dynamic array in C resizing

Question

I've got a problem with dynamic array. I have to write a program which adds a character to dynamic array starting with 0 elements going to 10. I mustn't use realloc and I have to free the array each time.

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

void add(int **ptab, int n, int new_elem)
{

    int *tab2, y;

    tab2 = malloc(n * sizeof(int));

    for(y = 0; y < n; y++)
    {
        tab2[y] = (*ptab)[y];
    }
    *ptab = tab2;   
    (*ptab)[n] = new_elem;
    free(ptab);
}

main()
{
    int *ptab, i, x;

    *ptab = NULL;
    for(i = 0; i < 10; i++)
    {
        scanf("%d", &x);
        add(&ptab, i, x);
    }
    for(i = 0; i < 10; i++)
    {
        printf("%d", ptab[i]);
    }
}

*ptab = NULL; --> ptab = NULL;, tab2=malloc (n * sizeof(int)); --> tab2=malloc ((n+1) * sizeof(int)); — BLUEPIXY
– BLUEPIXY, Commented Dec 4, 2014 at 18:31
function add() malloc, need to insert '+1' so will be room for an element to be added. Otherwise this line: (*ptab)[n] = new_elem; is writing past the end of the allocated memory. also when function add() parameter 'n' is 0, malloc returns NULL, so be sure to add the '+1' to the malloc. Note: checking the returned value from malloc for successful operation would have caught that problem. — user3629249
– user3629249, Commented Dec 4, 2014 at 22:42

Useless · Accepted Answer · 2014-12-04 18:27:01Z

3

*ptab=tab2;   
(*ptab)[n]=new_elem;
free(ptab);

should be

free(*ptab);
*ptab=tab2;   
(*ptab)[n]=new_elem;

Currently, you're overwriting the old array pointer before freeing it, so you no longer know what you're supposed to free.

answered Dec 4, 2014 at 18:27

Useless

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1 Comment

John Bollinger Over a year ago

Also, there is a tremendous difference between the original free(ptab) and this corrected version's free(*ptab).

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