[solved]prevent strings overflowing in c

Using C I want to allocate heap space of the maximum initial string length which is 6 in my case and then each time the string is about to overflow, double the available space. I want to build the string char by char.

size_t nstring = 6;
char *arr =  malloc(nstring);

int i;
for (i = 0; i < 55;i++){

        if (i == nstring) {
                printf("mAx sized!!");
                arr = realloc(arr, nstring*2);
                nstring = nstring*2;
            }
    arr[i] = 'f';

}
arr[i] = '\0';
printf("length %lu\n", strlen(arr));
free(arr);

via Chebli Mohamed