Tag Archives: compatibility

Coding Guidelines

Posted on by vivaclipper

Coding Guidelines

( by Greg Holmes )

Language Syntax 
The general rule of thumb is: built-in features in lowercase, and custom-written functions in mixed case. 
When specifying the complete syntax of a language element in documentation, the input items, parameters, and so on are referred to using the following symbols:

 Symbol  Description
< >  Indicates user input item
( )  Indicates function argument list
[ ]  Indicates optional item or list
{ }  Indicates code block or literal array
| |  Indicates code block argument list
–>  Indicates function return value
…  Repeated elements if followed by a symbol
Intervening code if followed by a keyword
,  Item list separator
|  Indicates two or more mutually exclusive options
@  Indicates that an item must be passed by reference
*  Indicates a compatibility command or function

For example:

    len(<cString>|<aArray>) --> nLength

Metasymbols provide a place holder for syntax elements, and they describe the expected data types. A metasymbol consists of one or more lowercase data type designators followed by a mixed case description. This is known as Hungarian Notation.

 Designator  Description
a  Array
b  Code block
c  Character expression
d  Date expression
exp  Expression of any type
id  Literal identifier
l  Logical expression
m  Memo field
n  Numeric expression
o  Object
x  Extended expression

In this example, dnLower and dnUpper can be either date or numeric:

    @...get...range <dnLower>, <dnUpper>
Filenames and Aliases 
All filenames, in any context, are in upper case. Filenames follow DOS naming conventions (preferably limited to letters, numbers, and the underscore).

    use CUSTOMER
    nHandle := fopen('DATAFILE.DAT')

When referring to specific file types in documentation, include the period.
e.g. “A program is stored in a text file with a .PRG extension.” 
Alias names follow the same conventions as filenames, but are limited to A-Z, 0-9, and the underscore. If a filename begins with a number or contains unusual characters, an alias must be specified when the file is opened or an error will result. 
Note that CA-Clipper does not natively support Windows 95 long filenames, although third-party libraries are available to add the capability.
Fieldnames 
Fieldnames are all uppercase, and always include the alias of the table. Fieldnames may contain underscores, but should not begin with one (because the underscore is generally used to indicate an internal symbol).

    @ 10, 10 say BANKS->BRANCH
    nAge := CUSTOMER->CUST_AGE
Memory Variables 
Memory variables consist of a lowercase type designator followed by a mixed case description (see Hungarian Notation). Although CA-Clipper only recognizes the first 10 characters as unique, variable names may be longer.

    cString := "Hello World"
    nYearlyAverage := CalcYearAvg()

If you use Hungarian Notation for your memory variable names and include the table alias with fieldnames, there will be no conflict between the two.
Commands, Functions, and Keywords 
All built-in commands, functions, and keywords are lowercase. In documentation, the font should be Courier or a similar font. If fonts are not available, then bold or CAPITALIZE the word for emphasis. 
Never use abbreviations — this practice is not necessary with a compiler, although it was common in the early days of dBase (which was an interpreter). 
There should never be a space between the function name and the opening parenthesis. Also, note that the iif() function should never be spelled if().

    replace CUSTOMER->CUSTNAME with cCustName
    nKey := inkey(0)

When specifying commands that have clauses in documentation, separate the keywords with an ellipsis (...) and do not include the to clause, unless it is followed by the file,print, or screen keywords.

    copy...sdf
    set message...center
    @...say...get
Programmer-Defined Functions & Procedures 
These begin with an uppercase letter, followed by mixed case letters as appropriate.

    ? StripBlanks("Hello there, this will have no spaces.")

Function and procedure names may contain underscores, but should not begin with one (they may conflict with internal functions which often start with an underscore). There should be only one return statement per function or procedure, and it should not be indented.

    function SomeFunc (...)
      .
      . <statements>
      .
    return cResult

The return value of a function is not enclosed in parentheses, although parentheses may be used to clarify a complex expression.

    return nValue
    return (nCode * 47) + nAnswer
Preprocessor Directives 
Preprocessor directives are lowercase and are preceded by the # sign.

    #include 'INKEY.CH'

Optionally, you may use single quotes around header files that come with CA-Clipper and double quotes around your own. This convention is purely voluntary, but it helps to distinguish between the two. For example:

    #include 'INKEY.CH'
    #include "MY_APP.CH"

Manifest constants are uppercase.

    #define ESCAPE   27
    if lastkey() == ESCAPE

Pseudo-function names should also be uppercase.

    #define AREA(length, width)   ((length)*(width))
Declarations 
Local variables are grouped according to functionality, and may be declared on one or more lines. The declarations appear as the first code at the beginning of a function or procedure.

    procedure Main ( )
    local nTop, nLeft, nBottom, nRight
    local cOldScreen, cOldColor, nOldCursor

Variables may be declared one per line and accompanied by a description.

    local nCount        // Number of records found.
    local nTotal        // Sum of dollars.

The description can be omitted if better variable names are chosen.

    local nRecordCount
    local nDollarTotal

Variables can be initialized when they are declared, although it is often clearer (and safer) to initialize them immediately before they are used.

    local nRecordCount:=0
    local nDollarTotal:=0
Logicals 
The .T. and .F. are typed in uppercase.
Operators 
The in-line assignment operator (:=) is used for all assignments, and the exact comparison operator (==) is used for all comparisons.

    lContinue := .T.
    nOfficeTotal := nRegionTotal := 0
    lDuplicate := (CUSTFILE->CUSTNAME == cCustName)
    if nLineCount == 4  ...
    if left(PRODUCT->CODE, 3) == left(cProdCode, 3)  ...

Although the compound assignment operators (+=, -=, *=, etc.) are convenient, they should not be used if readability suffers.

    // The traditional way to accumulate:
    nTotal := nTotal + INVDETAIL->PRICE
    // A good use of a compound assignment operator:
    nTotal += INVDETAIL->PRICE
    // But what does this do?
    nVal **= 2

The increment (++) and decrement (--) operators are convenient, but can lead to obscure code because of the difference between prefix and postfix usage.

    nRecCount++
    nY := nX-- - --nX        // Huh?
Spacing 
Whenever a list of two or more items is separated by commas, the commas are followed by a space.

    MyFunc(nChoice, 10, 20, .T.)

Spaces may be used between successive parentheses.

    DoCalc( (nItem > nTotal), .F. )
    cNewStr := iif( empty(cStr), cNewStr, cStr + chr(13) )

Spaces should surround all operators for readability.

    nValue := 14 + 5 - (6 / 4)

In declarations, often spaces are not used around the assignment operator. This tends to make searching for the declaration of a variable easier.

    local lResult:=.F., nX:=0

Thus, searching for “nX :=” would find the lines where an assignment is made, while searching for “nX:=” would find the declaration line (such as the local above).
Indentation 
Indenting control structures is one of the easiest techniques, yet it improves the readability the most. 
Indent control structures and the code within functions and procedures 3 spaces.

    procedure SaySomething
       do while .T.
          if nTotal < 50
             ? "Less than 50."
          elseif nTotal > 50
             ? "Greater than 50."
          else
             ? "Equal to 50."
          endif
          ...
       enddo
    return

Case statements in a do…case structure are also indented 3 spaces.

    do case
       case nChoice == 1
          ? "Choice is 1"
       case ...
          ...
       otherwise
          ...
    endcase
Tabs 
Do not use tabs in source code — insert spaces instead. Tabs cause problems when printing or when moving from one editor to another, because of the lack of a standard tab width between editors and printers. Typically, printers expand tabs to 8 spaces which easily causes nested control structures to fall off the right-hand side of the page. Commonly, a source code editing program will insert the appropriate number of spaces when the <TAB> key is hit.
Line Continuation 
When a line of code approaches the 80th column, interrupt the code at an appropriate spot with a semicolon and continue on the next line. Indent the line so that it lines up in a readable manner.

    set filter to CUSTFILE->NAME  == 'John Smith  ';
            .and. CUSTFILE->STATE == 'OR'

To continue a character string, end the first line with a quote and a plus sign and place the remainder on the next line. Try to choose a logical place in the string to break it, either at a punctuation mark or after a space.

    @ 10, 10 say "The lazy brown fox tripped over " + ;
                 "the broken branch."
Quotes 
Use double quotes for text that needs to be translated (will appear on the screen), and single quotes for other strings.

    ? "Hello World!"
    cColor := 'W+/B'
    SelectArea('PROP')

This is a simple but extremely effective technique because translation departments often want to see the messages in context (in the source code), so the different quote types indicate which messages are to be translated and which should be left alone.
Comments 
Comments are structured just like English sentences, with a capital letter at the beginning and a period at the end.

    // Just like a sentence.
    /* This comment is longer. As you
       can see, it takes up two lines */

You may encounter old-style comment indicators if you maintain older (Summer’87 and earlier) code.

    && This is an older-style of comment indicator.
    *  The asterisk is also old.

For in-line comments, use the double slashes.

    use CUSTOMER            // Open the data file.
    goto bottom             // The last record.

Note that the ‘//‘ of in-line comments begins at column 40, if possible. This leaves enough room for a useful comment.

Source :  http://www.ghservices.com/gregh/clipper/guide.htm

C5DG-7 DBFNTX Driver

Posted on by vivaclipper

Clipper 5.x – Drivers Guide

Chapter 7

DBFNTX Driver Installation and Usage

DBFNTX is the default RDD for Clipper. This new database driver replaces the DBFNTX database driver supplied with earlier versions of Clipper and adds a number of new indexing features. With DBFNTX, you can:

. Create conditional indexes by specifying a FOR condition

. Create indexes using a record scope or WHILE condition, allowing you to INDEX based on the order of another index

. Create both ascending and descending order indexes

. Specify an expression that is evaluated periodically during indexing in order to display an index progress indicator

In This Chapter 

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

. Overview of the DBFNTX RDD

. New Locking Scheme

. Conditional Indexing

. Installing DBFNTX Driver Files

. Linking the DBFNTX Driver

. Using the DBFNTX Driver

. Compatibility with dBASE III

Overview of the DBFNTX RDD

As an update of the default database driver, DBFNTX is linked into and used automatically by your application unless you compile using the /R option.

New Features

The replaceable driver lets you create and maintain (.ntx) files using features above and beyond those supplied with the previous DBFNTX driver. The new indexing features are supplied in the form of several syntactical additions to the INDEX and REINDEX commands. Specifically you can:

. Specify full record scoping and conditional filtering using the standard ALL, FOR, WHILE, NEXT, REST, and RECORD clauses

. Create an index while another controlling index is still active

. Monitor indexing as each record (or a specified record number interval) is processed using the EVAL and EVERY clauses

. Eliminate separate coding for descending order keys using the DESCENDING clause

Compatibility

Index files (.ntx) created with the original DBFNTX driver are compatible with DBFNTX and can be used in new applications without reindexing. Index files (.ntx) created with this version of DBFNTX will also work with previous Clipper applications provided that you use no FOR, WHILE, <scope>, or DESCENDING clauses.

Important! Indexes produced with DBFNTX using FOR or DESCENDING are incompatible with earlier version (.ntx) files. If you attempt to access them with the original DBFNTX database driver or programs compiled with versions earlier than Clipper 5.2, you will get an unrecoverable runtime error. In Clipper, this generates an “index corrupted” error message, causing the application to terminate.

New Locking Scheme

The DBFNTX database driver implements a new locking scheme to resolve several problems identified in previous versions of Clipper and to prevent potential problems that might arise when running Clipper applications in a network environment. This section discusses these changes and their implications, including compatibility issues.

Lock Time-outs

Problem: Index locking in previous versions of Clipper was handled automatically by the database driver, and had no time-out provision. This created the potential for problems in network environments if a workstation died while holding a lock. If this situation occurred all other workstations waiting for an index lock would appear to freeze while waiting to obtain their lock. This could also happen if a user placed a Clipper application in the background on a multitasking system without sufficient processing time allocated to it. Eventually, most network operation systems would clear a connection that had no activity for a specified period of time. This would free the lock and everything would resume as normal, but frustrated users may have rebooted their machines possibly causing file corruption.

Solution: In Clipper 5.2 the NTX driver will generate a recoverable runtime error if it fails to lock the index after a predetermined number of retries. The default error handler for this system simply returns (.T.) to retry the operation. This emulates the behavior of previous Clipper versions.

Error Handling

Time out handling: The handling of this error is problematic because the lock is issued from various internal index routines. Therefore the only safe recoveries are to retry or quit. Choosing to default from the error or issuing a break will more than likely leave the index in a corrupted state. If either of the options is employed, the application should immediately recreate the index. The preferred way to handle a time out such as this is to alert the user of the situation so they don’t think their machine has hung, and then have the network administrator determine what workstation is causing the problem. When the problem workstation is cleared, the users that have timed out can select retry and continue processing.

NTXERR.PRG: The file NTXERR.PRG contains the source code for the default error handler INIT procedure. This error handler can be modified to allow user-defined error handling for index lock time-outs. Care should be exercised when modifying the error handler as detailed above.

Compatibility: The lock time-out capability when used in conjunction with the default error handler is totally compatibility with previous versions of Clipper. No changes are made to the NTX file structure and no action is required by the developer to activate the time-out functionality.

New Lock Offset

Problem: Index locking, which is transparent to the developer, uses a single-byte semaphore locking system. This semaphore was placed at a virtual offset (beyond the physical end of file) in the index file. In previous versions of Clipper, this offset was located at one billion (1,000,000,000) which was adequate at the time. But many systems today are capable of producing indexes that are large enough to cause the actual data present at the lock offset to become physically locked. This leads to problems when trying to read or write to the data at that offset.

Solution: The solution is to move the offset where locking occurs to a location at a greater offset. We have chosen FFFFFFFF hex, which is the largest offset possible under the DOS operating system. The problem with this solution is that new applications using the index will be locking this new byte while old applications using the same index will lock the old position. Clearly this would cause both applications to fail because each could have a lock on the file at the same time.

To avoid this, the signature of the index (in the index header) is modified to prevent pre-Clipper 5.2 applications from being able to open the index. Clipper 5.2 applications can detect the correct offset to use by the flag in the header and will automatically use the correct one. In Figure 7-1 below, each bit represents a flag:

BIT  7 6 5 4 3 2 1 0
FLAG R R R O P I I C
R Reserved
I Index type - both bits set (NTX)
C Index created with a Condition, condition in header
T Created as a Temporary index
O New Offset for exclusive (semaphore) lock
Figure 7-1: Bit Field for the Signature Byte of a -Clipper 5.2 NTX File

Activation

If Clipper 5.2 automatically modified the signature in the header when it created indexes, programs with automatic reindexing routines would be creating indexes that appeared corrupt to pre-Clipper 5.2 applications. This has an obvious problem with backward compatibility. Therefore, in order to create indexes with the new signature, the developer must link in the module NTXLOCK2.OBJ with the full knowledge that this will create indexes that older applications will not be able to access.

Header Changes

The signature byte of a .NTX file is 6 for an unenhanced NTX index. The inclusion of the NTXLOCK2.OBJ will cause the signature to become 26 hex. (6 hex ORed with 20 hex). See Figure 7-1 for an illustration of all the possible values for the signature byte.

Error Handling

Clipper 5.2 applications will automatically recognize the signature byte of the header, and depending on the signature value, will use the correct index lock location. Applications built with previous versions of Clipper, however, do not have the capability to detect the optional new information in the signature byte. Therefore, when an order application tries to open a file that has been created with the NTXLOCK2.OBJ linked in it will produce a Corruption Detected error.

Compatibility

The new locking location, if used, is not backward compatible with applications compiled with previous versions of Clipper.

Indexes created by applications built with a previous version of Clipper can be used by Clipper 5.2 using the new location and will not be modified unless the index is recreated in application.

Since older applications have no knowledge of the new index locking scheme nor of the significance of the header signature, these applications will assume the index is corrupt and will produce an Index Corrupted error.

Conditional Indexing

Conditional indexes are a feature of the DBFNTX driver. This section discusses this feature of the DBFNTX driver in some detail, giving you specific information about the implementation of conditional indexes. Compatibility issues are also discussed.

Conditional Indexes

Conditional indexes are produced by using a FOR condition in the index creation process. These indexes are made fully maintainable by storing the FOR condition in the index header. This condition is subsequently retrieved and compiled each time the index in opened. During updates, items are added to the index only if they meet the criteria of the condition.

Since older applications do not have the ability to recognize and use the condition stored in the header, they must be prevented from opening the index since they corrupt the index. This is accomplished by modifying the signature of the index (in the index header) preventing pre-Clipper 5.2 applications from being able to open the index. Clipper 5.2 applications can detect the flag in the header and will automatically use the stored FOR condition correctly.

Temporary Indexes

Temporary indexes are produced by using any scoping clause other than the FOR condition in the index creation process. These indexes are not automatically maintainable because the condition is not stored for later use. These indexes can be made maintainable if the condition can be expressed as a FOR condition and is added using the FOR clause. But the main use of temporary indexes is for fast creation of indexes for read- only browses or reports that operate on a subset of the database.

Since older applications would not operate properly with indexes that do not contain all the keys in a given database, they must be prevented from using them. This is accomplished by modifying the index signature to prevent pre-Clipper 5.2 applications from being able to open the index.

Activation

Conditional Indexes

The developer need only specify the FOR condition when creating the index. In doing so he must be fully aware the index will no longer be accessible to pre–Clipper 5.2 applications.

Temporary Indexes

The developer need only specify a scope other than FOR when creating the index. In doing so he must be fully aware the index will no longer be accessible to pre-Clipper 5.2 applications and that the index created is not maintainable.

Header Changes

The signature byte of a .NTX file is 6 for a unenhanced NTX index. If the index is created as a conditional index it will have a signature of 7 hex (6 hex ORed with 1 hex). If the index is created as a temporary index it will have a signature of E hex. (6 hex ORed with 8 hex). See Figure 7-1 for an illustration of all the possible values for the signature byte.

Error Handling

Corruption Detected

Since older applications have no knowledge of the new index features nor how to interpret the additional flags in the header signature, these applications will assume the index is corrupt and will produce an Index Corrupted error.

EOF()

If an index is created with a FOR condition and an attempt is made to update the index with a key that does not match the condition, the update is suppressed and the index is placed at EOF(). This is consistent with the current behavior for indexes created with the unique flag when an update is attempted with a non-unique key.

Also if a navigational action is attempted (SKIP) and the current record is not found in the index, the index will place the record pointer at EOF(). This is true for both conditional and temporary indexes.

Compatibility

Backward Compatibility

If the conditional or temporary indexing features are used the index produced will not be backward compatible with applications compiled with previous versions of Clipper. Indexes that do not use the features, however, will be 100% compatible.

Forward Compatibility

Indexes created by applications built with a previous version of Clipper can be used by Clipper 5.2 and will not be modified unless the index is recreated using either the conditional or temporary index features.

Error Message Produced by Old Applications

Since older applications have no knowledge of the new index locking scheme nor of the significance of the header signature, these applications will assume the index is corrupt and will produce an Index Corrupted error.

Installing DBFNTX Driver Files

DBFNTX is supplied as the file DBFNTX.LIB.

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

Important! Before installing Clipper, you may want to rename the DBFNTX.LIB that currently resides in your \CLIPPER5\LIB directory to DBFNTX.001. The new version, when installed, will overwrite DBFNTX.LIB. If you do not rename or otherwise protect the old version of DBFNTX.LIB, you will lose it.

Linking the DBFNTX Database Driver

Since DBFNTX is the default database driver for Clipper, there are no special instructions for linking. Unless you specify the /R option when you compile, the new driver will be linked into each program automatically if you specify a USE command or DBUSEAREA() function without an explicit request for another database driver. The driver is also linked if you specify an INDEX or REINDEX command with any of the new features.

Using the DBFNTX Database Driver

In applications written for the new DBFNTX driver, you can use the INDEX and REINDEX commands exactly as you have used them in the past. The index files (.ntx) you create and maintain in this way are completely compatible with those created using previous versions of the driver.

Changes to existing code are necessary only if you use the new indexing features. The (.ntx) files you create using the new features will have a slightly different header file and cannot be used by programs linked with a previous version of the driver.

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

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

// (.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 (.ntx) and (.ndx) 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

The default DBFNTX driver makes Clipper programs behave differently than traditional dBASE programs. Some of these differences are discussed below.

Supported Data Types

The DBFNTX database driver supports the following dBASE III PLUS- compatible data types for key expressions:

. Character

. Numeric

. Date

. Logical

Supported Key Expressions

When you create (.ntx) files using the DBFNTX driver, you can use all Clipper or user-defined functions compatible with dBASE III PLUS as well as other functions accepted by the extended Clipper functionality.

Error Handling

The indexing behavior of DBFNTX and DBFNDX in a Clipper application is identical unless otherwise noted. With the default DBFNTX driver, you can handle most errors using BEGIN SEQUENCE…END SEQUENCE as illustrated in the next section.

FIND vs SEEK

In Clipper, you can use the FIND command only to locate keys in indexes where the index key expression is character data 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 DBFNTX driver lets you recover from data type errors 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 such SEEK or FIND data type errors. Within the error block for the current operation, issue a BREAK() using the error object that the DBFNTX 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

The DBFNTX driver provides file and record locking schemes that are different from dBASE III PLUS schemes. This means that if the same database and index files are open in Clipper and in 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. Therefore, concurrent use of the same database (.dbf) and index (.ndx) files by dBASE III PLUS and Clipper programs is strongly discouraged and not supported by Computer Associates.

Summary

In this chapter, you were given an overview of the new features of the default DBFNTX RDD. You learned how to this driver is automatically linked and how to use it in your applications, and were given an overview of the compatiblity issues.

Identifier length limit

Posted on by vivaclipper

What is variable and field name length limit in Harbour ?

Harbour support long field names without any problem.

TBrowse in Harbour also does not have any 10 character limit.

Variable and field names in Harbour can have up to 63 characters. This limit is defined in hbvmpub.h:

#define HB_SYMBOL_NAME_LEN 63

and can be changed to any bigger value if necessary.

All problems are result of DBASE III DBF format which has 10 character field name limit.

It’s not Harbour problem but DBF structure created many years ago which cannot be changed due to compatibility with other systems using such files.

If you want to use longer field names then you have to chose RDD which uses different low level storage format then DBASE III DBF and supports longer field names.

Przemek

(Source)