Dynamic Memory Management Recap: Understanding Memory Sizes and Allocation

1. July 11, 2012 Dynamic memory

2. Announcements • Homework #4 due today • HW #3 resubmit due tomorrow

3. How much memory do variables take up? • Depends on operating system, computer, and compiler, but can find out with sizeof() function.

4. On my system, variables take up the following • sizeof(char) = 8 bits / 1 byte • sizeof(int) = 32 bits / 4 bytes • sizeof(double) = 64 bits / 8 bytes

5. From yesterday's lab • Shervin used sizeof() for array counting. • Assume intfoo[10]. • What is sizeof(foo)?

6. Class question • How can we really calculate the size of an array using sizeof()?

7. Cubbyholes • Remember this?

8. Revisiting our cubbyholes • Cubbyholes have different widths!

9. Why does this matter? • As we begin to delve into dynamic memory allocation, we need to be keenly aware of the amount of memory that we need.

10. Dynamic Memory • Often, we don't know how large our arrays should be • What we've done thus far is to vastly over-estimate our array sizes

11. Drawback of fixed-size arrays • Consider the following array of strings: • char words[1024][1024]; • We've just taken up 1MB! • Most of this won't be used

12. Dynamically allocating memory with malloc() • malloc() is defined inside <stdlib.h> • Usage: malloc(<size>)

13. Malloc • malloc returns a "void" pointer. • In this case, void kind of means wildcard. As such, we have to cast the return value into something that we want. • Ex: (int *)malloc(sizeof(int));

14. Malloc • Where do we store the result of malloc()?

15. Malloc • Where do we store the result of malloc()? • Answer: In a pointer! double*foo; foo = (double *)malloc(sizeof(double));

16. Free • Just like fopen() and fclose() malloc() always needs an accompanying free() • Example: char *c = (char *)malloc(sizeof(char)); free(c);

17. Free • Calling free() returns the memory allocated by malloc() back to someone else that might need it. • VERY IMPORTANT that we give back memory that we ask for. • Class demonstration of what happens when you don't call free()

18. Free • Once we call free() we lose access to whatever data was stored inside the variable that we freed up. • However, we can use the variable to store new values in the future.

19. Putting it all together double dynamic_num; printf("%d", dynamic_num); dynamic_num = (double *)malloc(sizeof(double)); printf("%d", dynamic_num); *dynamic_num = 3.25; printf("%lf", *dynamic_num); free(dynamic_num); printf("%d", dynamic_num);

20. …But malloc() isn't for arrays! • That's why we use calloc() • How calloc() works: calloc(<size of array>, <size of data type>);

21. calloc() example int *array_ptr; array_ptr = (int *)calloc(10, sizeof(int)); for(i = 0; i < 10; i++) { printf("%d", array_ptr[i]); }

22. But we're still hard coding array sizes! • Another example, this time completely dynamic: int *array_ptr; intarray_size; array_size = read_int("Enter a number: "); array_ptr = (int *)calloc(array_size, sizeof(int)); for(i = 0; i < array_size; i++) { printf("%d", array_ptr[i]); }

23. Quiz questions

24. What is the value of "size"? intfoo = 0; int size = sizeof(foo);

25. What is the value of "size"? char foo[100]; int size = sizeof(foo);

26. Out of our simple data types, which takes up the most amount of memory?

27. How would we find the length of a double array? Assume that numbers is of type "double *" (an array)

28. How much memory does "foo" take up? • intfoo[10][10];

29. For tomorrow: • Learn how to use dynamic memory to return pointers from functions! • Improve our custom_io functions by using dynamic memory allocation!