System-Defined Files

Table 3-1 System-Defined File Designators

Input/Output sets

All file designators can be classified as those used for input files (Input Set), or those used for output files (Output Set). For your convenience, these sets are summarized in Table 3-2 “Input Set” and Table 3-3 “Output Set”.

An input file and a list file are said to be interactive if a real-time dialog can be established between a program and a person using the list file as a channel for programmatic requests, with appropriate responses from a person using the input file. For example, an input file and a list file opened to the same teleprinting terminal (for a session) would constitute an interactive pair. An input file and a list file are said to be duplicative when input from the former is duplicated automatically on the latter. For example, input from a magnetic tape device is printed on a line printer. You can determine whether a pair of files is interactive or duplicative with the FRELATE intrinsic call. (The interactive/duplicative attributes of a file pair do not change between the that time the files are opened and the time they are closed.)

The FRELATE intrinsic applies to files on all devices. To determine if the input file INFILE and the list file LISTFILE are interactive or duplicative, you could issue the following FRELATE intrinsic call:

   ABLE := FRELATE(INFILE,LISTFILE);

INFILE and LISTFILE are identifiers specifying the file numbers of the two files. The file numbers were assigned to INFILE and LISTFILE when the HPFOPEN/FOPEN intrinsic opened the files.

A half-word is returned to ABLE showing whether the files are interactive or duplicative. The half-word returned contains two significant bits, 0 and 15:

Passed files

Programmers, particularly those writing compilers or other subsystems, sometimes create a temporary disk file that can be automatically passed to succeeding MPE/iX commands within a job or session. This file is always created under the special name $NEWPASS. When your program closes the file, MPE/iX automatically changes its name to $OLDPASS and deletes any other file named $OLDPASS in the job/session temporary file domain. From this point on, your commands and programs reference the file as $OLDPASS. Only one file named $NEWPASS and/or one file named $OLDPASS can exist in the job/session domain at any one time.

The automatic passing of files between program runs is depicted in Figure 3-1 “Passing Files between Program Runs”. To illustrate how file passing works, consider an example where two programs, PROG1 and PROG2, are executed. PROG1 receives input from the actual disk file DSFILE (through the programmatic name SOURCE1) and writes output to an actual file $NEWPASS, to be passed to PROG2. ($NEWPASS is referenced programmatically in PROG1 by the name INTERFIL.) When PROG2 is run, it receives $NEWPASS (now known by the actual designator $OLDPASS), referencing that file programmatically as SOURCE2. Note that only one file can be designated for passing.

Figure 3-1 Passing Files between Program Runs

   .
   .
   .
   FILE SOURCE1=DSFIL
   FILE INTERFIL=$NEWPASS    <---
   RUN PROG1                    |-  Same File
   FILE SOURCE2=$OLDPASS     <---
   RUN PROG2
   .
   .
   .

A program file must pass through several steps as it is executed; passed files are most frequently used between these steps. A program file must be compiled and linked before it is executed. By default, the compiled form of a text file is written to $NEWPASS. When the compiler closes $NEWPASS, its name is changed to $OLDPASS; it is this file that is linked for execution. The linked form of the program file is written to a new $NEWPASS, which is renamed $OLDPASS when the file is closed; the old $OLDPASS is deleted. Now, this file is ready to be executed. This $OLDPASS may be executed any number of times, until it is overwritten by another $OLDPASS file.

The steps that a program takes as it is run are depicted in Figure 3-2 “Passing Files within a Program Run”.

Figure 3-2 Passing Files within a Program Run

$NEWPASS and $OLDPASS are specialized disk files with many similarities to other disk files. Comparisons of $NEWPASS to new files, and $OLDPASS to old files, are given in Table 3-4 “New Files Versus $NEWPASS” and Table 3-5 “Old Files Versus $OLDPASS”.