Answered: architecture x86_64

Database System Concepts

7th Edition

ISBN:9780078022159

Author:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan

Publisher:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan

Chapter1: Introduction

Section: Chapter Questions

Problem 1PE

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Question

The following code attempts to find all the prime numbers between 2 and n.

1. When compiled, why is the following error produced?:

Undefined symbols for architecture x86_64:
"_main", referenced from:
implicit entry/start for main executable
ld: symbol(s) not found for architecture x86_64

2. Fix code so that it compiles with no errors

#include <stdio.h>

#include <stdlib.h>

#include <math.h>

#include <sys/time.h>

//helper function: computes wall clock time

double getTime(struct timeval ts, struct timeval te){

doubletime = te.tv_sec - ts.tv_sec + (te.tv_usec - ts.tv_usec)/1.e6;

returntime;

}

//helper function: allocates an array of a specified length and returns a pointer

int * allocateArray(int len) {

int * result = malloc(len * sizeof(int));

returnresult;

}

//helper function: prints out elements of array separated by spaces

void printArray(int * arr, int len) {

inti;

for (i = 0; i < len; i++) {

printf("%d ", arr[i]);

}

printf("\n");

}

//helper function: checks to see if a number is prime

int isPrime(int x) {

inti;

for (i = 2; i < sqrt(x)+1; i++) { //no prime number is less than 2

if (x % i == 0) { //if the number is divisible by i

return0; //it is not prime

}

return1; //otherwise it is prime

}

// finds the next prime

int getNextPrime(int prev) {

intnext = prev + 1;

while (!isPrime(next)) { //while the number is not prime

next++; //increment and check again

}

returnnext;

}

// generates a sequence of primes

int genPrimeSequence(int * array, int limit) {

inti;

intlen = limit;

if (len == 0) return0;

array[0]=2; //initialize the first number to 2

for (i = 1; i < len; i++) {

array[i] = getNextPrime(array[i-1]); //fill in the array

if (array[i] > limit){

len = i;

returnlen;

}

returnlen;

}

int main(int argc, char ** argv) {

if (argc != 2) {

fprintf(stderr, "usage: %s <num>\n", argv[0]);

printf("where <num> is upper limit of the desired range of primes\n");

return1;

}

structtimevaltstart, tend;

intlimit = strtol(argv[1], NULL, 10);

gettimeofday(&tstart, NULL);

int * array = allocateArray(limit); //array can't be longer than the limit size

//gettimeofday(&tend, NULL);

//printf("Time to allocate: %g\n", getTime(tstart, tend));

//gettimeofday(&tstart, NULL);

intlength = genPrimeSequence(array, limit);

gettimeofday(&tend, NULL);

printf("Time to generate primes: %g\n", getTime(tstart, tend));

printf("%d primes found.\n", length);

//printf("The first %ld prime numbers are:\n", limit);

//printArray(array, length);

return0;

}

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