Attempting to dereference an invalid a NULL pointer results in undefined behavior, typically abnormal program termination. Given this, pointers should be checked to make sure they are valid before they are dereferenced.
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In this example, input_str is copied into dynamically allocated memory referenced by str. If malloc() fails, it returns an invalid (null) a NULL pointer that is assigned to str. When str is dereferenced in strcpy(), the program behaves in an unpredictable manner.
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