Introduction
This section contains functions that make many date computations easier
to perform. Date-type values have been a feature of the xBASE language
standard, and is preserved in Clipper. In spite of this, it is
always necessary to develop UDFs to execute certain technical financial
computations. Clipper Tools offers an alternative that not only
spares you the work of writing a UDF, but also provides implementation
in Assembler to guarantee fast execution.
This chapter also contains clock time functions. These include time
changes, time span computations, time control of loops, and memory-
resident clock display.
Please note that valid date tests are already carried out in Clipper.
Clipper internal date tests cannot be canceled, so it is impossible
to implement an individual UDF for error handling , if there is an
invalid date.
Clipper always replaces an invalid date with a null date. These
internal tests are carried out by keyboard input, as well as by CTOD()
allocation. For this reason, there are no examples with invalid dates
in the date function descriptions. Rather, the result of a function
with a null date is shown.
Daily Archives: July 1, 2013
Introduction Printer Functions
Introduction
This chapter discusses printer output. Although Clipper handles
printer errors with an error trap, it is always better to avoid them
completely. As a result, error traps for printer output will not become
superfluous, since errors also occur during output. You can also
determine a wide variety of information, such as the number of available
printers or the printer status.
This chapter also describes functions that support DOS print spoolers.
To implement these functions, prior to starting the respective
application, you must run the DOS PRINT program to make the print
spooler memory resident. Files can then be exclusively passed from a
program to the spooler. However, you must first reroute your print
output to a file (SET PRINTER TO <file>), before you can output it in
the background.
Note:
Since this section is about DOS-TEXT mode programming, considered as
obsolete and details skipped.
Introduction Disk Utilities
Fundamental to the disk functions
Fault tolerance and more
One of the biggest problems in stable application development is how to
handle errors. Errors that occur during hard or floppy disk operations
are especially frustrating. You may also encounter critical errors,
which disable the program or operating system. The result, known to all
Clipper programmers, are the DOS error choices: (A)bort, (R)etry,
(I)gnore.
Avoiding Errors
Error trap functions allow you to react to this type of error, but a
strategy to avoid them completely is better. Therefore, Clipper
Tools includes functions to make it easier to handle floppy and hard
disk errors.
Backup Systems
Backup systems, another component of the Clipper application, are
also discussed in this chapter. The recursive FILESEEK() system, which
allows you to format disks with DISKFORMAT(), is particularly useful.
Complete directories can be scanned recursively, and queries regarding
detailed information for every file can be made. The ideal time to
carry out this process is during source and target drive back ups,
because only the information that has changed is copied.
To avoid the annoying and not particularly fault-tolerant "RUN FORMAT",
you can format the file disks directly from the application in all
commonly available formats. Since the control UDF for DISKFORMAT() uses
a concept similar to DBEDIT(), you can display the format procedures on
the screen as desired.
You can also determine a good deal of varied information about disk
drives, files, and other things. This information far exceeds that of
the FILESEEK() system.
File Attributes
Following is the coding for each function where a file can be
designated:
Table 1:
------------------------------------------------------------------------
Value Symb. constants Assigned attribute
------------------------------------------------------------------------
0 FA_NORMAL
1 FA_READONLY READ ONLY
2 FA_HIDDEN HIDDEN
4 FA_SYSTEM SYSTEM
8 FA_VOLUME VOLUME
16 FA_DIRECTORY DIR
32 FA_ARCHIVE ARCHIVE
------------------------------------------------------------------------
If multiple attributes are implemented for a file, then the table
values are added accordingly. For example, if the HIDDEN and SYSTEM
attributes are implemented, the function must pass a 6 (2 + 4) as the
attribute mask.
All file attributes do not behave the same. To initiate a file into the
process, you must explicitly specify the HIDDEN, SYSTEM, VOLUME and DIR
attributes. However, if either no attribute, R/O, or ARCHIVE is
implemented with a file, it does not matter which value is passed.
These rules for attribute handling are grounded in DOS, which compares
the specified value with the actual file attributes in this way. Since
in some circumstances this may lead to problems, the FILESEEK() function
allows you to switch on an additional EXACT ATTRIBUTE MATCHING.
Share Modes
For DOS version 3.0 or higher, if a function uses the DOS OPEN FILE call
internally, it must pass a share mode. This way, one station determines
how all other stations in the network can access the open file. This is
valid for as long as the file remains open. The following variations
exist:
Table 3: Share Mode
------------------------------------------------------------------------
Code Symb. constants Share Modes
------------------------------------------------------------------------
0 SHARE_COMPAT Compatibility mode. Here, DOS or the
network software itself, determines the
share mode. In Novell networks, the
SHAREABLE attribute plays an important role
in this relationship.
1 SHARE_EXCLUSIVE Both read and write by other programs are
locked out
2 SHARE_DENYWRITE Write by other programs are locked out
3 SHARE_DENYREAD Read by other programs are locked out
4 SHARE_DENYNONE Not locked. Read and write by other
programs is allowed
------------------------------------------------------------------------
With the basic setting, all files are opened in the compatibility mode
(relating to older DOS). As a rule, the share mode depends on file
attributes or the network software setting.
Also in this chapter is the SETSHARE() function, which determines the
share mode that enables all Clipper Tools functions to open a file.
Clipper commands or functions are not affected in any way.
CSETSAFETY()
Use the CSETSAFETY() function as a safety switch to protect existing
files from unwanted overwriting during file operations with Clipper
Tools functions. You will find this function in the chapter on Switches
and Status Information, since it concerns all Clipper Tools file
operations.
Introduction Video Functions
Introduction
Working With Video Functions
This module discusses video functions that are either directly or
indirectly concerned with the screen. Included are all functions that
deal with such screen adapters as CGA, EGA and Hercules. Different
modes on various adapters are supported, such as a 40-column CGA, a 43-
line EGA, or a 50-line VGA.
DSETWINDOW()
Many of the functions in this chapter depend on setting the DSETWINDOW()
switch. This switch determines if the screen output of external
programs, as well as DOS, are redirected to a window.
Clipper Tools output functions are also valid as external. If the
default setting in DSETWINDOW() is .T. or on, this rerouting is carried
out. Some functions will then return inaccurate, or at the very least,
different results. Examples of this are ISANSI() or NUMCOL()
Attribute
Many of the functions in this module work with color attributes
designated as parameters. These arguments are carried out in three
different ways:
A numeric value, which corresponds to a combined color attribute
(e.g., 7).
A string in the "NN/NN" form, with two specified numeric values (e.g.,
"7/0").
A string in the "CC/CC" form, with two specified attributes are
designated in the form Clipper requires (e.g., "W/N").
With many functions, the attribute returns a combined numeric value.
Attributes for the foreground and background are tied together this way.
Color attributes are constructed as follows:
Bit 8 7 6 5 4 3 2 1
Attribute * R G B + R G B
_ _ _ _ _ _ / _ _ _ _ _ _
Background / Foreground
Each attribute consists of four bits, which represent a value in the
range of 0 to 15. Therefore, there are a total of 16*16, or 256,
different values from 0 to 255. These numeric values can be changed
into the "nn/nn" format, that can be used under Clipper with the
NTOCOLOR function. However, it is possible to directly influence this
combined attribute value. The following examples show this and relate
back to the chapter on number and bit manipulation:
NUMOR( nattr, 128) // Flashing on
NUMAND(nattr, 127) // Flashing off
NUMXOR(nattr, 128) // Change flashing
NUMOR(nattr, 8) // High intensity on
NUMAND(nattr, 247) // High intensity off
NUMXOR(nattr, 8) // Switch high intensity
Special Parameter Type
With many functions, a parameter may be of the <mIcCharacter|nCharacter>
type (e.g., an individual character). This can occur in two different
ways:
Numerically, as the ASCII code of the desired character (e.g., 7).
Alphanumeric as the character (e.g., ":").
Because of this, you must not use the CHR() function to change special
characters.
CLEARA and CLEARB
Some of the Clipper Tools functions use a standard attribute and
character to delete lines or screen areas. This attribute is described
as CLEARA; the character as CLEARB. You can query both CLEARA and
CLEARB with the corresponding functions. At the same time, certain
preset values are in effect for CLEARA and CLEARB. The attribute "W/N"
is the standard preset for CLEARA, while CHR(255) is the character used
for CLEARB. If you use this character for CLEARB, it fills the
background with the corresponding color for every deletion on every
screen adapter.
Use the following functions to set standard values for CLEARA and
CLEARB:
GETCLEARA()
SETCLEARA()
GETCLEARB()
SETCLEARB()
If you use the SETCLEARx() functions in conjunction with the
corresponding parameters (<Attribute> or <Character>), the currently
existing default value is replaced by a parameter.
Clipper Functions and Commands Which Delete
If you use the Clipper Extended Drivers, CHR(255) is used instead of
a space for all Clipper functions and commands that delete the screen
in one way or another (see CLEARB). A CHR(255) is helpful on many
screen adapters, since in contrast to a space, you can assign it a
color. Then, the screen will not appear so fuzzy.
More precisely, the functions and commands concerned always use the
delete character set by SETCLEARB(), which uses CHR(255) as the default
setting. If you want to use a space to clear in Clipper or
Clipper Tools, insert a SETCLEARB(32) into the program.
Re-implement a space for clear:
SETCLEARB(32)
CLEAR // The affected Clipper command
Video Modes
Occasionally, there is some confusion about video modes. You will not
be able to work in EGA mode just because your computer has an EGA
adapter built into it. By the same token, you will not get either EGA
or VGA modes, if you do not work with graphics.
EGA43 / VGA50 / VGA28, etc.
In this section, the concern is not to just get a mode "hardwired"' into
a card, but for Clipper Tools it is to generate corresponding fonts
and other settings. For these reasons, the GETMODE() and GETSCRMODE()
functions generate their own values that are greater than 255.
For example, changes to the screen mode concerned with line count can be
combined with a 40-column mode:
CGA40()
EGA43() // 43 lines and 40 columns
Video Functions and Windows
You cannot implement functions that change the base address for screen
memory while windows are open. This includes all mode changes, as well
as SETPAGE and SETSCRSTR.
Note:
Since this section is about DOS-TEXT mode programming, considered as obsolete and details skipped.
Introduction Number and Bit Manipulation
Introduction CT Number and Bit Manipulation
This module discusses number and bit manipulation. Specifically, the numeric section deals with conversions between two different number systems, and the creation of random numbers.
The bit manipulation section covers such required binary operations as AND, OR, XOR, and NOT, and how to test, set and clear bits. The NUMLOW() and NUMHIGH() functions are important for CA-Clipper Tools functions that take two 8-bit values and return them as combined 16-bit numbers. An example is the GETCURSOR() function from the Extended Drivers module.
Many of the functions in this module have a <value> parameter type. In this case, a number can be in the range of 0 to 65535 (or 4 294 967 295) and/or a hexadecimal string in the range of “0000” to “FFFF” (or “FFFFFFFF”).
CT Number and Bit Manipulation Functions
