1 Homework / Exam Finishing K&R Chapter 5 today –Skipping sections for now –Not covering section 5.12 Starting K&R Chapter 6 next Continue HW5 – Due at start of class 17 Exam Class 18 –Through end of K&R 6.4 plus MAKE

2 Using malloc( ) and free( ) To get a pointer p to a block of memory that is n characters in length, program calls p = malloc(n); When it is finished with that memory, the program returns it by calling free(p); Sounds simple, huh? It is NOT so simple!

3 Using malloc( ) and free( ) malloc returns a pointer to void (void *) that is the address of a memory block of n bytes If you need a pointer to n of a specific type, you must request a memory block in size of the type and cast pointer returned by malloc int *p; p = (int *) malloc(n * sizeof(int)); Contents of memory block are NOT initialized!

4 Using malloc and free If it can not provide the requested memory, malloc returns a NULL pointer value If you dereference a NULL pointer to access memory  System Crash!! Always check to be sure that the pointer returned by malloc is NOT equal to NULL If pointer is NULL, code must take appropriate recovery action to handle lack of memory

5 Using malloc and free Call to free does not clear the program’s pointer to the memory block, so it is now a “stale” pointer If program uses pointer after free( ) by accessing or setting memory via pointer, it could overwrite data owned by another program  System Crash! If program calls free again with the same pointer, it releases memory possibly owned by a different program now  System Crash! SHOULD set pointer to NULL after calling free( )

6 Using malloc and free However, if you set the pointer to a memory block to NULL before calling free, you have caused the system to lose the memory forever This is called a memory leak!! If it happens enough times  System Crash!! MUST not clear or overwrite a pointer to a memory block before calling free!!

7 Pointers to Functions What is &foobar in the following code fragment? int main () { … &foobar … } int foobar (void) { /* some code here */ }

8 Pointers to Functions &foobar is the address of the entry point to the function foobar - in code space, not in data space int main () { … &foobar … /* pointer to foobar code’s entry point */ } int foobar (void) { /* some code here */ }

9 Pointers to Functions Why would we need the address of the entry point to foobar and how would we use it? We can pass the address of foobar as an argument to another function that needs to call foobar: … result = function (i, j, &foobar); … int foobar (void) { /* some code here */ }

10 Pointers to Functions Define, initialize, and use a pointer to a function /* Define the function pointer fooptr */ int (*) (void ) fooptr; /* type is “function pointer” */ /* set function pointer fooptr = address of foobar */ fooptr = &foobar; /* Call the function foobar via the pointer fooptr */ result = (*fooptr) (void); /* Why first parens? */

11 Pointers to Functions, K&R 5.11 Function prototype with a pointer to a function void qsort ( …, int (*comp) (void *, void *)); Function call passing a pointer to a function and casting the data types of the argument variables qsort( …, (int (*) (void *, void *)) strcmp); /* strcmp is defined with char * argument variables*/ /* but qsort needs a function with void * as arguments */ qsort calling the function via passed the pointer if ((*comp) (v[i], v[left]) < 0) … Note: qsort here is NOT same as standard library qsort, but it uses the same type of function pointer argument!

12 structs, K&R 6 A struct is a collection of variables, possibly of different types, grouped under a single name for common reference as a unit. struct point {/* with optional tag */ int x; /* member x */ int y;/* member y */ } pt, q; /* variable names */ or struct {/* w/o optional tag */ int x, y;/* two members */ } pt, q;/* variable names */

13 structs The defined struct point is like a new “type” With the tag, point, can declare other variables: struct point pt1, maxpt = {320, 200}; Reference to struct members: pt1.x = 320; pt1.y = 200; /* alternate init */ printf("(%d, %d)\n", pt1.x, pt1.y); /* prints out as (320, 200)*/

14 structs Defining a struct inside a struct (nesting). struct rect { struct point pt1; /* lower left */ struct point pt2; /* upper right */ }; struct rect box;/* declare box as a rect */

15 structs pt1 is lower left hand corner of box pt2 is upper right hand corner of box: box.pt1.x < box.pt2.x box.pt1.y < box.pt2.y y xbox.pt1.xbox.pt2.x box.pt1.y box.pt2.y

16 structs /* Find area of a rectangle */ int area = rectarea (box); … int rectarea (struct rect x) { return (x.pt2.x - x.pt1.x) * (x.pt2.y - x.pt1.y); }

17 structs Memory allocation for structs –Two point structs pt1 and pt2 –One rect struct box containing two point structs pt1 pt1.xpt1.ypt2.xpt2.y pt2 pt1 pt1.xpt1.ypt2.xpt2.y pt2 box