324-lecture17 - Principles Principles of Programming...

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Principles of Programming Languages Lecture 17 University of Toronto Wael Aboulsaadat [email protected] http://portal.utoronto.ca/ 1
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Pointers Variables referencing memory address or NIL/NULL PL/I is the first high-level language to have pointer variables Operations: Assignment to memory address (allocation) Note that this could be done with/without allocation E.g. // C lang 1461 University of Toronto Reference to value stored in memory cell E.g. Release of memory address (de-allocation) Ada, ALGOL 68: no explicit de-allocation 2 1461
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Pointers cont’d Implementation: Usually 2 or 4 bytes Hardware restrictions (e.g. Intel architecture) Note that you can have pointer to pointer… to value University of Toronto 3 1461 2401 3003 2401 1461
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Pointers cont’d Why are pointers problematic? Type checking: PL/I allowed pointers to point to any type of object! Dangling Reference: Storage pointed to is freed, but pointer is not set to null. Then, you are able to access storage whose value are not meaningful. Garbage: Pointer itself is freed (perhaps by execution going out of scope) but heap locations pointed to are not freed University of Toronto Then, there is no way to access this heap storage Memory leaks: Gradual loss of available computer memory when a program repeatedly fails to return memory that it has obtained for temporary use. Then, the available memory for that application becomes exhausted and the program can no longer function. What can we do about pointer problems? 4
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Pointers cont’d Why are pointers problematic? Type checking: PL/I allowed pointers to point to any type of object! Dangling Reference: Storage pointed to is freed, but pointer is not set to null. Then, you are able to access storage whose value are not meaningful. Garbage: Pointer itself is freed (perhaps by execution going out of scope) but heap locations pointed to are not freed University of Toronto Then, there is no way to access this heap storage Memory leaks: Gradual loss of available computer memory when a program repeatedly fails to return memory that it has obtained for temporary use. Then, the available memory for that application becomes exhausted and the program can no longer function. What can we do about pointer problems?
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