COMP 320

1
COMP 320

• Memory management basics
• Allocation Deallocation

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Readings

• Read BO 3.8,3.11,3.13, 10.11.
• Read HS 5.2-5.3.
• Read VDL 10.

3
Allocation

• For all data, memory must be allocated.
• Allocated memory space reserved.
• Arrays provide good examples for all the issues.
• Two questions
• When do we know how much to allocate?
• When do we allocate?
• Two possible answers for each
• Compile-time (static)
• Run-time (dynamic)

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How Much to Allocate?

• Static size specified by declaration
• Dynamic size uses malloc()
• Finds specified of bytes of unused space and
  reserves it.
  
• Essentially equivalent to new in Java C.

char c int array10
include int array (int ) mall
oc(num_items sizeof(int))
5
Using malloc()
int i i (int ) malloc(sizeof(int))
i 3
int array array (int ) malloc(num_items
sizeof(int))
array3 5
Arrays ? pointers to the initial (0th) array
element.
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Using malloc()

• Always check the return value of system calls
  like malloc() for errors.
  
• For brevity, wont in class.

int a (int ) malloc(num_items
sizeof(int)) if (a NULL) fprintf(stderr
,Out of memory.\n)
exit(1)
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When to Allocate?

• Static time
• Typically global variables
• Only one copy ever exists, so can allocate at
  compile-time.

• Dynamic time
• Typically local variables
• One copy exists for each call. May be unbounded
  of calls, so cant allocate at compile-time.

int value int main()
int f() int value int main()


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When to Allocate?

• Static time
• Some local variables
• One copy exists for all calls. Allocated at
  compile-time.

int f() static int value int m
ain()

Not very common. Mainly an old efficiency hack.
Confusingly, local static has nothing to do with
global static!
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Allocation Summary
Global declaration
Local non-static declaration
Local static declaration
Constant-size malloc()
Nonsense
Non-constant-size malloc()
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Deallocation

• Space allocated via declaration (entering scope)
  is deallocated when exiting scope.
  
• Cant refer to y or array outside of f(), so
  their space is deallocated upon return.

f() int y int array10
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Deallocation

• Space allocated by malloc() is deallocated by
  free().
  
• Not automatically deallocated (garbage collected)
  as in Scheme Java.
  
• Forgetting to deallocate leads to memory leaks
  running out of memory.
  
• Must not use a freed pointer unless reassigned or
  reallocated.

int a (int ) malloc(num_items
sizeof(int)) free(a) a (int ) malloc(
2 num_items sizeof(int))
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Deallocation

• Space allocated by malloc() is freed when the
  program terminates.
  
• If data structure is used until program
  termination, dont need to free.
  
• Entire process memory is deallocated.
• Assumes virtual memory implementation is
  bug-free!
  
• Bad assumption.

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Common Memory Management Mistakes
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Whats Wrong With This Code?
int make_array() int array10
return array
int f() int i return i

• Consider j f().
• Leads to referencing deallocated memory.
• Never return a pointer to a local variable!
• Behavior depends on allocation pattern.
• Space not reallocated ? works.
• Space reallocated ? very unpredictable.

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One Solution
int f() int i_ptr (int ) m
alloc(sizeof(int))
return i_ptr
int make_array() int array
(int ) malloc(10 sizeof(int))

return array

• Allocate with malloc(), and return its pointer.
• Upon return, space for local pointer variable is
  deallocated.
  
• But the malloc-ed space isnt deallocated until
  it is free-d.

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Whats Wrong With This Code?
/ return y Ax / int matvec(int A, int
x) int y (int ) malloc(N sizeof(in
t)) int i, j for ( i for ( jj return y
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Whats Wrong With This Code?
char p int i / Allocate space for
MN matrix / p malloc(M sizeof(char)) fo
r (i0 iar))
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Whats Wrong With This Code?
char p int i / Allocate space for
MN matrix / p (char ) malloc(M sizeof(cha
r )) for (i0 i) malloc(N sizeof(char))
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Whats Wrong With This Code?
char s 1234567 char t7 strcpy(t
, s)
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Whats Wrong With This Code?
int binheap_delete(int binheap, int
size) int packet packet binheap
0 binheap0 binheapsize - 1 size
- 1 heapify(binheap, size, 0) return(p
acket)
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Whats Wrong With This Code?
/ Search memory for a value. /
/ Assume value is present. /
int search(int p, int value)
while (p 0 p ! value)
p sizeof(int) return p
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Whats Wrong With This Code?
x (int ) malloc(N sizeof(int))
free(x) y (int ) malloc(M sizeof(int)
) for (i0 ii 1
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Whats Wrong With This Code?
void foo() int x (int ) malloc(N siz
eof(int))
return
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Whats Wrong With This Code?
struct ACons int first stru
ct ACons rest typedef struct ACons Li
st void foo() List list cons(1,cons(2
,cons(3,NULL))) free(list) return

A peek at how well define lists.