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Lecture08: Static Variable, Memory Model, Top-Down Program Development

4/22/2013

Outline

• Midterm results• Static variables• Memory model• Program development

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Midterm results

• Good job – most of you have done well.• Avg: 84.68• Std: 36

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Static Local VariablesA permanent variable inside a function whose value lifetime lasts through the entire program execution (vs. local variable, its storage is gone at the end of the function)

#include <stdio.h>

void f1(){ static int k=0; int j = 10; printf("value of k=%d j=%d\n", k, j); k=k+10;}

void main(){ int i = 0; f1(); printf("after first call\n"); f1(); printf("after second call\n"); f1(); printf("after third call\n");}

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Exercise 8-1

Complete the following function sumit() that reads an integer from the user, computes an accumulative sum, and print it out.

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void sumit(){ static int sum = 0; ..int num; printf("\nEnter a number: "); ..scanf("%d", &num); ..sum+=num; printf("The current sum is: %d", sum);}

int main(){ int i =0; printf("Enter 5 numbers to be summed\n"); for(i = 0; i<5; ++i) sumit(); printf("Program completed\n"); return 0;}

C Memory Model – where are variables & program code stored in the computer memory?

• Program code• Function variables

– Arguments– Local variables– Return location

• Global Variables– Statically allocated– Dynamically allocated

int fact (int n){

return(n*fact(n-1));}

void main() { … fact(5); …}

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The Stack

• Stores– Function local variables– Temporary variables– Arguments for next function call– Where to return when function ends

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The Stack

• Managed by compiler– One stack frame each time function called– Created when function called– Stacked on top (under) one another– Destroyed at function exit

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What can go wrong?

• Recall that local variables are stored on the stack• Memory for local variables is deallocated when function

returns• Returning a pointer to a local variable is almost always a bug!

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char *my_strcat(char *s1, char *s2){

char s3[1024];strcpy(s3, s1);strcat(s3, s2);return s3;

}

What Can Go Wrong?

• Run out of stack space• Unintentionally change values on the stack

– In some other function's frame– Even return address from function

• Access memory even after frame is deallocated

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The Heap

• C can use space in another part of memory: the heap– The heap is separate from the execution stack– Heap regions are not deallocated when a function returns

• The programmer requests storage space on the heap– C never puts variables on the heap automatically– But local variables might point to locations on the heap– Heap space must be explicitly allocated and deallocated by the

programmer

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malloc()

• Library function in <stdlib.h>– Stands for memory allocate

• Requests a memory region of a specied size– Syntax: void *malloc(int size)– void * is generic pointer type

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Usage

int main(){

int* p1 = (int *) malloc(10 * sizeof(int));if (p1 == NULL){

// do cleanup}// do somethingfree(p1);return 0;

}

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• Good to check the return value from malloc()• Must explicitly free memory when no longer in use

What Can Go Wrong?

• Run out of heap space: malloc returns 0• Unintentionally change other heap data• Access memory after free'd• free memory twice

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Multidimensional Array

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• On the stack: int a[10][20];– Initialization: int a[][] = {{1, 2, 3}, {4, 5, 6}};– Accessing the array: a[1][0]

• On the heap: int **a = (int **) malloc()– The array size is not known until runtime and stored in a variable x

int **a = (int **) malloc(x * sizeof(int *));for (int i = 0; i < 10; i++){a[i] = (int *) malloc(20 * sizeof(int));

}

• Don't forget to free them!

Exercise 8-2

Write a program that (1) asks for the number of friends, (2) asks for a name, and (3) checks a series of strings to see which one matches the names of friends. Please use •Use malloc() to create this multi-dimensional array to stores friends’ names. Assume that each name is less than 10 characters. •Use <string.h> library•Use scanf(“ %s”, ..) to read a name separated by one or more whitespacesYou can modify from the skeleton code on the next slide.

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#include <stdio.h>#include <string.h>int main(){ char** friends; char name[10]; int n, i;

printf("Input the number of your friends: "); scanf("%d", &n);

/* call malloc() to allocate dynamic memory for friends and friends[1..n-1] */ …

for (;;) { printf("Input a name: "); scanf(" %s", name); for (i=0; i<n; i++) { if (strcmp(friends[i], name) == 0) { printf("%s is friend #%d\n", name, i+1); break; } } if (i == n) { printf("%s is not a friend\n", name); } }

/* call free() to free memory for friends and friends[1..n-1] */…

}

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Program development (important!)

• A Common problem– Know all C commands but don’t know how to use them to write a

program.• Top-down approach – always work!

– Start by writing down the main steps (in words/comments, not code) of a program. Like a paragraph in an essay.

– For each difficult main step, break it down into smaller steps. Again in words/comments, not code.

– Apply this “divide-and-conquer” approach until the steps are simple enough to code.

– Write code

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Last Year’s Midterm Problem 1

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Top-Down Approach to Program Development• Start by writing down the main steps (in words/comments, not code)

of a program. Like a paragraph in an essay.• For each difficult main step, break it down into smaller steps.

Again in words/comments, not code. • Apply this “divide-and-conquer” approach until the steps are

simple enough to code. • Write code

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Top-Down Approach to Program Development• Start by writing down the main steps (in words/comments, not code)

of a program. Like a paragraph in an essay.• For each difficult main step, break it down into smaller steps.

Again in words/comments, not code. • Apply this “divide-and-conquer” approach until the steps are

simple enough to code. • Write code

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Top-Down Approach to Program Development• Start by writing down the main steps (in words/comments, not code)

of a program. Like a paragraph in an essay.• For each difficult main step, break it down into smaller steps.

Again in words/comments, not code. • Apply this “divide-and-conquer” approach until the steps are

simple enough to code. • Write code

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Systematic way of writing programs

• Like writing an essay.• Start with an outline that describes what you are going to

write.• Okay to write the comments in Chinese.

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Exercise 8-3

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Top-Down Approach to Program Development• Start by writing down the main steps (in words/comments, not code)

of a program. Like a paragraph in an essay.• For each difficult main step, break it down into smaller steps.

Again in words/comments, not code. • Apply this “divide-and-conquer” approach until the steps are

simple enough to code. • Write code

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Top-Down Approach to Program Development• Start by writing down the main steps (in words/comments, not code)

of a program. Like a paragraph in an essay.• For each difficult main step, break it down into smaller steps.

Again in words/comments, not code. • Apply this “divide-and-conquer” approach until the steps are

simple enough to code. • Write code

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Top-Down Approach to Program Development• Start by writing down the main steps (in words/comments, not code)

of a program. Like a paragraph in an essay.• For each difficult main step, break it down into smaller steps.

Again in words/comments, not code. • Apply this “divide-and-conquer” approach until the steps are

simple enough to code. • Write code

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Top-Down Approach to Program Development• Start by writing down the main steps (in words/comments, not code)

of a program. Like a paragraph in an essay.• For each difficult main step, break it down into smaller steps.

Again in words/comments, not code. • Apply this “divide-and-conquer” approach until the steps are

simple enough to code. • Write code one step at a time• Debug each step if necessary

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