Wednesday Week 04

Wednesday Week 04 1/16

In This Lecture ...

Memory Management (Sec.10.1)

Coming Up ...

Linked Lists (Ch.10)

Dynamic Memory Allocation 2/16

So far, we have considered static memory allocation

all objects completely defined at compile-time
sizes of all objects are known to compiler

Examples:

int   x;     // 4 bytes containing a 32-bit integer value
char *cp;    // 4 bytes containing address of a char
typedef struct {float x; float y;} Point;
Point p;     // 8 bytes containing two 32-bit float values
char  s[20]; // array containing space for 20 1-byte chars

... Dynamic Memory Allocation 3/16

In many applications, fixed-size data is ok.

In many other applications, we need flexibility.

Examples:

char name[MAXNAME];  // how long is a name?
char line[MAXLINE];  // what's the longest line?
char words[MAXWORDS][MAXWORDLENGTH];
                     // how many words are there?
                     // how long is each word?

With fixed-size data, we need to guess sizes ("large enough").

... Dynamic Memory Allocation 4/16

Fixed-size memory allocation:

allocate as much space as we might ever possibly need

Dynamic memory allocation:

allocate as much space as we actually need
determine size based on inputs

But how to do this in C?

all data allocation methods so far are "static"
however, stack data (when calling a function) is created dynamically (size is known)

Dynamic Data Example 5/16

Problem:

read integer data from stdin
first number tells how many numbers follow
rest of numbers are read into a vector
subsequent computation uses vector (e.g. sorts it)

Example input: 6 25 -1 999 42 -16 64

How to define the vector?

... Dynamic Data Example 6/16

Suggestion #1: allocate a large vector; use only part of it

#define MAXELEMS 1000

// how many elements in the vector
int numberOfElems;
scanf("%d", &numberOfElems);
assert(numberOfElems <= MAXELEMS);

// declare vector and fill from stdin
int i, vector[MAXELEMS];
for (i = 0; i < numberOfElems; i++) {
	scanf("%d", &vector[i]);
}

Works ok, unless too many numbers; usually wastes space.

assert() terminates program with standard error message if test fails.

... Dynamic Data Example 7/16

Suggestion #2: use variables to give object sizes

// how many elements in the vector
int numberOfElems;
scanf("%d", &numberOfElems);

// declare vector and fill from stdin
int i, vector[numberOfElems];
for (i = 0; i < numberOfElems; i++) {
	scanf("%d", &vector[i]);
}

Produces compiler error (compiler needs to know object sizes)

... Dynamic Data Example 8/16

Suggestion #3: create vector after count read in

// how many elements in the vector
int numberOfElems;
scanf("%d", &numberOfElems);

// declare vector and fill from stdin
int i, *vector;
size_t numberOfBytes;
numberOfBytes = numberOfElems * sizeof(int);
vector = (int *)malloc(numberOfBytes);
assert(vector != NULL);
for (i = 0; i < numberOfElems; i++) {
	scanf("%d", &vector[i]);
}

Works unless the heap is already full (very unlikely)

The malloc() function 9/16

Recall memory usage within C programs:

... The malloc() function 10/16

malloc() function interface

void *malloc(size_t n);

What the function does:

attempts to reserve a block of n bytes in the heap
returns the address of the start of this block
if insufficient space left in the heap, returns NULL

Note: size_t is essentially an unsigend int

but has specialised interpretation of applying to memory sizes measured in bytes

... The malloc() function 11/16

Example use of malloc:

... The malloc() function 12/16

Things to note about void *malloc(size_t):

it is defined as part of stdlib.h
its parameter is a size in units of bytes
its return value is a generic pointer (void *)
the return value must be cast to an appropriate type
the return value must always be checked (may be NULL)

Size is determined by #Elements * sizeof(ElementType)

... The malloc() function 13/16

malloc() returns a pointer to a data object of some kind.

Things to note about objects allocated by malloc():

they exist until explicitly removed (program-controlled lifetime)
they are accessible while some variable references them
if no active variable references an object, it is garbage

The function free() releases objects allocated by malloc()

... The malloc() function 14/16

Usage of malloc() should always be guarded:

int *vector, length, i;
...
vector = (int *)malloc(length*sizeof(int));
// but malloc() might fail to allocate
assert(vector != NULL);
// now we know its safe to use vector[]
for (i = 0; i < length; i++) {
	... vector[i] ...
}

Alternatively:


int *vector, length, i;
...
vector = (int *)malloc(length*sizeof(int));
// but malloc() might fail to allocate
if (vector == NULL) {
	fprintf(stderr, "Out of memory\n");
	exit(1);
}
// now we know its safe to use vector[]
for (i = 0; i < length; i++) {
	... vector[i] ...
}

Exercise: Creating Objects via malloc() 15/16

Give variable declarations and statements that on the heap will create:

a string buffer to hold strings of length ≤ K
a vector of L double-precision floating point values

an object of type Car, defined as:


typedef struct {char rego[7]; float premium;} Car;

For each object:

draw a diagram of what it might look like in memory
show how you would initialise it to some plausible values

Tips for Next Week's Lab 16/16

Structured data types

3 exercises

Define structured data types, initialise and print elements


boss.salary = 675000.00;
strcpy(boss.name, "John Elliot");
printf("Name: %s\n", boss.name);

Handle arrays of structured data


typedef struct {...} WorkerT;
WorkerT worker[1500];
for (i = 0; i < 1500; i++) {
	... worker[i].salary ...

Dynamically allocate memory using malloc()


*** test data file ***
20
Xiang 5
Bin 1
...


#define NAMLEN 50
typedef struct { char name[NAMELEN]; int colour; } Preferences;

Preferences *firstMember;
scanf("%d", &number);
firstMember = (Preferences *)malloc(number * sizeof(Preferences));

Use of structptr->elem
```
... firstMember->name ...
```

Produced: 12 Aug 2016