HP 3000 Manuals

HP Pascal/iX Programmer's Guide

General File Information 

You need the general file information in this section to understand the
rest of this chapter.  Examine Figure 3-2 , and then read the
explanations of the entities in italics, whose relationships it shows.

Figure 3-2  illustrates the relationship between physical files (in
the operating system environment) and logical files (in the program
environment).  It also shows how logical files, textfiles, and the
standard textfiles input and output are related.


          Figure 3-2.  File Relationships 

Physical Files 

A physical file is a program-independent entity that the operating system
controls.  It can be a file on a disk or other medium, or an interactive
file created at a terminal (refer to your operating system manual for
information on creating and controlling physical files).

Your program can manipulate a physical file if the physical file is
associated with one of the program's logical files.  In this case, the
physical file assumes the characteristics of the logical file.

Logical Files 

A logical file is a data structure that a program declares and controls.
It is a sequence of components of the same type.

The declaration of a logical file determines the type of its components
but not their number.  A logical file that is declared FILE OF x has
components of type x.  File operations can change the number of file
components.

A logical file does not exist outside the main program or routine that
declares it.  If it is associated with a physical file, however, anything
that happens to the logical file within the program also happens to the
physical file.  This is how a program can manipulate its external
environment.


NOTE  In subsequent sections of this chapter, the term file refers to a
      logical file unless otherwise noted.


Textfiles 

A textfile is a logical file that is subdivided into lines, each of which
ends with an end-of-line marker.  The components of a textfile are of
type char, but a textfile declaration specifies the type text, not FILE
OF char.

The standard files input and output are textfiles.  If you declare them
in the program header, they are the default file parameters for all of
the sequential input and output procedures, respectively.

Example 

     PROGRAM prog (input,output);

     VAR
        tfile : text;
            c : char;

     BEGIN
        .
        .
        .
        read(tfile, c);   {Reads from tfile}
        read(c);          {Reads from input}
        write(c);         {Writes to output}
     END.

The preceding program has three textfiles:  the standard textfiles input
and output, and the file tfile.

End-of-line markers are not file components, and are not of type char.
The predefined procedure writeln writes them to the file (see "Textfile
Input/Output" ).  An end-of-line marker always precedes the
end-of-file mark in a textfile, whether writeln wrote the last component
to the file or not.

Current Position Indexes 

Every logical file has a current position index that indicates either its
current component, an end-of-file marker, or (in a textfile) an
end-of-line marker.  This index is an integer--the ordinal number of the
current component or marker.  A file's current component is the component
that the next I/O operation on that file will input or output.

Figure 3-3  illustrates the relationship between current position 
index and current component. 


          Figure 3-3.  Relationship Between Current 
                       Position Index and Current Component 

File Buffer Variables and Selectors 

Every logical file has a file buffer variable, or buffer, which is a
variable of the same type as the file components.  Some file operations
assign the value of the current component to the buffer; other operations
leave the buffer undefined.

When the buffer is defined, you can access its value with its file buffer 
selector.  The file buffer selector for the file f is f^ or f@.

Accessing an undefined buffer causes an error.

MPE/iX 5.0 Documentation