Memory Management

Question 1

inaccessible memory

Answer 1

data that can have no use to the program but still takes up memory

Question 2

manual memory management

Answer 2

no garbage collector, the programmer must manually allocate and deallocate memory

Question 3

use after free

Answer 3

overwrites data used somewhere else

Question 4

double free

Answer 4

more than one free in different places

Question 5

invalid free

Answer 5

passing a non-malloced pointer to free

Question 6

not checking for allocation fail

Answer 6

always check malloc != 0

Question 7

memory leak

Answer 7

not freeing data after use

Question 8

reading uninited memory

Answer 8

security attack vector

Question 9

dangling pointers

Answer 9

pointers to freed memory

Question 10

probabilistic memory safety: replacement MM

Answer 10

• random location of objects in heap
• heap n times larger than required

Question 11

probabilistic memory safety: replicated mode

Answer 11

execute multiple copies concurrently, high chance correct programs output same values

different random seeds for allocators

Question 12

garbage collection properties

Answer 12

• intrusive, stops computation to recover memory
• real-time, interleave computation and recovery
• identification, reference counts or tracing+marking

Question 13

reference counting

Answer 13

keep count of the number of pointers to an object, when reference count == 0 add object to free list

Question 14

reference counting pros

Answer 14

• immediate recovery
• simple implementation

Question 15

reference counting cons

Answer 15

• fragmentation
• overhead on basic operations
• cyclic structures not recovered
• limited size of reference count field

Question 16

mark+sweep

Answer 16

mark all live data, sweep to collect inactive data

associate a mark with each active heap object via depth-first, in-place traversal

Question 17

mark+sweep pros

Answer 17

• can recover cyclic garbage
• uses all of memory

Question 18

mark+sweep cons

Answer 18

• can be costly for large heaps
• pauses entire program during collection
• reclaimed memory might be fragmented

Question 19

mark+sweep+compact

Answer 19

uses mark+sweep and then makes all live data contiguous

copies data breadth first, uses forwarding pointers

Question 20

forwarding pointer

Answer 20

when data is moved a forwarding pointer is placed at the old location which stores the datas new position

Question 21

2-space copy

Answer 21

memory is in two equal regions, FROM and TO

data is allocated in FROM, when FROM fills active data is copied to TO and they are swapped

Question 22

2-space copy pros

Answer 22

• recovers cycles
• compacts live data
• only scans live data

Question 23

2-space copy cons

Answer 23

wastes memory