MDAC 2.5 SDK - ODBC Programmer's Reference
Chapter 4: ODBC Fundamentals

Allocating and Freeing Buffers

All buffers are allocated and freed by the application. If a buffer is not deferred, it need only exist for the duration of the call to a function. For example, SQLGetInfo returns the value associated with a particular option in the buffer pointed to by the InfoValuePtr argument. This buffer can be freed immediately after the call to SQLGetInfo, as shown in the following code example:

SQLSMALLINT   InfoValueLen;
SQLCHAR *     InfoValuePtr = malloc(50);   // Allocate InfoValuePtr.

SQLGetInfo(hdbc, SQL_DBMS_NAME, (SQLPOINTER)InfoValuePtr, 50,
            &InfoValueLen);

free(InfoValuePtr);                        // OK to free InfoValuePtr.

Because deferred buffers are specified in one function and used in another, it is an application programming error to free a deferred buffer while the driver still expects it to exist. For example, the address of the *ValuePtr buffer is passed to SQLBindCol for later use by SQLFetch. This buffer cannot be freed until the column is unbound, such as with a call to SQLBindCol or SQLFreeStmt as shown in the following code example:

SQLRETURN    rc;
SQLINTEGER   ValueLenOrInd;
SQLHSTMT     hstmt;

// Allocate ValuePtr
SQLCHAR * ValuePtr = malloc(50);

// Bind ValuePtr to column 1. It is an error to free ValuePtr here.
SQLBindCol(hstmt, 1, SQL_C_CHAR, ValuePtr, 50, &ValueLenOrInd);

// Fetch each row of data and place the value for column 1 in *ValuePtr. 
// Code to check if rc equals SQL_ERROR or SQL_SUCCESS_WITH_INFO 
// not shown.
while ((rc = SQLFetch(hstmt)) != SQL_NO_DATA) {
   // It is an error to free ValuePtr here.
}

// Unbind ValuePtr from column 1.  It is now OK to free ValuePtr.
SQLFreeStmt(hstmt, SQL_UNBIND);
free(ValuePtr);

Such an error is easily made by declaring the buffer locally in a function; the buffer is freed when the application leaves the function. For example, the following code causes undefined and probably fatal behavior in the driver:

SQLRETURN   rc; 
SQLHSTMT    hstmt;

BindAColumn(hstmt);

// Fetch each row of data and try to place the value for column 1 in 
// *ValuePtr. Because ValuePtr has been freed, the behavior is undefined 
// and probably fatal. Code to check if rc equals SQL_ERROR or 
// SQL_SUCCESS_WITH_INFO not shown.
while ((rc = SQLFetch(hstmt)) != SQL_NO_DATA) {}

   .
   .
   .

void BindAColumn(SQLHSTMT hstmt)  // WARNING! This function won't work!
{
   // Declare ValuePtr locally.
   SQLCHAR      ValuePtr[50];
   SQLINTEGER   ValueLenOrInd;

   // Bind rgbValue to column.
   SQLBindCol(hstmt, 1, SQL_C_CHAR, ValuePtr, sizeof(ValuePtr),
               &ValueLenOrInd);

   // ValuePtr is freed when BindAColumn exits.
}