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LEC20081016 - Introduction to Computer Programming CSC180...

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Introduction to Computer Programming October 16, 2008 CSC180 Fall 2008, University of Toronto
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Null pointers A null pointer points to nothing. It contains a special value that is not the address of any object. A null pointer constant: an integral constant expression with value 0 , or such an expression cast to type void * : #define NULL 0 #define NULL (1-1) #define NULL ((void *)0) /* typical */ #define NULL ((void *)(1-1)) NULL is defined in stdio.h , stdlib.h , string.h , stddef.h , time.h , and locale.h . CSC180 Fall 2008, University of Toronto 1
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How to get a null pointer? Assign a null pointer constant to the pointer. A null pointer compares unequal to a pointer to any object. Example: #include <stdio.h> char *p, *q; int *r, *s; /* * 0 in a pointer context is converted to a null pointer constant */ p = 0; q = NULL; /* preferred way */ if (p == NULL && q == 0) /* p and q are null pointers of type char */ printf(‘‘p and q are null pointers\n’’); CSC180 Fall 2008, University of Toronto 2
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if (r != NULL) printf(‘‘r is not a null pointer\n’’); s = NULL; if (q == s)
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