Problems using realloc for decreasing

Question

I am trying to delete with realloc but it isn't working the way I expected...

void deallocates(int** v, int size, int original_size) {
    int i;

    *v = (int*)realloc(*v, sizeof(int) * size);
    printf("\nAFTER REALLOC FOR SIZE = %d\n", size);

    for (i = 0; i < original_size; i++) {
        printf("%d ", (*v)[i]);
    }
}

int main() {
    int i, *v, size, original_size;

    srand(time(NULL));

    printf("size of the vector: ");
    scanf("%d", &original_size);

    v = (int *)malloc(sizeof(int) * original_size);

    printf("before realloc...\n");

    for (i = 0; i < original_size; i++) {
        v[i] = rand() % 100;
        printf("%d ", v[i]);
    }
    size = original_size;

    for (i = 1; i < size; i++)
        deallocates(&v, size - i, original_size);
}

The values that I wanted to be deleted sometimes remains. Please see this photo with the output of my code. I painted a red mark at the lines that are annoying me: https://ibb.co/C1TMHF5

Just because you reallocate a vector doesn't mean the values are deleted/erased. You'll have to manually "delete" (set to zero or whatever value seem appropriate) the values before reallocation. Accessing memory locations outside the allocated vector is `Undefined behaviour`. — Sani Huttunen, Mar 31 '19 at 15:59
Accessing deallocated objects invokes *UB*. Your program may coredump, may print the value the object had when it was valid, may print 0, may open the CD tray, ... may behave different if there is a full moon ... — pmg, Mar 31 '19 at 15:59

score 0 · Answer 1 · answered Mar 31 '19 at 16:15

Your code has undefined behavior because you access memory beyond the end of the allocated block. You cannot safely examine the bytes beyond the new size of the reallocated object.

Here is a modified version:

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

void deallocates(int **v, int size) {
    int *newptr;
    int i;

    newptr = realloc(*v, sizeof(int) * size);
    printf("After realloc for size=%d:", size);
    if (newptr == NULL) {
        printf(" reallocation failure\n");
    } else {
        *v = newptr;
        for (i = 0; i < size; i++) {
            printf(" %d", (*v)[i]);
        }
        printf("\n");
    }
}

int main() {
    int i, *v, size, original_size;

    srand(time(NULL));

    printf("size of the vector: ");
    if (scanf("%d", &original_size) != 1)
        return 1;

    v = malloc(sizeof(int) * original_size);
    if (v == NULL)
        return 1;

    printf("Before realloc: ");
    for (i = 0; i < original_size; i++) {
        v[i] = rand() % 100;
        printf(" %d", v[i]);
    }
    printf("\n");

    size = original_size;
    for (i = 1; i < size; i++)
        deallocates(&v, size - i);
    free(v);
    return 0;
}

Output:

size of the vector: 10
Before realloc:  64 90 47 15 62 4 19 67 95 5
After realloc for size=9: 64 90 47 15 62 4 19 67 95
After realloc for size=8: 64 90 47 15 62 4 19 67
After realloc for size=7: 64 90 47 15 62 4 19
After realloc for size=6: 64 90 47 15 62 4
After realloc for size=5: 64 90 47 15 62
After realloc for size=4: 64 90 47 15
After realloc for size=3: 64 90 47
After realloc for size=2: 64 90
After realloc for size=1: 64

Problems using realloc for decreasing

1 Answers1