cs 314 principles of programming languages lecture 10 · cs 314 principles of programming languages...

CS 314 Principles of Programming Languages

Lecture 10

Zheng Zhang

Department of Computer ScienceRutgers University

Friday 7th October, 2016

Zheng Zhang 1 CS@Rutgers University

Class Information

I Homework 1 grades and sample solution released.

I Homework 4 will be released by the end of today.

I First project will be posted by the end of this week.

I Midterm exam: Friday October 28 (tentative), in class, closedbook, closed notes.


Homework 2 Extra-credit

I Section 1: Min Chai, Krishna Patel, Karl Xu.

I Section 2: Akhila Narayan, Azizur Rahman, BhavinkumarHirpara, Christian Suasi, Elina Chhabra, Heman Gandhi, RajPatel, Renard Tumbokon.

I Section 3: Xinran Shi, David Domingo


Review: Syntax Directed Translation

Examples:

1. Interpreter

2. Code generator

3. Type checker

4. Performance estimator

Use hand-written recursive descent LL(1) parser


Review - Example: LL(1) Parser For Prefix Expression

1: <expr> ::= + <expr> <expr> |2: <digit>3: <digit> :: = 0 | 1 | 2 | 3 | . . . | 9

+ 0..9 other

<expr> r1 r2 error<digit> error r3 error

void expr( ): // returns value of expressionint val1, val2; // valuesswitch token {

case +: token := next token( );expr( ); expr( );

case 0..9: digit( );. . .

}

void digit( ): // returns value of constantswitch token {

case 1: token := next token( );case 2: token := next token( );. . .

}Zheng Zhang 5 CS@Rutgers University

Review - Example: Simple Code Generation

<expr> ::= + <expr> <expr> |<digit>

<digit> :: = 0 | 1 | 2 | 3 | . . . | 9

int expr: // returns target register of operationint target reg;// “fresh” registerint reg1, reg2; // other registersswitch token {

case +: token := next token( );target reg = next register( );reg1 = expr( ); reg2 = expr( );CodeGen(ADD, reg1, reg2, target reg);return target reg;

case 0..9: return digit( );. . .

}

int digit: // returns target register of operationint target reg; // “fresh” registerswitch token {

case 1: token := next token( );target reg = next register( );CodeGen(LOADI, 1, target reg);return target reg;

case 2: token := next token( );target reg = next register( );CodeGen(LOADI, 2, target reg);return target reg;

. . .}


Review - Example: Simple Code Generation

What happens when you parse subprogram“+ 2 + 1 2” ?

Assumption:first call to next register( ) will return 1

The parsing produces:

LOADI 2 => r2

LOADI 1 => r4

LOADI 2 => r5

ADD r4, r5 => r3

ADD r2, r3 => r1


Review: Imperative Programming Languages

Imperative: Sequence of state-changing actions.I Manipulate an abstract machine with:

1. Variables naming memory locations2. Arithmetic and logical operations3. Reference, evaluate, assign operations4. Explicit control flow statements

I Key operations: Assignment and “Goto”I Fits the von Neumann architecture closely

Von Neumann Architecture


Review - C: An Imperative Programming Language

Expressions: include procedure and function calls andassignments.

Control Structures:

I if statements, with and without else clauses

I loops, with break and continue exits

while ( <expr> ) <stmt>do <stmt> while ( <expr> )for ( <expr> ; <expr> ; <expr> ) <stmt>

I switch statements

I goto with labelled branch targets


Review: Pointers in C

Pointer: Variable whose R-values (content) is the L-value(address) of a variable

I “address-of” operator &

I dereference (“content-of”) operator ∗

5p x

*p = 5;

p = &x;

p xint *p, x;

x = 12;p x

12


Stack vs. Heap

Stack:

I Procedure activations, statically allocated local variables,parameter values

I Lifetime same as subroutine in which variables are declared

I Stack frame is pushed with each invocation of a subroutine,and popped after subroutine exit

Heap:

I Dynamically allocated data structures, whose size may not beknown in advance

I Lifetime extends beyond subroutine in which they are created

I Must be explicitly freed or garbage collected


Maintaining Free List

I allocate: continuous block of memory; remove space fromfree list (here: singly-linked list).

I free: return to free list after coalescing with adjacent freestorage (if possible); may initiate compaction.


Maintaining Free List

free space pointerALLOCATE

free space pointer

free space pointerFREE

free space pointer


Problems with Explicit Control of Heap

I Dangling referencesI Storage pointed to is freed, but pointer (or reference) is not

set to NULLI Able to access storage whose values are not meaningful

I GarbageI Objects in heap that cannot be accessed by the program any

moreI Example

int *x, *y;

x = (int *) malloc(sizeof(int));

y = (int *) malloc(sizeof(int));

x = y;

I Memory leaksI Failure to release (reclaim) memory storage builds up over time


Example: Singly-linked list

Let’s deallocate, i.e., free all list elements.

#include "list.h"

/* GLOBAL VARIABLES */

listcell *head, *new_cell, *current_cell;

int main (void) {

int j;

/* CREATE FIRST LIST ELEMENT */

. . .

/* CREATE 9 MORE ELEMENTS */

. . .

/* DEALLOCATE LIST */

for (current_cell = head;

current_cell != NULL;

current_cell = current_cell->next)

free(current_cell);

}

. . .

}

Does this work?Zheng Zhang 15 CS@Rutgers University

What went wrong?

Uninitialized variables and “dangerous” casting

#include <stdio.h>

#include <stdlib.h>

int main(void)

{

int *a;

*a = 12;

printf("%x %x: %d\n", &a, a, *a);

a = (int *) 12;

printf("%d\n", *a);

}

> a.outeffff60c effff68c: 12

Segmentation fault (core dumped)

Note: Segmentation faults result in the generation of a core filewhich can be rather large. Don’t forget to delete it.Zheng Zhang 16 CS@Rutgers University

What went wrong?

That’s better!

#include <stdio.h>

#include <stdlib.h>

int main(void)

{

int *a = NULL; /* good practice */

a = (int *) malloc(sizeof(int));

*a = 12;

printf("%x %x: %d\n", &a, a, *a);

}

> a.outeffff60c 20900: 12


What went wrong?

The machine or compiler must be broken!?!!

#include <stdio.h>

#include <stdlib.h>

int main (void)

{

int i;

char *string = "Hello, how are you today.";

printf("\n%s\n", string);

for (i=0; string[i] != ’.’; i++) {

if (string[i] = ’ ’)

for (; string[i] = ’ ’;i++);

printf("%c", string[i]);

}

printf(".\n");

}

> a.outHello, how are you today.

Segmentation fault (core dumped)Zheng Zhang 18 CS@Rutgers University

What went wrong?

“=” is not the same as “==”

#include <stdio.h>

#include <stdlib.h>

int main (void)

{

int i;

char *string = "Hello, how are you today.";

printf("\n%s\n", string);

for (i=0; string[i] != ’.’; i++) {

if (string[i] == ’ ’)

for (; string[i] == ’ ’;i++);

printf("%c", string[i]);

}

printf(".\n");

}

> a.outHello, how are you today.

Hello,howareyoutoday.Zheng Zhang 19 CS@Rutgers University

What went wrong?“Aliasing” and freeing memory#include <stdio.h>

#include <stdlib.h>

int main(void)

{

int *a = NULL; int *b = NULL; int *c = NULL;

a = (int *) malloc(sizeof(int));

b = a; *a = 12;

printf("%x %x: %d\n", &a, a, *a);

printf("%x %x: %d\n", &b, b, *b);

free(a);

printf("%x %x: %d\n", &b, b, *b);

c = (int *) malloc(sizeof(int));

*c = 10;

printf("%x %x: %d\n", &c, c, *c);

printf("%x %x: %d\n", &b, b, *b);

}

> a.outeffff60c 209d0: 12effff608 209d0: 12effff608 209d0: 12

effff604 209d0: 10 effff608 209d0: 10


What went wrong?

Use a subroutine to create an object

#include <stdio.h>

#include <stdlib.h>

/* TYPE DEFINITION */

typedef struct cell listcell;

struct cell

{ int num;

listcell *next;

};

listcell *head = NULL;

listcell *create_listcell() {

listcell new;

new.num = -1; new.next = NULL;

return &new;

}

int main (void) {

head = create_listcell();

printf("head->num = %d\n", head->num);

}

> gcc stack.cstack.c: In function ‘create listcell’:stack.c:17: warning: function returns address of local variable

> ./a.out

head→num = -1


What went wrong?

Use a subroutine to create an object: malloc

#include <stdio.h>

#include <stdlib.h>

/* TYPE DEFINITION */

typedef struct cell listcell;

struct cell

{ int num;

listcell *next;

};

listcell *head = NULL;

listcell *create_listcell() {

listcell *new;

new = (listcell *) malloc(sizeof(listcell));

new->num = -1; new->next = NULL;

return new;

}

int main (void) {

head = create_listcell();

printf("head->num = %d\n", head->num);

}

> gcc heap.c> ./a.out

head→num = -1


Pointers and Arrays in C

Pointers and arrays are similar in C:I array name is pointer to a[0]:

after

int a[10];int *pa;pa = &a[0];

pa and a have the same semantics

I pointer arithmetic is array indexing

pa+1 and a+1 point to a[1]

I exception: an array name is a constant pointer

a++ is ILLEGALa=pa is ILLEGAL (pa=a is LEGAL!)


Next Lecture

Things to do:Continue programming in C.

Read Scott: Chap. 3.1-3.4 ;


cs 314 principles of programming languages lecture 10 · cs 314 principles of programming languages...

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