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C5DG-8 DBPX Driver

Posted on by vivaclipper

Clipper 5.x – Drivers Guide

Chapter 8

DBPX Driver Installation and Usage

DBPX is the Paradox 3.5 compatible RDD for Clipper. It connects to the low-level database management subsystem in the Clipper architecture. When you use the DBPX RDD, you add a number or features, including the ability to:

. Create access to and modify Paradox tables, records, and fields

. Create, select, and activate secondary indexes on Paradox tables

. Create and modify Paradox table structures, including primary index fields

. Use explicit record and file locks with concurrent execution of other Clipper applications

. Import Paradox tables directly into Clipper arrays

In This Chapter

This chapter explains how to install DBPX and how to use it in your applications. The following major topics are discussed:

. Overview of the DBPX RDD

. Installing DBPX Driver Files

. Linking the DBPX Driver

. Using the DBPX Driver

Overview of the DBPX RDD

The DBPX driver lets you create and maintain (.db), (.px), (.x??), and (.y??) files with features different from those supplied with the original DBFNTX driver and compatible with files created under Paradox 3.5. The new features are supplied in the form of several syntactical additions to database and indexing commands and functions. Specifically you can:

. Create tables that recognize the standard Clipper data types as well as Currency ($) and Short (S) numbers between -32,767 to +32,767

. Create equally efficient keyed and unkeyed tables

. Create, select, and activate secondary indexes on Paradox tables

The DBPX driver provides simple, seamless access to the Paradox database system. The Clipper application programmer who intends to access Paradox tables with the “VIA” clause need only include the RDD header file at compile time and make the appropriate libraries available at link time.

Paradox stores data in tables (known to Xbase developers as data files (.db)’s), consisting of fields and records. Unlike Xbase databases, a Paradox database refers to a group of files that are related to each other in some way, rather than to one file.

Also, Paradox employs the concept of companion files, known as objects, that are related to the table. Some examples of object files are report forms, indexes, and data entry forms. A table and its accompanying objects are referred to as a family.

It is easy to identify objects belonging to a particular family since they all have the same base filename and are distinguished by their extensions as shown in the table below.

Paradox File Descriptions
——————–  —————————————————
Extension         Object

.DB                       Table

.PX          Primary Index 
.X?? or Y??  Secondary Index
.F or F??    Data Entry Forms
.R or R??    Report Formats
.G or G??    Graph Specifications
.SET         Image Settings
.VAL         Field Validity Specifications
------------ ------------------------------------------------------------

The DBPX driver only deals with the table and index files (.db, .px, .x?? and y??) so only these files are discussed here.

Though Paradox tables are limited to 8 character filenames, each table can contain an unlimited number of records in files up to 256M in size. Paradox records in nonkeyed tables can be up to 4000 bytes each while keyed tables have a 1350 byte limitation. Each record can contain up to 255 fields of up to 255 characters each.

There are some field naming restrictions you must observe. Field names may:

. Although the Paradox file structure allows fields to be up to 25 characters long, since Clipper symbols can only be 10 characters, DBPX truncates the field name to 10 characters.

. The Paradox file structure allows embedded spaces in field names. Since this is illegal in Clipper, the DBPX driver converts spaces into underscores (_).

. Not be duplicated in the same table.

Also, most Paradox data types directly match data types in standard Xbase data files, with these differences:

. Paradox tables support both the Numeric (N) data type as well as a more specific Currency ($) data type. Both the N and $ data types can have 15 significant digits. Numeric types that exceed this length are rounded and stored as scientific notation. Also, DBPX supports the Short (S) data type to represent numbers between -32,767 and +32,767.

. The Alphanumeric field type allows all ASCII characters except embedded nulls (ASCII 0). The Alphanumeric type is identical to the Character (C) data type in Xbase. Paradox limits this field type to 255 characters.

. Paradox also supports a Date (D) field type, stored as a long integer. It can contain any value between January 1, 100 A.D. and December 31, 9999.

Installing DBPX Driver Files

The DBPX RDD is supplied as the file, DBPX.LIB:

The Clipper installation program installs this driver in the \CLIPPER5\LIB subdirectory on the drive that you specify, so you need not install the driver manually.

Linking the DBPX Database Driver

To link the DBPX driver, you must specify DBPX.LIB to the linker along with your application object (.OBJ) modules.

1. To link with .RTLink using positional syntax:

C>RTLINK <appObjectList> ,,,DBPX

2. To link with .RTLink using freeformat syntax:

C>RTLINK FI <appObjectList> LIB DBPX

Note: These link commands all assume the LIB, OBJ, and PLL environment variables are set to the standard locations. They also assume that the Clipper programs were compiled without the /R option.

Using the DBPX Database Driver

To use Paradox files in a Clipper program:

1. Place REQUEST DBPX at the top of each program file (.prg) that opens a database file using the DBPX driver.

2. Specify the VIA “DBPX” clause if you open the database file with the USE command.

-OR-

3. Specify “DBPX” for the <cDriver> argument if you open the database file with the DBUSEAREA() function.

-OR-

4. Use RDDSETDEFAULT( “DBPX” ) to set the default driver to DBPX.

Except in the case of REQUEST, the RDD name must be a literal character string or a variable. In all cases it is important that the driver name be spelled correctly using uppercase letters.

The following program fragments illustrate:

REQUEST DBPX 
. 
. 
. 
USE Customers INDEX Name, Address NEW VIA "DBPX"
-OR-
REQUEST DBPX RDDSETDEFAULT( "DBPX" )
.
. 
. 
. 
USE Customers INDEX Name, Address NEW

Index Management 

The greatest variation from the standard Xbase database design in Paradox tables is index management. As in other systems, Paradox indexes are an efficient method of dynamically sorting or locating specific data within a table without forcing a search of all data in that table. Paradox tables take two forms: unkeyed and keyed.

An unkeyed table has no fields in its structure that have been identified as specific index keys. Therefore, records are maintained in natural order. New records are added to the end of an existing table, and the unique identity for each record is a record number.

Unlike Xbase data files, unkeyed tables are not more efficient in design or faster to traverse than keyed tables. This is because Paradox tables are built as linked lists rather than fixed-length, sequential tables. Therefore, it is actually less efficient to SKIP through a unkeyed table than it is through a keyed table.

A keyed table, on the other hand, can be lightning fast as long as the data you seek is part of the key. Otherwise, just as in an unkeyed table, you are forced to do a sequential search through the table’s data fields.

Paradox tables support two types of keys or indexes.

. Primary

. Secondary

Primary Indexes

Primary indexes are directly tied to keyed tables because a primary index indicates the table is keyed. Simply, it is impossible to have a keyed table without a primary index. If you remove the primary index from a keyed table it becomes an unkeyed table.

When you identify one or more of the table’s fields as a key field (by placing an asterisk (*) at the end of the field name) during table creation/restructuring, these fields are used to create a primary index. (Note that all key fields must be together as the first fields in a table). This invisibly rebuilds the table’s structure, though in operation it only seems to change or create the key index.

Once you identify this primary key, the table is automatically maintained in the key field order and all new records are checked to make sure that no duplicate keys are added to the table. This type of index is called a unique key index. You may have only one primary key per table, but this key can be a composite of many fields in the table. You may only modify by restructuring the table.

If it is necessary to change a primary key and restructure a table, all data in the table will still be bound to the unique key restriction. This is important if you change the primary key by adding a new field to it and there is already data in the table where this new composite key would have duplicates.

DBPX handles this situation by generating a runtime error and removing every record that violates this unique key and moving it to another table named KEYVIOL.db which has the identical structure of the offending table.

The KEYVIOL.db is automatically generated whenever this situation occurs. If there is already a KEYVIOL table, it is overwritten. Because of this you should always check for the existence of a KEYVIOL.db table immediately after any type of table restructuring.

Secondary Indexes

Secondary indexes are more like common Xbase-type indexes because they can be generated or modified on the fly without having any effect on the data or table structure and aren’t restricted to unique key data.

Unlike Xbase indexes, secondary indexes can only contain a single field as their key. As mentioned earlier, primary indexes are automatically maintained so that they are always up to date. Secondary indexes are created in two different types.

. Incremental (for keyed tables)

. Independent (for unkeyed tables)

Independent indexes are created only for unkeyed tables and are not dynamically maintained in any way. Because of this they can only be considered accurate at the time of their creation. If data changes inside the table that affects the index, the index must be completely regenerated before it can be considered useful again.

Alternately, incremental indexes are created only for keyed tables and are automatically maintained similarly to primary indexes except that instead of a complete rebuild at every change, only the portion of the index affected is updated. Incremental indexes are preferable when you are handling large tables since they take considerably less time and energy to keep accurate.

Temporary Indexes

ALL, NEXT, RECORD, and REST are all supported in the scoping expressions. The syntax of these keywords is identical to that used in Clipper. Note that you can only use one scope keyword at a time. If more than one of these keywords is encountered in a scoping expression, then the last keyword in the expression is the option used.

The ALL keyword (default) specifies that all records in the table should be included in the operation, starting at the first record.

NEXT processes the specified number of records, starting with the current record. For example, NEXT 5 would process the current record and the four records following it.

The RECORD keyword identifies a specific record to process. The desired record number should follow the keyword RECORD. To process record number 3, you would include “RECORD 3” in the expression.

The REST keyword causes processing to begin with the current record, instead of starting at the beginning of the table.

Sorting

In the event that you want to reorder a table based on field data but don’t need or want to have an index attached to it, you have the option of sorting the table based on the current index. This entails a simple copy from a keyed table to an unkeyed table using the table sort function.

Passwords and Security

Although the Paradox DBMS cannot be considered a data dictionary system, it does have some special characteristics that make it more suitable to networks than the standard Xbase tables. One of these features is the level of security available.

There are two methods to make sure that your data is secure: master passwords and auxiliary passwords. As the owner of a table, you can limit access by attaching a master password to it. Auxiliary passwords can also be identified to establish access to the table and its family.

Once any type of password is identified for a table, its is encrypted. This protects it not only from unauthorized Paradox users but also from anyone trying to dissect it at the DOS file level. The encryption method used by Paradox is literally unbreakable and if you (or your users) forget a table password, there is no way to recover that information.

Auxiliary passwords allow access control at the table and field levels. Access to tables can be restricted to:

. ReadOnly: No changes to the table can be made

. Update: Changes to nonkey fields are allowed, no records can be added or deleted

. Entry: Same as update except that new records can be added

. InsertDelete: Same as Entry except that records can be inserted and deleted

. All: Full access including restructuring and table deletion

Access to the fields can be identified as:

. None: This field data cannot be displayed to the user

. ReadOnly: User can see the field value, but cannot change it

. All: Full access

With DBPX, you may perform basic database operations on Paradox tables without code changes.

Note that because Paradox tables can have primary indexes which are actually part of the table structure specification, when you open a Paradox table, its associated primary index (if applicable) is also opened and activated. The only exception to this rule is if you indicate that you want a secondary index to be activated at the time you open the table. If no primary index is available and no secondary index is specified, the table is opened in natural sequence order.

You can have up to twenty-four Paradox tables open simultaneously. These may be separate tables or the same table repeatedly or any variation in between. This might be important if you want to have more than one secondary index active for a single table, allowing you to move from one work area to another with the only change being the index order of the data in the table. Be careful with this type of multiviewed approach, however, since you will be eating up memory for each work area, despite the fact that they refer to the same table.

Sharing Data in Networks

The DBPX driver supports the native Clipper single-lock locking scheme. Therefore, in a shared environment, your application and Paradox will not see each other’s record locks. This may result in some concurrency corruption and errors.

In a shared environment, DBPX performs no record buffering; immediately writing all changes to disk.

Concurrency is an issue whenever your application is running either on a network or in some other shared environment. One example of a non- network shared environment is when your application is called from another program (like Paradox, Quatro Pro, etc.) that also has access to the Paradox tables. Even if you don’t have any plans to use your program on a network, you should design it to be smart enough not to become a problem if faced with this type of shared example.

Also be aware that many networks have different rights and privilege restrictions and you should know what they are and how to handle them.

Using (.px) and (.ntx) Files Concurrently

You can use both (.px), as well as (.x), (.y) and (.ntx) files concurrently in a Clipper program like this:

REQUEST DBPX
// (.ntx) file using default DBFNTX driver
USE File1 INDEX File1 NEW
// (.idx) files using DBPX driver
USE File2 VIA "DBPX" INDEX File2 NEW

Note, however, that you cannot use (.px) and (.ntx) files in the same work area. For example, the following does not work:

USE File1 VIA "DBFNTX" INDEX File1.ntx, File2.px

Summary

In this chapter, you were given an overview of the features and benefits of the DBPX RDD. You also learned how to link this driver and how to use it in your applications.

C5DG-6 DBFNDX Driver

Posted on by vivaclipper

Clipper 5.x – Drivers Guide

Chapter 6

DBFNDX Driver Installation and Usage

DBFNDX is the dBASE III PLUS compatible RDD for Clipper. The DBFNDX driver uses the Clipper driver architecture to access dBASE III PLUS compatible index files within a Clipper program.

In This Chapter

This chapter explains how to install DBFNDX and how to use it in your applications. The following major topics are discussed:

. Overview of the DBFNDX RDD

. Installing DBFNDX Driver Files

. Linking the DBFNDX Driver

. Using the DBFNDX Driver

. Compatibility with dBASE III PLUS

Overview of the DBFNDX RDD

The DBFNDX database driver allows creation, access, and updating of dBASE III and dBASE III PLUS compatible index (.ndx) files. Index files (.ndx) created with Clipper are exactly the same as those created by dBASE III PLUS. All operations that can be performed on standard Clipper index (.ntx) files can be performed on (.ndx) files using the DBFNDX database driver.

In a network environment, the DBFNDX driver supports the Clipper file and record locking scheme. The multiuser behavior is the same as the default DBFNTX driver. This means that the DBFNDX database driver supports concurrent access to (.ndx) files between Clipper applications only. Concurrent access to (.ndx) files between dBASE III PLUS and Clipper programs is not supported.

Important! Updating database (.dbf) and index (.ndx) files shared between dBASE III PLUS and Clipper programs may corrupt the (.dbf) and any of its associated (.ndx) files.

Installing DBFNDX Driver Files

The DBFNDX database driver is supplied as the file, DBFNDX.LIB.

The Clipper installation program installs this driver in the \CLIPPER5\LIB subdirectory on the drive that you specify, so you need not install the driver manually.

Linking the DBFNDX Database Driver

To link the DBFNDX database driver into an application program, you must specify DBFNDX.LIB to the linker in addition to your application object files (.OBJ).

1. To link with .RTLink using positional syntax:

C>RTLINK <appObjectList> ,,, DBFNDX

2. To link with .RTLink using freeformat syntax:

C>RTLINK FI <appObjectList> LIB DBFNDX

Note: These link commands all assume the LIB, OBJ, and PLL environment variables are set to the standard locations. They also assume that the Clipper programs were compiled without the /R option.

Using the DBFNDX Database Driver

To use (.ndx) files in a Clipper program:

1. Place a REQUEST DBFNDX at the beginning of your application or at the top of the first program file (.prg) that opens a database file using the DBFNDX driver.

2. Specify the VIA “DBFNDX” clause if you open the database file with the USE command.

-OR-

3. Specify “DBFNDX” for the <cDriver> argument if you open the database file with the DBUSEAREA() function.

-OR-

4. Use RDDSETDEFAULT(“DBFNDX”) to set the default driver to DBFNDX.

Except in the case of REQUEST, the RDD name must be a literal character string or a variable. In all cases it is important that the driver name be spelled correctly.

The following program fragments illustrate:

REQUEST DBFNDX 
. 
. 
. 
USE Customers INDEX Name, Address NEW VIA "DBFNDX"

-OR-

REQUEST DBFNDX RDDSETDEFAULT( "DBFNDX" ) 
.
.
. 
USE Customers INDEX Name, Address NEW

Using (.ntx) and (.ndx) Files Concurrently

You can use (.ndx) and (.ntx) files concurrently in a Clipper program like this:

REQUEST DBFNDX

// (.ntx) file using default DBFNTX driver USE File1 INDEX File1 NEW

// (.ndx) files using DBFNDX driver USE File2 VIA “DBFNDX” INDEX File2 NEW

Note, however, that you cannot use (.ndx) and (.ntx) files in the same work area. For example, the following does not work:

USE File1 VIA "DBFNDX" INDEX File1.ntx, File2.ndx

Compatibility with dBASE III PLUS

When accessing dBASE III PLUS (.ndx) files, there are several compatibility issues of which you must be aware. These issues are discussed below.

Supported Data Types

The DBFNDX database driver supports the following data types for key expressions:

. Character

. Numeric

. Date

This is consistent with dBASE III PLUS.

The DBFNDX database driver does not support indexing with logical key expressions as does the default DBFNTX database driver. This is actually a dBASE III PLUS limitation and is not supported by the DBFNDX driver in order to enforce compatibility with dBASE III PLUS.

To work around this limitation, index logical values by converting them to character values like this:

INDEX ON IIF(<lExp>, "T", "F") TO <logicalIndex>

Supported Key Expressions

When you create (.ndx) files using the DBFNDX driver, you must use only Clipper or user-defined functions compatible with dBASE III PLUS. Use of the other functions will render the (.ndx) file unreadable in dBASE III PLUS.

FIND vs SEEK

In Clipper, you can use the FIND command only to locate keys in indexes where the index key expression is character type. This differs from dBASE III PLUS where FIND supports character and numeric key values.

Note: In Clipper programs, always use the SEEK command or the DBSEEK() function to search an index for a key value.

The DBFNDX driver lets you recover from a data type error raised during a FIND or SEEK. However, since Error:canDefault, Error:canRetry, or Error:canSubstitute are set to false (.F.), you should use BEGIN SEQUENCE…END to handle a SEEK or FIND data type error. Within the error block for the current operation, issue a BREAK() using the error object the DBFNDX database driver generates, like this:

bOld := ERRORBLOCK({|oError| BREAK(oError)})
.
.
 .
BEGIN SEQUENCE
SEEK xVar
RECOVER USING oError
// Recovery code
END
.
.
 .
ERRORBLOCK(bOld)

There is an extensive discussion of the effective use of the Clipper error system in the Error Handling Strategies chapter of the Programming and Utilities guide.

Sharing Data on a Network

As mentioned above, the DBFNDX driver does not support dBASE III PLUS file and record locking schemes. Instead, the DBFNDX driver supports the DBFNTX file and record locking scheme. This means that if the same database and index files are open in Clipper and dBASE III PLUS, Clipper program locks are not visible to dBASE III PLUS and vice versa.

Warning! Database integrity is not guaranteed and index corruption will occur if Clipper and dBASE III PLUS programs attempt to write to a database or index file at the same time. For this reason, concurrent use of the same database (.dbf) and index (.ndx) files by dBASE III PLUS and Clipper programs is strongly discouraged and not supported.

Compatibility with dBASE IV

Specific compatibility with dBASE IV is provided through the DBFMDX driver. It includes (.dbf), (.mdx), and (.dbt) file format compatibility and is described in detail in the previous chapter.

Summary

In this chapter, you were given an overview of the features and benefits of the DBFNDX RDD. You learned how to link this driver and how to use it in your applications, and were given an overview of the compatibility issues.

C5DG-4 DBFCDX Driver

Posted on by vivaclipper

Clipper 5.x – Drivers Guide

Chapter 4

DBFCDX Driver Installation and Usage

DBFCDX is the FoxPro 2 compatible RDD for Clipper. As such, it connects to the low-level database management subsystem in the Clipper architecture. When you use the DBFCDX RDD, you add a number of new features including:

. FoxPro 2 file format compatibility

. Compact indexes

. Compound indexes

. Conditional indexes

. Memo files smaller than DBFNTX format

In This Chapter

This chapter explains how to install DBFCDX and how to use it in your applications. The following major topics are discussed:

. Overview of the DBFCDX RDD

. Installing DBFCDX Driver Files

. Linking the DBFCDX Driver

. Using the DBFCDX Driver

Overview of the DBFCDX RDD

The DBFCDX driver lets you create and maintain (.cdx) and (.idx) files with features different from those supplied with the original DBFNTX driver and is compatible with files created under FoxPro 2. The new features are supplied in the form of several syntactical additions to the INDEX and REINDEX commands. Specifically, you can:

. Create indexes smaller than those created with the DBFNTX
driver. The key data is stored in a compressed format that
substantially reduces the size of the index file.

. Create a compound index file that contains multiple indexes
(TAGs), making it possible to open several indexes under one file
handle. A single (.cdx) file may contain up to 99 index keys.

. Create conditional indexes (FOR / WHILE / REST / NEXT).

. Create files with FoxPro 2 file format compatibility.

Compact Indexes

Like FoxPro 2, The DBFCDX driver creates compact indexes. This means that the key data is stored in a compressed format, resulting in a substantial size reduction in the index file. Compact indexes store only the actual data for the index keys. Trailing blanks and duplicate bytes between keys are stored in one or two bytes. This allows considerable space savings in indexes with much empty space and similar keys. Since the amount of compression is dependent on many variables, including the number of unique keys in an index, the exact amount of compression is impossible to predetermine.

Compound Indexes

A compound index is an index file that contains multiple indexes (called tags). Compound indexes (.cdx)’s make several indexes available to your application while only using one file handle. Therefore, you can overcome the Clipper index file limit of 15. A compound index can have as many as 99 tags, but the practical limit is around 50. Once you open a compound index, all the tags in the file are automatically updated as the records are changed.

Once you open a compound index, all the tags contained in the file are automatically updated as the records are changed. A tag in a compound index is essentially identical to an individual index (.idx) and supports all the same features. The first tag (in order of creation) in the compound index is, by default, the controlling index.

Conditional Indexes

The DBFCDX driver can create indexes with a built-in FOR clause. These are conditional indexes in which the condition can be any expression, including a user-defined function. As the database is updated, only records that match the index condition are added to the index, and records that satisfied the condition before, but don’t any longer, are automatically removed.

Expanded control over conditional indexing is supported with the revised INDEX and REINDEX command options as in the new DBFNTX driver.

Installing DBFCDX Driver Files

The DBFCDX driver is supplied as the file, DBFCDX.LIB.

The Clipper installation program installs this driver in the \CLIPPER5\LIB subdirectory on the drive that you specify, so you need not install the driver manually.

Linking the DBFCDX Database Driver

To link the DBFCDX database driver into an application program, you must specify DBFCDX.LIB to the linker in addition to your application object files (.OBJ).

1. To link with .RTLink using positional syntax:

C>RTLINK <appObjectList> ,,,DBFCDX

2. To link with .RTLink using freeformat syntax:

C>RTLINK FI <appObjectList> LIB DBFCDX

Note: These link commands all assume the LIB, OBJ, and PLL environment variables are set to the standard locations. They also assume that the Clipper programs were compiled without the /R option.

Using the DBFCDX Database Driver

To use FoxPro 2 files in a Clipper program:

1. Place REQUEST DBFCDX at the beginning of your application or at the top of the first program file       (.prg) that opens a database file using the DBFCDX driver.

2. Specify the VIA “DBFCDX” clause if you open the database file with the USE command.

    -OR-

3. Specify “DBFCDX” for the <cDriver> argument if you open the database file with the DBUSEAREA()       function.

   -OR-

4. Use ( “DBFCDX” ) to set the default driver to DBFCDX.

    Except in the case of REQUEST, the RDD name must be a literal character string or a variable. In all       cases it is important that the driver name be spelled correctly.

The following program fragments illustrate:

  REQUEST DBFCDX
  .
  .
  .
  USE Customers INDEX Name, Address NEW VIA "DBFCDX"

     -OR-

  REQUEST DBFCDX
  RDDSETDEFAULT( "DBFCDX" ) .
  . 
  .
  USE Customers INDEX Name, Address NEW

Using (.idx) and (.ntx) Files Concurrently

You can use both (.idx) and (.ntx) files concurrently in a Clipper program like this:

// (.ntx) file using default DBFNTX driver
 USE File1 INDEX File1 NEW
// (.idx) files using DBFCDX driver
 USE File2 VIA "DBFCDX" INDEX File2 NEW

Note, however, that you cannot use (.idx) and (.ntx) files in the same work area. For example, the following does not work:

USE File1 VIA "DBFNTX" INDEX File1.ntx, File2.idx

Using (.cdx) and (.idx) Files Concurrently

You may use (.cdx) with (.idx) files concurrently (even in the same work area); however, in most cases it is easier to use a single (.cdx) index for each database file or separate (.idx) files. When using both types of index at the same time, attempting to select an Order based on its Order Number can be confusing and will become difficult to maintain.

File Maintenance under DBFCDX

When an existing tag in a compound index (.cdx) is rebuilt using INDEX ON…TAG… the space used by the original tag is not automatically reclaimed. Instead, the new tag is added to the end of the file, increasing file size.

You can use the REINDEX command to “pack” the index file. REINDEX rebuilds each tag, eliminating any unused space in the file.

If you rebuild your indexes on a regular basis, you should either delete your (.cdx) files before rebuilding the tags or use the REINDEX command to rebuild them instead.

DBFCDX and Memo Files

The DBFCDX driver uses FoxPro compatible memo (.fpt) files to store data for memo fields. These memo files have a default block size of 64 bytes rather than the 512 byte default for (.dbt) files.

DBFCDX memo files can store any type of data. While (.dbt) files use an end of file marker (ASCII 26) at the end of a memo entry, (.fpt) files store the length of the entry. This not only eliminates the problems normally encountered with storing binary data in a memo field but also speeds up memo field access since the data need not be scanned to determine the length.

Tips For Using DBFCDX

1. Make sure index extensions aren’t hard-coded in your application. The default extension for DBFCDX indexes is (.idx), not (.ntx). You can still use (.ntx) as the extension as long as you specify the extension when you create your indexes. The best way to determine index extensions in an application is to call ORDBAGEXT().

For example, if you currently use the following code to determine the existence of an index file:

IF .NOT. FILE("index.ntx")
    INDEX ON field TO index
ENDIF

Change the code to include the INDEXEXT() function, as follows:

IF .NOT. FILE("index"+ORDBAGEXT())
   INDEX ON field TO index
ENDIF

2. If your application uses memo fields, you should convert your (.dbt) files to (.fpt) files.

There are some good reasons for using (.fpt) files. Most important is the smaller block size (64 bytes). Clipper’s (.dbt) files use a fixed block size of 512 bytes which means that every time you store even 1 byte in a memo field Clipper uses 512 bytes to store it. If the data in a memo field grows to 513 bytes, then two blocks are required.

When creating (.fpt) files, the block size is set at 64 bytes to optimize it for your needs. A simple conversion from (.dbt) files to (.fpt) files will generally shrink your memo files by approximately 30%.

3. Add DBFCDX.LIB as a library to your link command or link script.

Summary

In this chapter, you were given an overview of the features and benefits of the DBFCDX RDD. You also learned how to link this driver and how to use it in your applications.