ITEC 320 2018fall nokie ibarland

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ITEC320 closing thoughts

Some last examples on memory-allocation

Three areas of memory:

• The heap (grows upward, in address-numbers)
• The stack (grows down)
• Static constants (at the bottom)

• infrec.adb
• exhaust-heap.adb
• aliasedLocal2b.adb

So: What did we talk about, this semester?

See Dr Okie's page for a list of study-topics.

Is 320 about Ada?

No. Well, not much.
ITEC320 is about providing perspective on what a programming language can do, and can look like, to be contrasted with your (perhaps) first language/paradigm (Java and Object-Oriented).

• A rich type system.
• The C memory model (stack-allocating records/objects, in addition to heap-allocation).
• The “value semantics” this effects, as opposed to “reference semantics”.¹
• memory allocation & de-allocation—not a feature that is unique to procedural languages, but definitely one of the the goals of this class.
• parameter modes (in, out, and in-out), as another way of passing information around in a program. functions vs. (void) procedures.

Review: stack- vs. heap-allocation

Cons of heap-allocation

• Every field-access takes two memory accesses (fetching the pointer, and then de-referencing it).² This incurs both time overhead.
• There is also the space-overhead: in addition to the object itself, there is a pointer to it, and next to the object tends to be extra tag-information (e.g. every Java object includes 12B of overhead).
• Garbage collection (extra time overhead, perhaps at unpredictable times).

Cons of stack-allocation

• harder to manage items of dynamic-size.

Having a language which supports both immediate objects and object-references is its own mixed blessing: The programmer needs to be remembering/aware of whether each value can be aliased or not. Also, there can be mismatches between functions that want objects vs. references-to-objects, that must be converted. (This can be in functions written by your team, as well as language-wide APIs.)

Review: manual vs. automatic memory management

Allocation and freeing memory are critical concepts to know and be aware of; they form the basis of our mental-model of real machines. See Garbage collection strategies (Wikipedia). (Every CS major should know the basic ideas of garbage-collection, even though only our OS class actually covers it.)

If somebody asks “do you know C”, they're not asking about whether you can use if and struct and typedef; they are really asking if you understand the what is necessary whenever a string is allocated and de-allocated, passing values vs. references (memory-addresses), aliasing and dangling pointers, etc..

However, in practice, even top-notch programmers have difficulties getting manual memory-management correct.

upshot: Avoid manual de/allocation! Either use a language that does it automatically (like Java), or use a library for new/malloc/free which does it itself.

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