                     Chapter 14 - Example Programs


                             WHY THIS CHAPTER?

             Although  every  program  in this tutorial has  been  a 

        complete  program,  each  one  has also been  a  very  small 

        program intended to teach you some principle of  programming 

        in  C.   It  would do you a disservice to leave you at  that 

        point  without introducing you to a few larger  programs  to 

        illustrate  how  to  put together the  constructs  you  have 

        learned  to create a major program.   This chapter  contains 

        four  programs  of increasing complexity,  each designed  to 

        take  you  into a higher plateau of  programming,  and  each 

        designed to be useful to you in some way.

             DOSEX will illustrate how to make DOS system calls  and 

        will teach you,  through self-study, how the system responds 

        to  the  keyboard.   WHATNEXT  reads commands input  on  the 

        command line and will aid you in setting up a variable batch 

        file,  one  that requests an operator input and responds  to 

        the  input  by branching to a different part  of  the  batch 

        file.

             LIST  is  the source code for the program you  used  to 

        print  out the C source files when you began studying C with 

        the aid of this tutorial.  Finally we come to VC, the Visual 

        Calculator,  which  you should find to be a  useful  program 

        even  if you don't study its source code.   VC uses most  of 

        the  programming  techniques we have studied in this  course 

        and  a few that we never even mentioned such  as  separately 

        compiled subroutines.

             We  will  take a look at the example programs one at  a 

        time  but  without  a complete explanation of  any  of  them 

        because  you  have  been studying C for some  time  now  and 

        should be able to read and understand most of these programs 

        on  your  own.   One other thing must  be  mentioned,  these 

        programs  use  lots of nonstandard constructs and  you  will 

        probably need to modify some of them to get them to  compile 

        with  your  particular  compiler.  That will be left  as  an 

        exercise for you.

                     DOSEX.C - The DOS Example Program

             The  copy of DOS that you received with your IBM-PC  or 

        compatible has about 50 internal DOS calls that you can  use 

        as  a programmer to control your peripheral devices and read 

        information  or status from them.   Some of the earlier  IBM 

        DOS manuals, DOS 2.0 and earlier, have these calls listed in 

        the back of the manual along with how to use them.   Most of 

        the  manuals  supplied  with compatible  computers  make  no 

        mention  of  these  calls even  though  they  are  extremely 

        useful.   These  calls  can  be  accessed  from  nearly  any 



                                  Page 94









                     Chapter 14 - Example Programs


        programming  language but they do require some initial study 

        to learn how to use them.   This program is intended to  aid 

        you in this study.

             Display the program on your monitor or print it out for 

        reference.   It  is  merely a loop watching for  a  keyboard 

        input or a change in the time.  If either happens, it reacts 

        accordingly.   In line 23,  the function "kbhit()" returns a 

        value  of 1 if a key has been hit but not yet read from  the 

        input buffer by the program.  This is a nonstandard function 

        and  may require a name change for your particular compiler.  

        There will probably be several similar calls that will  need 

        changed  for  your compiler in order to compile and run  the 

        programs in chapter 14.

             Look at the function named "get_time" for an example of 

        a  DOS call.   An interrupt 21(hex) is called after  setting 

        the  AH  register to 2C(hex) =  44(decimal).   The  time  is 

        returned in the CH,  CL, and DH registers.  Refer to the DOS 

        call  definitions in your copy of DOS.   If the  definitions 

        are  not included there,  Peter Nortons  book,  "Programmers 

        Guide  to  the  IBM PC" is recommended as a  good  reference 

        manual   for   these  calls  and  many   other   programming 

        techniques.

             Another useful function is the "pos_cursor()"  function 

        that  positions the cursor anywhere on the monitor that  you 

        desire  by  using  a  DOS  interrupt.   In  this  case,  the 

        interrupt  used  is  10(hex) which is  the  general  monitor 

        interrupt.   This particular service is number 2 of about 10 

        different  monitor  services  available.    This  particular 

        function  may  not be needed by your compiler  because  some 

        compilers  have a cursor positioning function predefined for 

        your use.  This function is included here as another example 

        to you.

             The  next  function,  service  number  6  of  interrupt 

        10(hex)  is the window scroll service.   It should  be  self 

        explanatory.

             In this program, the cursor is positioned and some data 

        is  output  to the monitor,  then the cursor is "hidden"  by 

        moving  it  to line 26 which is not  displayed.   After  you 

        compile and run the program, you will notice that the cursor 

        is  not  visible on the monitor.   This is possible  in  any 

        program,  but  be  sure  to put the cursor  in  view  before 

        returning  to  DOS  because  DOS does not  like  to  have  a 

        "hidden" cursor and may do some strange things. 

             Some time spent studying this program will be  valuable 

        to  you as it will reveal how the keyboard data is input  to 



                                  Page 95









                     Chapter 14 - Example Programs


        the  computer.   Especially of importance is how the special 

        keys such as function keys, arrows, etc. are handled.

                  WHATNEXT.C - The Batch File Interrogator

             This  is  an  example of how to read the  data  on  the 

        command line following the function call.  Notice that there 

        are  two variables listed within the  parentheses  following 

        the main() call.   The first variable is a count of words in 

        the entire command line including the command itself and the 

        second  variable  is  a  pointer to  an  array  of  pointers 

        defining the actual words on the command line.

             First the question on the command line, made up of some 

        number of words, is displayed on the monitor and the program 

        waits for the operator to hit a key.   If the key hit is one 

        of  those  in the last "word" of the group of words  on  the 

        command  line,  the number of the character within the group 

        is  returned to the program where it can be tested with  the 

        "errorlevel" command in the batch file.   You could use this 

        technique  to  create a variable AUTOEXEC.BAT  file  or  any 

        other  batch  file  can  use this for  a  many  way  branch.  

        Compile  and  run this file with TEST.BAT for an example  of 

        how  it  works in practice.   You may  find  this  technique 

        useful  in  one  of  your batch files and  you  will  almost 

        certainly  need  to  read in  the  command  line  parameters 

        someday.

             An  interesting alternative would be for you to write a 

        program  named "WOULD.C" that would return a 1 if a  "Y"  or 

        "y"  were typed and a zero if any other key were hit.   Then 

        your batch file could have a line such as;

        WOULD YOU LIKE TO USE THE ALTERNATIVE METHOD (Y/N)

             Dos would use "WOULD" as the program name,  ignore  the 

        rest  of  the  statement  except for displaying  it  on  the 

        screen.   You  would  then respond to the  question  on  the 

        monitor  with a single keyhit.   Your batch file would  then 

        respond   to  the  1  or  0  returned  and  either  run  the 

        alternative  part  of  the batch file or  the  primary  part 

        whatever each part was.

        WOULD YOU LIKE PRIMARY (Y/N)
        IF ERRORLEVEL 1 GOTO PRIMARY
        (secondary commands)
        GOTO DONE
        :PRIMARY
        (primary commands)
        :DONE




                                  Page 96









                     Chapter 14 - Example Programs


                        LIST.C - The Program Lister

             This program is actually composed of two files,  LIST.C 

        and  LISTF.C  that must be separately  compiled  and  linked 

        together  with your linker.   There is nothing new here  and 

        you  should  have  no  trouble compiling  and  linking  this 

        program  by  reading the documentation  supplied  with  your 

        compiler.

             The  only  thing  that is new in this  program  is  the 

        inclusion   of  three  "extern"  variables  in  the  LISTF.C 

        listing.   The only purpose for this is to tie these  global 

        variables  to  the main program and tell the  compiler  that 

        these  are not new variables.   The compiler will  therefore 

        not  generate any new storage space for them but simply  use 

        their names during the compile process.   At link time,  the 

        linker  will  get  their actual storage locations  from  the 

        LIST.OBJ  file and use those locations for the variables  in 

        the  LISTF part of the memory map also.   The  variables  of 

        those  names in both files are therefore the same  identical 

        variables and can be used just as any other global variables 

        could be used if both parts of the program were in one file.

             There is no reason why the variables couldn't have been 

        defined  in the LISTF.C part of the program and declared  as 

        "extern"  in the LIST.C part.   Some of the variables  could 

        have  been  defined  in one and some in the  other.   It  is 

        merely a matter of personal taste.   Carried to an  extreme, 

        all of the variables could have been defined in a third file 

        and  named "extern" in both of these files.   The third file 

        would then be compiled and included in the linking process.

             It would be to your advantage to compile, link, and run 

        this  program to prepare you for the next program  which  is 

        composed of 5 separate files which must all work together.

                        VC.C - The Visual Calculator

             This  program  finally ties nearly everything  together 

        because  it uses nearly every concept covered in the  entire 

        tutorial.   It  is so big that I will not even try to  cover 

        the finer points of its operation.   Only a few of the  more 

        important points will be discussed.

             The  first  thing  you  should do  is  go  through  the 

        tutorial  for  VC included in the file  VC.DOC.   There  are 

        several  dozen  steps  for you to execute,  with  each  step 

        illustrating some aspect of the Visual Calculator.  You will 

        get  a  good feel for what it is capable of doing  and  make 

        your study of the source code very profitable.  In addition, 

        you  will  probably  find  many  ways  to  use  the   Visual 



                                  Page 97









                     Chapter 14 - Example Programs


        Calculator  to  solve problems involving calculations  where 

        the  simplicity  of  the problem at hand  does  not  warrant 

        writing a program.

             Notice that the structure definitions,  used in all  of 

        the  separate parts of the program,  are defined in the file 

        STRUCT.DEF.   During  program development,  when  it  became 

        necessary  to change one of the structures slightly,  it was 

        not  necessary to change it in all of the  files,  only  one 

        file  required modification which was then "included" in the 

        source files.   Notice that the transcript data is stored in 

        a doubly linked list with the data itself being stored in  a 

        separate  dynamically allocated char string.   This line  is 

        pointed to by the pointer "lineloc".

             For  ease  of development,  the similar functions  were 

        grouped together and compiled separately.   Thus, all of the 

        functions  involving the monitor were included in  the  file 

        named  VIDEO.C,  and all of the functions involving the data 

        storage were grouped into the FILE.C  collection.   Dividing 

        your  program  in  a  way similar to  this  should  simplify 

        debugging and future modifications. 

             Of special interest is the "monitor()" function.   This 

        function  examines  the  video mode through  use  of  a  DOS 

        command  and  if it is a 7,  it assumes it is  a  monochrome 

        monitor,  otherwise it assumes a color monitor.   The colors 

        of  the various fields are established at this time and used 

        throughout  the  program.   Most  of  the  data  is  written 

        directly  to the video memory,  but some is written  through 

        the standard BIOS routines.

             The file DEFIN.C is simply a catalogue of the functions 

        to aid in finding the functions.  This file was generated as 

        one  of the first files and was maintained and  updated  for 

        use during the entire design and coding lifetime.

             Feel free,  after understanding this code, to modify it 

        in any way you desire for your own use.














                                  Page 98

