Weeks 7-8

1
Weeks 7-8

• Memory allocation
• Pointers functions
• Complex structures with pointers, structures, etc
• Dynamic structures

2
Memory Allocation

• Where we have dealt with memory issues
• Address arithmetic we had to use array to get
  memory
• Return characters strings It is incorrect to
  return a local char array.

char encode(int x) char temp4
temp0A x return temp / wrong /
buff256
cp
malloc(256)
3
Memory Interface

• Interfaces from ltstdlib.hgt
• malloc void malloc(int size)
• calloc void calloc(int num, int size)
• realloc void realloc(void ptr, int size)
• free void free(void ptr)
• Miscellaneous
• memset void memset(void ptr, int c, int size)
• bzero void bzero(void ptr, int size)

4
Usages

• To get a dynamic memory area, which exists until
  free
• To avoid a really large array
• To avoid static arrays (array with fixed sizes)
• malloc/free

int main() int i struct student s
scanf(d d, s.id, s.num) s.bookids
(int )malloc(sizeof(int)num) while (i0
ilt s.num i) scanf(d,
s.bookidsi) free(s.bookids)
return 0
struct student int id int num /
wrong / int bookidsnum int
bookids
5
Memory issues

• Memory Leak
• Pointer aliases
• Dangling pointers
• Garbage collection

6
Rules

• Do NOT dereference a pointer before its assigned
  a memory area
• Dereference only pointers with type than void
• Always free the memory you allocated
• Be cautious of your pointer aliases and dangling
  pointers.
• Initialize your pointers with NULL and do sanity
  checks.

7
Pointer functions

• Syntax
• type ( name) (argument list)
• Compare function prototypes
• type name (argument list)
• Usage

for ( i0 ilt20 i) max max gt ai
? max ai return max int main()
int job, result, score20 int (func)(int
) scanf(d, job) func job 1 ?
find_average find_max result
func(score) return 0
int find_average(int a) int i, sum for
( i0 ilt20 i) sum ai return
sum/20 int find_max(int a) int
maxa0
8
Passing Function pointers
int main() int job, result, score20
scanf(d, job) result process (job,
score, find_average, find_max) return 0
int process(int job, int score, int ()a(int
), int()b(int )) int (func)(int
) func job 1? ab return
func(score)
9
Pointer functions in Structures
struct course int course_num int
scores20 int (find_min)(int ) int
(average)(int )
int main() int result struct course
c459 resultc459.find_min(c459.scores)
resultc459.average(c459.scores)
return 0