Visual Basic

This section describes the Visual Basic source modules for the Btrieve API.I

Visual Basic, when compiling a 32-bit application, aligns members of a UDT (user-defined data type) on 8-, 16-, or 32-bit boundaries, depending on the size of that particular member. Unlike structures, database rows are packed, meaning there is no unused space between fields. Because there is no way to turn alignment off, there must be some method to pack and unpack structures so that Visual Basic applications can access a database. The Pervasive Btrieve Alignment DLL, PALN32.DLL, is designed to handle this alignment issue.

In the case of Visual Basic, this language aligns elements at various multiples of bits. The following table provides various data types and shows how Visual Basic handles them:


Visual Basic Type	Type Constant	Typical Size (in bytes)	Boundary
Byte	FLD_BYTE	Any	1 byte (none)
String	FLD_STRING	Any	1 byte (none)
Boolean	FLD_LOGICAL	2	2 bytes
Integer	FLD_INTEGER	2	2 bytes
Currency	FLD_MONEY	4	4 bytes
Long	FLD_INTEGER	4	4 bytes
Single	FLD_IEEE	4	4 bytes
Double	FLD_IEEE	8	4 bytes

Programs access Btrieve calls in Visual Basic by calling the BTRCALL function. Access to this function is accomplished by including the BTRAPI.BAS module in your project. The rest of the functions required reside in PALN32.DLL.

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To include PALN32.DLL in your project:

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Select Project -> References and check the Pervasive Btrieve Alignment Library module. If it is not shown, add it to the list first by selecting the browse button and locating the file.

The following table includes each function and the specific module it requires.

To perform Btrieve operations

BTRAPI.BAS

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OP As Integer
The Btrieve operation number as listed in the Btrieve API Guide.

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Pb$ As String
Stores the position block in a string used to retrieve or store records or to pass structures to Btrieve.

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Db As Any
Data buffer. This parameter is used to retrieve or store records or to pass structures to Btrieve.

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DL As Long
Length of data buffer.

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Kb As Any
Key buffer.

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Kl As Integer
Length of key buffer.

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Kn As Integer
Key number.

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Integer
The Btrieve status code returned by the operation. See Status Codes and Messages for more information about a specific code.

RowToStruct

Converts a row of bytes into a Visual Base UDT.

PALN32.DLL

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row (1 to n) As Byte
Input array to retrieve packed data.

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fld (1 to n) As FieldMap
A FieldMap array used to determine data types of individual fields.

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udt As Any
The UDT to store the data.

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udtSize As Long
The size of the UDT. Generate this value using LenB( ).

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Integer
0 if okay. Otherwise an error occurred.

SetFieldMap

Sets the members of a FieldMap element.

PALN32.DLL

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map As FieldMap
An element of a FieldMap array.

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dataType As Integer
The field type, passes on the following constants:
- FLD_STRING
- FLD_INTEGER
- FLD_IEEE
- FLD_MONEY
- FLD_LOGICAL
- FLD_BYTE
- FLD_UNICODE1

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size As Long
Size of the field, in bytes, as stored in the database.

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Nothing

SetFieldMapFromDDF

Sets all the members of an array of the FieldMap type.

PALN32.DLL

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path As String
A full path name to the data source.

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table As String
Name of the table.

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userName As String
Reserved, pass a null string (e.g. “ “).

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passwd As String
Reserved, pass a null string (e.g. “ “).

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map (1 to n) As FieldMap
The destination FieldMap array to fill. It must contain the exact number of elements as the number of fields in the record.

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unicode As Integer
0 if strings are stored in record as ASCII. Otherwise, they are stored as Unicode.

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Integer
0 if okay. Otherwise, an error occurred.

StructToRow

Converts a Visual Basic UDT to a row of bytes

PALN32.DLL

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row (1 to n) As Byte
Output array to store packed data.

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fld (1 to n) As FieldMap
A FieldMap array used to determine data types of individual fields.

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udt As Any
The UDT from which to retrieve data.

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udtSize As Long
The size of the UDT. Generate this value using LenB( ).

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Integer
0 if okay. Otherwise, an error occurred.

1The field type FLD_UNICODE is used to specify a field of Visual Basic Type ‘String’ that is to be stored in UNICODE both within the database row ( packed structure ) as well as within the UDT (user-defined data type). If the field type FLD_STRING is used, it will be converted into the default ANSI code page character set of the system in the database row, although UNICODE will be used in the UDT (user-defined data type). In short, if you wish to store the string field in UNICODE in your Pervasive.SQL database, choose field type FLD_UNICODE. If you wish to store the string field in the database in the default ANSI code page character set of the system, choose FLD_STRING .