CS61 Section Notes. Section 5 (Fall 2011) Topics to be covered Common Memory Errors Dynamic Memory Allocation Assignment 3: Malloc

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1 CS61 Section Notes Section 5 (Fall 2011) Topics to be covered Common Memory Errors Dynamic Memory Allocation Assignment 3: Malloc

2 Common Memory Errors In lecture, we learned about several errors programmers commonly make when dealing with memory. Here are some examples: Dereferencing bad pointers Reading uninitialized memory Off-by-one errors Not checking buffer size Pointer arithmetic bugs Confusing pointers and objects (e.g. manipulating a pointer instead of an object) Referencing nonexistent variables (e.g. local variables from another function) Freeing blocks multiple times Referencing freed blocks Memory leaks These bugs can be very subtle and tricky to debug, but can cause dangerous and unpredictable behavior, so it is important to be able to recognize them. Question 1. Point out the problem in the following code that may yield unexpected results. int* Hamlet() { int Ophelia = 42; return &Ophelia; void main() { int *Horatio = Hamlet(); printf( The answer is %d\n, *Horatio); Question 2: What is the problem with the code below? typedef struct { double Jaques; int Pages[10]; char *Duke; ExiledCourt; void main() { ExiledCourt *Rosalind; int counter = 0; Rosalind = (ExiledCourt*) malloc(sizeof(exiledcourt)); for (counter = 0; counter < 10;counter++) { Rosalind->Pages[counter] = 0x2a; strcpy(rosalind->duke, As You Like It ); Question 3: Can the following code lead to memory leaks?

3 void main() { ExiledCourt *Rosalind; int counter = 0; Rosalind = (ExiledCourt*) malloc(sizeof(exiledcourt)); Rosalind->Duke = (char*) malloc(0x2a); free (Rosalind); Now, let s get some experience finding memory errors in real (by which we mean fake) code. Question 4. Jack, a federal agent, is on the trail of a terrorist who has planted a nuclear device somewhere in Los Angeles. To find the culprit before the bomb detonates, Jack needs a piece of software that will copy an FBI watch list (in the form of a singly-linked list of person structs) to his PDA and search the list for suspects living in Los Angeles. Chloe, his expert data analyst, is taking the day off, so the task falls to a less experienced programmer working for the Counter-Terrorist Unit. This programmer has written the following code with a number of memory errors. Find them all and suggest solutions, but do it quickly. We only have ten minutes before the bomb goes off! 1 typdef struct person_struct { 2 char name[20]; 3 int age; 4 char *city; 5 struct person_struct *next; 6 person; 7 8 person *search_suspects (person* first_person, int n) { 9 //Allocate new memory, copy list of people person *new_first = malloc(n*sizeof(person)); 12 person *new_current_person = NULL; 13 person *current_person = first_person; 14 int i = 0; 15 for (i = 0; i <= n; i++) { 16 new_current_person = new_first + i*sizeof(person); 17 strcpy(new_current_person->name, current_person->name); 18 new_current_person->age = current_person->age; 19 new_current_person->city = malloc(strlen(current_person->city)); 20 strcpy(*new_current_person->city, *current_person->city); 21 if(current_person->next == NULL) { 22 new_current_person->next = NULL;

4 23 else { 24 new_current_person->next = new_current_person+sizeof(person); 25 current_person = current_person->next; //Search new list for people in Los Angeles person *last_person = NULL; 32 new_current_person = new_first; 33 do { 34 if(strcmp(new_current_person->city, "Los Angeles") == 0) { 35 strcat(new_current_person->name, " (POSSIBLE SUSPECT!)"); 36 if(last_person == NULL) { 37 //This is the new head of the linked list 38 new_first = new_current_person; 39 else { 40 //Update pointer in the last node. 41 last_person->next = new_current_person; last_person = new_current_person; 44 else { 45 //Free this struct. 46 free(new_current_person); new_current_person = new_current_person->next; 49 while(new_current_person!= NULL); return new_first; 52 Question 5. Which common memory errors are avoided in a language that with garbage collection? What other features can a language provide that will help prevent these bugs? Dynamic Memory Allocation Question 6: Consider a memory manager that uses implicit free lists, and headers and footers are stored in four-byte words. Determine the minimum block size for each of the following combinations of alignment requirements and block formats. Assume that zero-sized payloads are not allowed. Alignment (of payload) Allocated Block Free block Min block size (bytes) Single-word (i.e., 4 byte) Header and footer Header and footer

5 Single-word Header but no footer Header and footer Double-word (i.e., 8 byte) Header and footer Header and footer Double-word Header but no footer Header and footer Question 7: What is the motivation for having footers in free blocks? How can we avoid the need to have footers in allocated blocks? Question 8: In class we saw three techniques for managing the free list: Implicit List: Keep track of allocated length of each block in a header. Free blocks are linked implicitly by the size fields in the headers (and maybe footers). Simple, but the search of the free list will be linear with the total number of allocated and free blocks in the heap. Explicit List: Use an explicit data structure to organize free blocks, stored within the bodies of the free blocks. E.g., a doubly linked free list with a predecessor and successor pointer in each free block. Search reduces to linear in time with total number of free blocks, but has a larger minimum block size than an implicit free list. Segregated Free List: Maintain multiple free lists, where each list holds blocks that are roughly the same size, ordered by increasing size. When the allocator needs a certain block size, it searches the appropriate free list. If it cannot find a block that fits, it searches the next list. What do you think is the relative performance of these three techniques, in terms of throughput? What about in terms of memory utilization? Assignment 3: Malloc implementation In Assignment 3, you are implementing your own dynamic memory manager. There are a number of issues to think about, and a number of design choices you will have to make. The lists below raise some of these issues and choices. Allocator Requirements and Goals: Handle arbitrary request sequences: The allocator cannot make any assumptions about the ordering of allocate and free requests. Make immediate responses to requests: The allocator is not allowed to reorder or buffer requests. Use only the heap: Any non-scalar data structures used by the allocator must be stored in the heap itself. Align blocks: The allocator must align blocks in such a way that they can hold any type of data object. Do not modify allocated blocks: Allocators can only change free blocks. They are not allowed to modify or move blocks once they are allocated. Be fast and efficient: An allocator should ideally be both fast (handle requests for allocating and freeing memory quickly) and efficient in its utilization of memory. Design choices/implementation Issues: Free block organization: How do we keep track of free blocks?

6 Placement: How do we choose an appropriate free block for a newly allocated block? Some possibilities are: First Fit: Searches the free list from the beginning and chooses the first free block that fits. Next Fit: Similar to first fit, but instead of starting each search from the beginning of the list, it starts each search where the previous search left off. Best fit: Examines every free block and chooses the free block with the smallest size that fits. Splitting: After we place a newly allocated block in some free block, what do we do with the remainder of the free block? Coalescing: Do we combine adjacent free blocks? If so, how, and when? Some possibilities are when the block is freed, or when we search the free list to perform a new allocation. Question 9: What are some of the trade-offs between first fit, next fit, and best fit? Question 10: In what ways are speed and efficient utilization of memory at odds with each other? Question 11: How do you intend to go about designing and implementing Assignment 3? What are good practices for designing and implementing robust correct code? What tools do you intend to use (e.g., which editor (and why), debugger, profiler, version control,...)

CS61, Fall 2012 Midterm Review Section

CS61, Fall 2012 Midterm Review Section (10/16/2012) Q1: Hexadecimal and Binary Notation - Solve the following equations and put your answers in hex, decimal and binary. Hexadecimal Decimal Binary 15 +