CODEWITHSUNDEEP
carraysloopssegmentation-faultquicksort
Daniel Wasserlauf
Daniel Wasserlauf

Reputation: 11

Segmentation Fault Loop C99

This code is a part of a much larger program. There is a main function within the code and it runs fine, sorry if its messy. Within the function Quick_mode_time in the second while loop I get a segmentation error, and I dont understand how I'm messing the array up to cause this. If someone could point me in the right direction I would be very happy! 

#include <stdio.h>
#include <time.h>
#include <math.h>
#include <ctype.h>
#include <stdlib.h>
#include <stdbool.h>

#define timing_start() start = clock();
#define timing_end(f) msec = (double)(clock()-start) * 1000000.0 /CLOCKS_PER_SEC; \
    printf("Calling "#f " takes %10.2f microseconds!\n", msec);

void Quick_mode_time()
{
    srand(time(NULL));
    int end=500,final=5000;
    int a[end],beginning=0;
    double t1,t2,t_tot;
    printf("***************COMPARISON***************\n");

    printf("                quicksort       bubblesort      (in microseconds)\n");
    while(end!=final){

        while(beginning<end)
        {
            a[beginning]=rand()%end;  // <--- This causes a segmentation fault
            beginning++;
        }
        printf("N=%d",end);

        t1=clock();
        quicksort_time(a,0,end);

        t2=clock();
        t_tot= (double)(t2-t1)*1000/CLOCKS_PER_SEC;
        printf("\t%7.3f\t", t_tot);
        end+=500;
    }
}

void quicksort_time(int x[],int first,int last){
    int pivot,j,temp,i;
    // The Quick Sorting algorithim is below!
    if(first<last){
        pivot=first;
        i=first;
        j=last;

        while(i<j){
            while(x[i]<=x[pivot]&&i<last)
                i++;
            while(x[j]>x[pivot])
                j--;
            if(i<j){
                temp=x[i];
                x[i]=x[j];
                x[j]=temp;
            }
        }

        temp=x[pivot];
        x[pivot]=x[j];
        x[j]=temp;
        quicksort_time(x,first,j-1);
        quicksort_time(x,j+1,last);
    }
}

Upvotes: 0

Views: 75

Answers (2)

The Dark
The Dark

Reputation: 8514

This line: 

int a[end]

creates an array of 500 ints (end is set to 500 in the line before)

This line

a[beginning]=rand()%(end+1-end)+end;

writes to a[beginning] which is OK as long as beginning is less than 500.

This seems to be checked by this line just above:

while(beginning<end)

but later on in your code, you have:

end+=500;

so suddenly, end can be > 500, which means beginning can be > 500. Which means you write outside your array boundaries.

One fix would be to change your declaration so that a is big enough:

int a[final]

Upvotes: 2

rwaeng
rwaeng

Reputation: 26

Your outer while loop never exits because you never modify the values of end or final.

Upvotes: 0

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