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Pointers and Expressions

Prof. Evan Chang

Meeting 16, CSCI 3155, Fall 2009

Announcements

• HW5 due Thu Oct 22 at 11:55pm

• How was Project 1?

• How are recitations?

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Finish Unions

Unions

• What are they for?

• Kinds of unions?

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Unions

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Exercise

• Define types and code for evaluating an expression language in C (and in SML):

e ::= n | -e | e + e | e * e

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Pointers

Pointers

• Dangling pointer bug

• Memory leak bug

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Pointers

• A pointer type include values that range over memory addresses

• ... and often an additional distinguished value null.

– “The Billion Dollar Mistake”

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Pointers

• A pointer type include values that range over memory addresses

• ... and often an additional distinguished value null.– SML analog?

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Pointer Operations

• What are they?

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Problem: Dangling Pointer

• What is it?

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Problem: Dangling Pointer

• What is it?

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Problem: Dangling Pointer

• What problems can it cause?

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Problem: Dangling Pointer

• What problems can it cause?

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Problem: Dangling Pointer

• Possible solutions?

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Problem: Memory Leak

• What is it?

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Problem: Memory Leak

• What is it?

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Problem: Memory Leak

• What problems can it cause?

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Problem: Memory Leak

• What problems can it cause?

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Problem: Memory Leak

• Possible solutions?

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Garbage Collection

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Garbage Collection: Reference Counting

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Garbage Collection: Reference Counting

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Garbage Collection: Reference Counting

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Garbage Collection: Mark-and-Sweep

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For Next Time

• Reading

• Online discussion forum– ≥1 substantive question, comment, or answer each week

• Work on HW3

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