Article: Q110470
Product(s): Open Database Connectivity (ODBC)
Version(s): WINDOWS:1.0,2.0,3.0
Operating System(s):
Keyword(s):
Last Modified: 12-JUN-2001
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The information in this article applies to:
- Microsoft Open Database Connectivity, versions 1.0, 2.0
- Microsoft Visual Basic Standard Edition for Windows, versions 2.0, 3.0
- Microsoft Visual Basic Professional Edition for Windows, versions 2.0, 3.0
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SUMMARY
=======
Visual Basic developers can call Windows DLLs provided that Basic function
declarations for the functions in the DLL are included in the Visual Basic
project. When calling ODBC drivers from Visual Basic, there are a few
considerations to keep in mind.
This article discusses issues surrounding the use of the SQLBindCol ODBC function
from Visual Basic.
The SQLBindCol function is used to bind a variable to a column of a result set.
The function binds an address in memory to a column of data and subsequent calls
to SQLFetch place the data for that column and row into the specified memory
location.
It is possible to pass the address of a Visual Basic string to SQLBindCol but as
strings may be moved in memory by VB, it is not safe to use SQLBindCol without
first locking the memory. This article discusses using VB strings with
SQLBindCol and calling the Windows API functions GlobalAlloc, GlobalLock, and
hmemcpy to allocate, lock, and retrieve data from memory into Visual Basic
strings.
MORE INFORMATION
================
After submitting a SQL query, an ODBC application binds the results of that
query into variables. Typically one variable is associated with each column in
the results set. The following is the SQLBindCol declaration that must be used
by a Visual Basic program:
' function must be declared on one line!<BR/>
Declare Function SQLBindCol Lib "odbc.dll" Alias "SqlBindCol"<BR/>
(ByVal hstmt As Long, ByVal icol As Integer, ByVal fCType As Integer,
rgbValue As Any , ByVal cbValueMax As Long, pcbValue As Long)<BR/>
As Integer
Essentially, a pointer to memory is passed to SQLBindCol. The ODBC driver then
places data in that memory location specified by the pointer on each fetch. The
application can then retrieve the data in the recordset by referencing that
memory location.
To pass a pointer to a string from Visual Basic to a DLL, you use the ByVal
keyword. This topic will not be discussed in this article; the following
Knowledge Base article discusses passing strings from VB to DLLs:
Q118643 How to Pass a String or String Arrays Between VB and a C DLL.
When you pass a string using ByVal to SQLBindCol (or any DLL) VB passes a pointer
to the string. It is possible to use VB strings with SQLBindCol as long as you
are not growing any other strings (including the text of controls) during the
time that the strings are bound. During normal processing, VB will periodically
reorganize its string space. This means that strings will be moved in memory and
their address will change. As a result, the address you passed to SQLBindCol is
no longer valid and you will see invalid data in your strings.
NOTE: This is behavior is by design for Visual Basic and is documented in the
Control Development Kit. Microsoft does not recommend using SQLBindCol with
Visual Basic strings and cannot guarantee that strings will not be moved in
memory. Using SQLBindCol with VB strings will result not only in incorrect data
but also General Protection Faults.
Calling SQLBindCol with VB Strings
----------------------------------
If you choose to use VB strings with SQLBindCol the following are some of the
rules that you must follow for SQLBindCol in your Basic code:
1. Do not manipulate any other strings between the call(s) to SQLBindCol and
SQLFreeStmt with SQL_UNBIND. A common example is during a SQLFetch loop
concatenating the bound variables into one long string and adding it to a
text box.
2. When binding to strings, bind to variable-length Visual Basic strings, not
fixed-length strings.
Variable-length strings are declared thus: Dim mystr as String
3. Do not bind to arrays of strings. Arrays of strings are stored in 64K
segments and Visual Basic may move string elements in memory to recover
fragmented string space. As a result, the array element that a column is
bound to may not be valid during a subsequent SQLFetch.
As SQLExtendedFetch specifically requires binding to arrays, it is recommended
that you do not use SQLExtendedFetch from Visual Basic.
4. Before binding variable-length strings, initialize them as the following,
mystr = String$(255,0).
This allocates a string.
5. When calling SQLBindCol and binding to a string (i.e., if rgbValue is going
to be a string), use the Byval keyword. For instance,
SQLBindCol(hstmt&, 1, SQL_C_CHAR, Byval mystr, cbMax&, cbVal&)
Allocating Global Memory
------------------------
It is possible to use the Windows API function GlobalAlloc, GlobalLock, and
hmemcpy to obtain a fixed chunk of memory, bind columns with SQLBindCol to this
memory, and copy the data from the memory into a Visual Basic string.
GlobalAlloc: allocates a block of memory and returns a handle to that
memory if successful. You can allocate a chunk of
memory for each column you wish to bind.
GlobalLock: locks the memory allocated with GlobalAlloc and returns a
pointer to the memory. You can use GlobalLock to obtain a
pointer which you can use with SQLBindCol.
hmemcpy: copies memory from one address to another a specified
number of bytes. You can use hmemcpy to copy the bound
memory to a variable length string.
NOTE: When finished with the memory you must call GlobalUnlock and GlobalFree to
free the memory and the selector used by GlobalAlloc. If you are performing a
large number of GlobalAlloc calls you should allocate fewer but larger chunks of
memory and break them apart manually.
The following demonstration assumes that you have a database table created and
populated called Customer. It also assumes that you have a data source already
created. The structure of the Customer table is:
Field Type Null?
--------- ------------ --------
CustNum int not null
Company varchar(30) null
Address varchar(30) null
City varchar(30) null
1. Start Visual Basic, or if VB is already running, create a new project. Form1
is created by default.
2. Add two command buttons (Command1, Command2) to Form1. Add a text box (Text1)
to Form1.
3. Set the following properties of the controls. Text1 properties must be see at
design time:
Command1.Caption = "Log On" Command2.Caption = "Execute Query"
Text1.MultiLine = True Text1.ScrollBars = 3
4. Add the following to the <general> <declarations> section of
Form1. (All Declare statements must be on 1 line.):
Option Explicit
'
' ODBC constants
'
Const SQL_NULL_HENV = 0
Const SQL_NULL_HDBC = 0
Const SQL_NULL_HSTMT = 0
Const SQL_DRIVER_PROMPT = 2
Const SQL_ERROR = -1
Const SQL_INVALID_HANDLE = -2
Const SQL_NO_DATA_FOUND = 100
Const SQL_SUCCESS = 0
Const SQL_SUCCESS_WITH_INFO = 1
Const SQL_CHAR = 1
Const SQL_NUMERIC = 2
Const SQL_DECIMAL = 3
Const SQL_INTEGER = 4
Const SQL_SMALLINT = 5
Const SQL_FLOAT = 6
Const SQL_REAL = 7
Const SQL_DOUBLE = 8
Const SQL_VARCHAR = 12
Const SQL_C_CHAR = SQL_CHAR
Const SQL_C_LONG = SQL_INTEGER
Const SQL_C_SHORT = SQL_SMALLINT
Const SQL_C_FLOAT = SQL_REAL
Const SQL_C_DOUBLE = SQL_DOUBLE
Const SQL_CLOSE = 0
Const SQL_DROP = 1
Const SQL_UNBIND = 2
Const SQL_RESET_PARAMS = 3
'
' GlobalAlloc Constants
'
Const GMEM_FIXED = &H0
Const GMEM_MOVEABLE = &H2
'
' ODBC function declarations
'
Declare Function SQLAllocConnect Lib "odbc.dll" (ByVal hEnv&, phdbc&)
As Integer
Declare Function SQLAllocEnv Lib "odbc.dll" (phenv&) As Integer
Declare Function SQLAllocStmt Lib "odbc.dll" (ByVal hdbc&, phstmt&)
As Integer
Declare Function SQLBindCol Lib "odbc.dll" (ByVal hStmt&, ByVal icol%,
ByVal fCType%, rgbValue As Any, ByVal cbValueMax&, pcbValue&)
As Integer
Declare Function SQLExecDirect Lib "odbc.dll" (ByVal hStmt&, ByVal
szSqlStr$,
ByVal cbSqlStr&) As Integer
Declare Function SQLDisconnect Lib "odbc.dll" (ByVal hdbc&) As Integer
Declare Function SQLDriverConnect Lib "odbc.dll" (ByVal hdbc&, ByVal hWnd%,
ByVal szCSIn$, ByVal cbCSIn%, ByVal szCSOut$, ByVal cbCSMax%,
cbCSOut%,
ByVal fDrvrComp%) As Integer
Declare Function SQLFetch Lib "odbc.dll" (ByVal hStmt&) As Integer
Declare Function SQLFreeConnect Lib "odbc.dll" (ByVal hdbc&) As Integer
Declare Function SQLFreeEnv Lib "odbc.dll" (ByVal hEnv&) As Integer
Declare Function SQLFreeStmt Lib "odbc.dll" (ByVal hStmt&, ByVal fOption%)
As Integer
'
' Memory function declarations
'
Declare Sub hMemCpy Lib "Kernel" (hpvDest As Any, hpvSource As Any,
ByVal cbCopy As Long)
Declare Function GlobalAlloc Lib "Kernel" (ByVal wFlags As Integer,
ByVal dwBytes As Long) As Integer
Declare Function GlobalLock Lib "Kernel" (ByVal hMem As Integer) As Long
Declare Function GlobalUnlock Lib "Kernel" (ByVal hMem As Integer) As
Integer
Declare Function GlobalFree Lib "Kernel" (ByVal hMem As Integer) As Integer
Dim hEnv As Long ' environment handle
Dim hConnect As Long ' connect handle
Dim hStmt As Long ' statement handle
Dim rc As Integer ' return code
5. Add the following code to the Click event of Command2:
Sub Command1_Click ()
Dim icbConnStrOut As Integer ' connect vars
Dim sConnectStr As String
Dim sConnectBuffer As String
' initialize handles
hConnect = SQL_NULL_HDBC
hEnv = SQL_NULL_HENV
hStmt = SQL_NULL_HSTMT
' initialize strings
sConnectStr = ""
sConnectBuffer = Space$(255)
' allocate the environment and connect to the source
rc = SQLAllocEnv(hEnv)
rc = SQLAllocConnect(hEnv, hConnect)
' the following should be all on 1 line
rc = SQLDriverConnect(hConnect, Me.hWnd, sConnectStr, Len(sConnectStr),
sConnectBuffer, Len(sConnectBuffer), icbConnStrOut,
SQL_DRIVER_PROMPT)
If rc <> SQL_SUCCESS And rc <> SQL_SUCCESS_WITH_INFO Then
MsgBox "Unable To Connect"
End
Else
MsgBox "Connected To Database"
Me.Command1.Enabled = False
End If
End Sub
6. Add the following code to the Unload event of Form1:
Sub Command2_Click ()
' strings to hold column data
Dim sCompany As String
Dim sAddress As String
Dim sCity As String
' length of column data returned
Dim lAddressLen As Long
Dim lCityLen As Long
Dim lCompanyLen As Long
' ptrs to memory to bind to columns
Dim lpAddress As Long
Dim lpCompany As Long
Dim lpCity As Long
' mem handles
Dim hMemAddress As Integer
Dim hMemCompany As Integer
Dim hMemCity As Integer
Dim sQuery As String ' sql statement
' Run query
sQuery = " select Company, Address, City From Customer "
rc = SQLAllocStmt(hConnect, hStmt)
rc = SQLExecDirect(hStmt, sQuery, Len(sQuery))
If rc = SQL_SUCCESS Or rc = SQL_SUCCESS_WITH_INFO Then
'
' initialize string space
'
sCompany = String$(30, " ")
sAddress = String$(30, " ")
sCity = String$(30, " ")
Text1.Text = ""
'
' allocate memory and get ptr to it for each column
'
hMemAddress = GlobalAlloc(GMEM_MOVEABLE, Len(sAddress))
lpAddress = GlobalLock(hMemAddress)
hMemCompany = GlobalAlloc(GMEM_MOVEABLE, Len(sCompany))
lpCompany = GlobalLock(hMemCompany)
hMemCity = GlobalAlloc(GMEM_MOVEABLE, Len(sCity))
lpCity = GlobalLock(hMemCity)
'
' bind the columns to the memory we allocated. The must all be
' on one line!
'
rc = SQLBindCol(hStmt, 1, SQL_C_CHAR, ByVal lpCompany, 29,
lCompanyLen)
rc = SQLBindCol(hStmt, 2, SQL_C_CHAR, ByVal lpAddress, 29,
lAddressLen)
rc = SQLBindCol(hStmt, 3, SQL_C_CHAR, ByVal lpCity, 29, lCityLen)
'
' loop and fetch all records into memory
'
rc = SQLFetch(hStmt)
Do While rc = SQL_SUCCESS Or rc = SQL_SUCCESS_WITH_INFO
'
' copy the string from memory to a VB string
'
Call hMemCpy(ByVal sCompany, ByVal lpCompany, lCompanyLen)
Call hMemCpy(ByVal sAddress, ByVal lpAddress, lAddressLen)
Call hMemCpy(ByVal sCity, ByVal lpCity, lCityLen)
Text1.Text = Trim(Text1.Text) & sCompany & sAddress
Text1.Text = Trim(Text1.Text) & sCity & Chr$(13) & Chr$(10)
rc = SQLFetch(hStmt)
Loop
'
' unbind columns and free memory
'
rc = SQLFreeStmt(hStmt, SQL_UNBIND)
rc = GlobalUnlock(hMemCity)
rc = GlobalUnlock(hMemAddress)
rc = GlobalUnlock(hMemCompany)
rc = GlobalFree(hMemCity)
rc = GlobalFree(hMemAddress)
rc = GlobalFree(hMemCompany)
Else
Text1.Text = "No Records Returned"
End If
'
' free statement handle
'
rc = SQLFreeStmt(hStmt, SQL_CLOSE)
MsgBox "Done"
End Sub
7. Save the project and run.
Additional Considerations
-------------------------
1. Use cbcoldef appropriately. Refer to Appendix D of the "Programmer s
Reference" for the ODBC SDK.
2. Use a LONG variable for pcbValue, and not a literal value. Remember that you
are passing by reference.
Additional query words:
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Keywords :
Technology : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbODBCSearch kbVB300Search kbVB300 kbODBC200 kbODBC100 kbVB200
Version : WINDOWS:1.0,2.0,3.0
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