Ken Hirsch ([log in to unmask]) wrote:
: Mark Landin ([log in to unmask]) wrote:
: : Is there an example of how to do mapped file access in COBOL
: : floating around on the net somewhere? I'd like to play around with
: : it.
 
: : --
: : Mark Landin                             "If you choose not to decide,
: : Systems Manager                         you still have made a choice"
: : Superstar Satellite Entertainment          -Neil Peart, RUSH
 
: I must note that it is possible to do SHORT-mapped file access in COBOL.
: This has the following disadvantages:
:   (1) File size is limited (to 4MB, I think)
:   (2) You can only open a limited number of files this way.
:       Does anybody have details on this? Is this adjustable in SYSGEN?
:   (3) While you have a file opened for short-mapped access, it cannot
:       be opened in any other way.
 
: While COBOL (unfortunately) has no support for pointers per se, you can
: lie to your subprograms since COBOL has no type-checking for subprogram
: parameters.
 
: You need to open the file for short-mapped access with a 32-bit pointer.
: 01 short-pointer     pic s9(9) comp.
 
Pointers are unsigned 32 bit integers. By declaring s9(9) comp the
sign-bit is ignored causing problems. It would be safe to make it X(04).
 
Milind Gole
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: Then, when you call your program, pass this by VALUE.
 
: call "MYSUBPROG" using \short-pointer\
 
: In your subprogram linkage section, you can make this whatever you want:
 
:  id division.
:  program-id. mysubprog.
:  data division.
:  linkage section.
:  01 large-array.
:     05 my-record occurs 4096 times.
:        10 my-name               pic x(30).
:        10 my-addr               pic x(30).
:        10 your-states-name-here pic x(30).
:    etc.
 
:  procedure division using large-array.
:  your-code-goes-here.
 
: You can use this technique to use pointers in general including memory
: that you malloc()!
 
: If the mapped file does actual disk I/O, Scott Gavin is right that
: mapped-access is not much faster.  However, if the file is present in
: memory, mapped-file access is MUCH faster.
 
: You still must handle EOF via intrinsics.  And, if more than one process
: accesses the file at the same time, you still must contol access
: somehow.
 
: Ken Hirsch
: Carrboro, NC