How to realloc based on size of array?

Question

Say you malloc enough memory space to hold an array of size 20. The program is running and now I need enough memory for an array of size say 40. I tried to do this using realloc but it doesn't seem to be working. My code is the following(I'm trying to find the sum of all even-valued fibonacci terms below 4million):

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char** argv){
    int i,sum,size;
    int *fibo;

    size = 20; //initial size of array
    fibo = (int *) malloc(size*sizeof(int));
    fibo[0]=1;
    fibo[1]=1;
    i=2;
    sum=0;

    while(fibo[i-1]<4000000){
            fibo[i] = fibo[i-1]+fibo[i-2];
            printf("fibo[%d] = %d\n", i, fibo[i]);
            if(fibo[i]%2 == 0){
                    sum+= fibo[i];
            }
            i++;
            if(i>size){
                    fibo = (int *) realloc(fibo, (size *= 2)*sizeof(int));
            }
    }

    printf("Sum = %d\n", sum);
    return 0;

}

Anyone know why realloc is failing, and how I can fix it?

What leads you to believe that it is realloc that is failing (rather than something else)? What symptom are you seeing? — psmears
– psmears, Commented Jan 26, 2015 at 22:18
I know I'm going to reach about 23 fibonacci terms before I go over 4 million. Sof if I allocate accordingly in the beginning, the proIgram works fine. When I use realloc, it reaches 20 and fails to add any more integers to the fibo array. ie program stops working. — modsoussi
– modsoussi, Commented Jan 26, 2015 at 22:20
(NB you probably want "if (i >= size)" rather than "if (i > size)" - otherwise when you do the realloc you've just written off the end of the array...) — psmears
– psmears, Commented Jan 26, 2015 at 22:20
@modsoussi it should be if (i == size). Greater-than should never become a possibility or you're already invoking undefined behavior. — WhozCraig
– WhozCraig, Commented Jan 26, 2015 at 22:24
As an aside, Don't cast the result of malloc (and friends). Also, you should get into the habit of checking for failure to allocate, parse, open files and connections, whatever. — Deduplicator
– Deduplicator, Commented Jan 26, 2015 at 22:25

Lamaseed · Accepted Answer · 2015-01-26 22:56:39Z

2

During the last iteration, i equals 20 but the expression

if(i>size)

is false, so you do not actually use realloc, then by writing to

fibo[20]

the program is accessing part of the memory that does not belong to it. Changing the expression to

if(i>=size)

should fix it :)

answered Jan 26, 2015 at 22:56

Lamaseed

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