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December
2004
Boosting your
e3000 productivity
MPE for UNIX
Users, Part 3
The Complex
World of Files
By Robert
Green
Last month we
explored the directory structure of MPE a bit to explain about
logging on. This month we look at files to see how they are different
from files in Unix.
The File
System is one area where MPE is really different from Unix. And much
more complicated. Lets start by building a file, using the
command :build abc.
Seems just
like touch abc in Unix, right? Wrong the build command has
lots of optional parameters, all with defaults. To see what we
built, we use the listf command (this is like ls l
in Unix):
:listf abc,2
ACCOUNT= GREEN GROUP= BOB
FILENAME CODE ------------LOGICAL RECORD----------- ----SPACE----
SIZE TYP EOF LIMIT R/B SECTORS #X MX
ABC 128W FB 0 1023 1 0 0 *
The listf,2
option shows the most common attributes of an MPE file. Other options
show access dates, creator name, and other attributes.
FILENAME
file names are limited to 8 characters and case is not
significant; just as explained for user and account names in part
2. CODE a
file type number that tells you what type of content the file should
hold. Some of these have mnemonic labels, but underneath all are
integer values. For example, 111 is a QEDIT workfile, 1030 is an
NMPROG file. The default code is 0 (i.e., nothing special). For a
list of file codes used by HP and third-party vendors, go to www.robelle.com/smugbook/filecode.html
A negative
file code makes the file a privileged file which can only
be opened by a program running in privileged mode (in user mode,
programs cannot access or change data that is not their own; in
privileged mode, programs can get at data outside the program; the
operating system runs in a special privileged mode). The most well
known example of privileged files in MPE is database files; they can
only be accessed by going through the database API (or by going into
privileged mode, access to which is restricted by system
security).
SIZE
specifies the record size. Unix files do not have records; a file is
just a stream of bytes. But MPE files have a record structure (except
for Posix bytestream files, which are like Unix
files).
The record
size is the amount of data that is read or written by a single call
to MPEs FREAD or FWRITE intrinsic. An intrinsic is an MPE
system routine, just like fread and fwrite in Unix.
The default
record size is 128 words (which is 256 bytes, because the original HP
3000 was a 16 bit machine).
TYP FB
means fixed, binary. Fixed is the record format, meaning
that each record contains exactly 128 words. Another option is V for
Variable. Binary means that if a fixed record is incomplete, it is
filled out with binary zero. The alternative A for ASCII means that
incomplete records are filled with blanks. Also, when a file is
ASCII, the SIZE column will show the record size in Bytes (128B for
example).
EOF
tells you how many logical records have been written into the file
(actually, it shows you the highest record number that has been
written). An empty file like this always shows 0 for the EOF.
LIMIT
tells you the maximum number of logical records that can be written
into the file (for variable length records this number is based on
writing maximum length records, so it is often not the real limit).
The LIMIT concept is completely alien to Unix. In MPE, a program can
fail because it attempts to append a record to a file that has
reached its LIMIT, even though the system still has lots of free disk
space.
R/B
Records Per Block. In the original MPE you specified the number of
logical records to be stored in a physical block of disk storage (a
block was the amount normally transferred to/from disk). Over the
years, the internals of MPE have changed so much that this attribute
does not mean much anymore, although it is maintained for
compatibility.
SPACE,
SECTORS Logical disk space in MPE is measured in
sectors, which are 256 bytes each. As records are written
into a new file, disk space is allocated in chunks called
extents and this SECTOR value goes up. Again this unit of
measure is just for historical compatibility, since the current
version of MPE actually manages disk space in pages, which are much
larger. If you would like more explanation, please see
www.robelle.com/smugbook/pages.html
SPACE, #X MX
disk space is allocated in extents, so #X is the number of
extents currently allocated to the file and MX is the maximum number
that can be allocated. In the old days, MX was limited to 32 and #X
went from 0 to 32, at which point the file was full. And all extents,
except the last one, were the same size.
Now the
default MPE file has * for MX, which means that there is no effective
limit on the number of extents. However, you still must have a fixed
number for the LIMIT on logical records, even if that number is 2
billion!
Be warned
that if you create a file with a fixed number of extents and a large
LIMIT in logical records (for example, build def;disc=123456789,32),
you may very well get an error such as extent size too
large.
Here are some
additional file attributes for MPE:
USER LABELS
are optional 256 byte areas for control information that are tacked
onto the start of the file space. Typical examples are
Self-Describing files and Powerhouse subfiles, where the data
dictionary is stored in the user labels; this leaves all the logical
records with the same data format. Historically they come from old
mainframe systems, even going back to tape-based systems where the
tapes had labels on them! They are most commonly used in COBOL
systems that were migrated from these old IBM systems. The typical
file on MPE has 0 user labels.
CCTL
stands for Carriage Control. A file with the CCTL attribute has an
extra byte at the beginning of each logical record which tells
whether to do a page eject, single space, double space, or more
obscure function before (or after) printing the record. These are not
the same as Spool files; they are more like antique portable print
files. This capability goes back to old line printers, which had
program tapes, which could be customized for special forms such as
pre-printed invoices. What is interesting is that most of MPEs
simulated line printer functionality still works on Deskjet printers
and terminal screen emulators because MPE converts the CCTL codes
into terminal codes. The default for
an MPE file is build abc;nocctl. For more on CCTL files: see www.robelle.com/smugbook/cctl.html.
TEMP
stands for temporary files. On Unix, temporary files are just
permanent files that are built in a special directory, such as
/var/temp. Unix does not normally have an automatic cleanup for
temporary files after a user logs off. MPE temp files are different
because they are actually temporary. They are not recorded in the
regular file directory, but in a temp directory that is attached to
the user logon (interactive session or batch job). Any process
created during the logon can access the temp files of that
session/job and when the session/job logs off, the temporary file
space is returned to the system.
MPE even has
process-temporary files that are private to a program and go away
when that process terminates or aborts. These are called NEW files
and can be saved as Temporary or Permanent files by the
process.
CIR, MSG
stands for Circular or Message file. MPE has special files
where the current record pointer either wraps around (CIR) or moves
in such a way that a fread erases the record that is read (MSG).
KSAM, KSAMXL
these are indexed sequential files, with KSAM being the
original, classic MPE version and KSAMXL being the rewrite for the
RISC processors and MPE/XL (now called MPE/iX).
So, you can
see that our simple build abc is actually a much more complex
command:
:build
abc;rec=128,1,F,binary;disc=1024;code=0; &
std;ulabel=0;nocctl
Tune in next
month for details on how to work with the content of all these file
types!
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