray = psa;
rs->Fields->Item["BinData"]->Value = varChunk;

Step 4: release the array

Caution we need to set the varChunk to NULL first. Or else error will happen when destruction of varChunk is called.

if (FAILED(SafeArrayDestroy(psa)))
    return FALSE;
varChunk.vt = VT_EMPTY;   
varChunk.parray = NULL;