Re allocating C array for more space

Question

Im writing a program with a function add(a , i, n) which will add 'i' as an element to 'a', but if the array 'a' runs out of space, then I need to realloc more memory to the array. Im stuck here:

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

int add(int* a, int i, int n);

int main(){
    int n = 20;
    int *a = (int*) malloc(n*sizeof(int));
    int i;

    for (i = 0; i < 100000; i++){
        n = add(a, i, n);
        printf("a[%d]=%d\n",i,(int)a[i]);
    }
    return 0;
}

int add(int *a, int i, int n){
    if (i >= n){
        n++;
        int* b = (int*) realloc(a, n*sizeof(int));
        a[i]=i;
        return n;
    }else{
    }
}

Im not very experienced so please be gentle...

Answer 1

realloc tries to reallocate the given memory, but sometimes, it can't and gives you a new memory pointer.

It must be used like:

int *b;
b = realloc(a, <newsize>);
if (b) {
    /* realloc succeded, `a` must no longer be used */
    a = b;
    /* now a can be used */
    printf("ok\n");
} else {
    /* realloc failed, `a` is still available, but it's size did not changed */
    perror("realloc");
}

---

Now, you still have some trouble in your code:

The idea of function add() is to reallocate a when needed, but a is given by copy, so its value won't be changed in main.

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

int add(int** a, int i, int n);

int main(){
    int n = 20;
    int *a = malloc(n*sizeof(int));
    int i;

    for (i = 0; i < 100000; i++) {
        /* note how `a` is passed to `add` */
        n = add(&a, i, n);
        printf("a[%d]=%d\n",i,a[i]);
    }
    /* and finally free `a`*/ 
    free(a);
    return 0;
}

/* changed `a` type to make its new value visible in `main` */
int add(int **a, int i, int n){
    if (i >= n){
        /* want to write a[i], so size must be at least i+1*/
        n = i+1;
        /* realloc memory */
        int *b = realloc(*a, n*sizeof(int));

        /* check that `realloc` succeded */
        if (!b) { 
            /* it failed!*/
            perror("realloc"); 
            exit(1);
        } 
        /* store new memory address */
        *a = b;
    }

    /* update memory */        
    (*a)[i]=i;        

    /* return new size */
    return n;
}

---

Note: I removed malloc/realloc cast, see: Do I cast the result of malloc?