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Homework / Exam

• Finishing K&R Chapter 5 today– Skipping sections 5.7-5.9 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

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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!

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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 mallocint *p;p = (int *) malloc(n * sizeof(int));

• Contents of memory block are NOT initialized!

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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

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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( )

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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!!

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Pointers to Functions

• What is &foobar in the following code fragment?int main ()

{

… &foobar …

}

int foobar (void)

{

/* some code here */

}

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Pointers to Functions

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

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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 */}

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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? */

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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 variablesqsort( … , (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!

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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 */

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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) */

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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 */

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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.x box.pt2.x

box.pt1.y

box.pt2.y

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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);

}

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structs• Memory allocation for structs

– Two point structs pt1 and pt2

– One rect struct box containing two point structs

pt1

pt1.x pt1.y pt2.x pt2.y

pt2

pt1

pt1.x pt1.y pt2.x pt2.y

pt2box