malloc and realloc array giving unexpected output

Question

When input is '1 2 3 4 5 6 7 8 9 10 11' Expected output should be the same as input. Yet the output is '1 2 3 4 5 6 7 8 9 10 -1850774484' . Only happens when there are more than 10 integers being inputted. Where did I go wrong with my realloc line

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

#define INITIAL_SIZE 10

int
main(int argc, char **argv){
    int i = 0, num, size = 0, n = INITIAL_SIZE;
    int *A;

    A = malloc(n * sizeof(int));

    if(A == NULL){
        printf("OUT OF MEMORY\n");
        exit(EXIT_FAILURE);
    }

    while(scanf("%d",&num) == 1 && getchar()!='\n'){
        A[i] = num;
        i++;
        size++;
        if (size >= n){
            n = n * 2;
            A = realloc(A,n * sizeof(int));
        }
    }

    printf("Int Array: ");
    for (i = 0; i <= size; i++){
        printf("%d ", A[i]);
    }
    printf("\n");

    return 0;
}

Spikatrix · Accepted Answer · 2015-06-01 12:32:15Z

5

There is nothing wrong with the realloc. You are printing an extra uninitialized index in the last loop. Fix it by changing

for (i = 0; i <= size; i++)

to

for (i = 0; i < size; i++)

The only problem with the realloc is that you don't check if it was successful just like you did for the malloc. Another problem is that you don't free the allocated memory.

edited Jun 1, 2015 at 12:32

answered Jun 1, 2015 at 12:27

Spikatrix

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12 Comments

Kyuu Over a year ago

yep my input is '1 2 3 4 5 6 7 8 9 10 11\n' and output gives '1 2 3 4 5 6 7 8 9 10' . it seems if i remove the 'getchar()...' part and end by using control-D instead gives me the correct output though

Spikatrix Over a year ago

@Qbb , Use BLUEPIXY's solution instead or add A[i]=num; size++; i++; after the while loop.

Persixty Over a year ago

@MOehm Read i as next index and size as current size! When we pass A[i] = num; with i == 9 both i and size are incremented to 10 and the realloc is triggered because it uses size >= n (or equal to). So I think the loop is fine - except it could perform an unnecessarily reallocation which is why we would normally reallocate before assignment. However this code is valid.

M Oehm Over a year ago

@DanAllen: Yes, you're right. i is accessed before it is incremented. I got that wrong, because I usually enlarge the space just before I need it, not after it. Sorry for that.

Spikatrix Over a year ago

@Qbb , I'm getting too confused. Here is the solution which uses BLUEPIXY's method. Note that I've not freed or checked the return value of realloc in the code.

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