Opening Files

Except when using input and output files, your program must open files before it can use them. A call to a predefined file-opening procedure has the following syntax and parameters.

Syntax

Parameters

Example 1

reset(logfile);
rewrite(logfile2,physfile2);
append(lfile1,pfile1,'SHARED');  {HP-UX operating system ignores 'SHARED'}
open(lfile1);

If you specify physical_file, the system associates it with logical_file. If logical_file was previously associated with another physical file, the system closes the other physical file with its data intact and opens a new physical file.

Example 2

PROGRAM prog;
 
VAR
   datafile : FILE OF integer;
 
BEGIN
   open (datafile, 'file1');  {Logical file datafile is associated with
                               physical file file1.}
 
   open (datafile, 'file2');  {Physical file file1 is closed.
                               Logical file datafile is associated with
                               physical file file2.}
END.

If logical_file is not a program parameter, and physical_file is not specified, logical_file remains associated with its previously associated physical file. If logical_file was not previously associated with a physical file, the system associates logical_file with a temporary, nameless physical file.

Example 3

PROGRAM prog;  {Logical files logfile1 and logfile2 are not
                program parameters}
VAR
   logfile1,
   logfile2 : text;
 
BEGIN
   reset(logfile1,'file1'); {Logical file logfile1 is associated with
                             physical file file1.}
 
   rewrite(logfile1); {No physical file is specified, so logical file
                       logfile1 remains associated with physical file file1.}
 
   rewrite(logfile2); {No physical file is specified, and logical file
                       logfile2 is not associated with a physical file,
                       so logfile2 is associated with a temporary,
                       nameless physical file.}
END.

If logical_file is a program parameter, and physical_file is not specified, the system opens the physical file that has the same name as logical_file (with the lowercase letters upshifted — see Appendix B “HP-UX Dependencies ” for HP-UX implications). If no such physical file exists, the result depends on whether either append or rewrite opened the logical file. If so, the system creates the physical file. If not, it is an error.

Example 4

For this example, assume that the physical file file1 exists, but the physical file file2 does not.

PROGRAM prog (file1,file2);  {Logical files file1 and file2
                              are program parameters.}
VAR
   file1,
   file2 : text;
 
BEGIN
   rewrite(file1);  {Logical file file1 is associated with the
                     physical file file1.}
 
   rewrite(file2);  {Logical file file2 is associated with a
                     physical file file2.  }
END.

A temporary, nameless physical file cannot be saved. It becomes inaccessible when the main program or routine that declared logical_file terminates, or when you associate logical_file with a new physical file.

Your program does not need to open the standard textfiles input and output. When they are program parameters, the operating system opens them with reset and rewrite, respectively.

The standard textfiles input and output are bound to specific system files. For the MPE/iX operating system, see Appendix A “MPE/iX Dependencies ”; for the HP-UX operating system, see Appendix B “HP-UX Dependencies ”.

Table 3-2 “Characteristics of File-Opening Procedures” summarizes the characteristics of the four predefined file-opening procedures.