Subj : Garbage collection
To   : Mike Luther
From : David Noon
Date : Thu Feb 22 2001 01:02 am

Hi Mike,

Replying to a message of Murray Lesser to Mike Luther:

The "ml" quote is from Mike Luther, the "ML" quote is from Murray Lesser.

ml>> Does PL/I have automatic garbage collection?  Is the fact
  >> that you don't consider PL/I to be a "modern" language, the
  >> reason you didn't include it?

ML>     To my knowledge, PL/I does not have automatic garbage collection.
ML> AFAIK, no language that requires a "FREE" (or equivalent) statement
ML> to recover memory otherwise lost in "the heap" has automatic garbage
ML> collection.  I've never written a program (except in C) that required
ML> a "free" statement, so I really don't know.

Murray has given you the complete answer here, but phrased somewhat obliquely.
... :-)

PL/I garbage collection varies with "storage class". Variables in virtual
memory are each allocated in one of 4 storage classes: STATIC, AUTOMATIC,
CONTROLLED and BASED. The garbage collection varies with each.

Under STATIC allocation the variables exist for the entire life of the
program's execution and garbage collection is neither required nor possible.

Under AUTOMATIC allocation [the usual default] the variables are constrained by
their "scope", and once they go out of scope they are *freed automatically*.

Under CONTROLLED allocation the variables come into existence and go out of
existence based on explicit program statements. Not only must these variables
be declared, but they must be explicitly allocated and freed.

Variables in the BASED class are pointer located. They can be heap-allocated,
like controlled variables, or their pointers can simply be set to the address
of of some other variable and they become aliases for those other storage
locations. In the first case the program should free the memory explicitly; in
the second case the original allocation's garbage collection procedure should
be followed.

If you keep things simple -- specifically, stick to AUTOMATIC and STATIC
variables -- then your garbage will be collected automatically. You need only
consider garbage collection when you move to more exotic memory allocation
schemes.

Basically, what Murray has said is that he sticks to AUTOMATIC and STATIC
allocation and so never needs to code explicit garbage collection.

Regards

Dave
<Team PL/I>

--- FleetStreet 1.25.1
* Origin: My other computer is an IBM S/390 (2:257/609.5)

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