Fix C input

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define GROW_FACTOR 2
#define INIT_SIZE 8
#define NEW(kind, size) (kind*)reallocate(0, sizeof(kind) * size, NULL)
#define GROW(kind, ptr, o, n) (kind*)reallocate(sizeof(kind) * o, sizeof(kind) * n, ptr)

void* reallocate(int oldsize, int newsize, void* ptr) {
    /* Allocates/reallocates/deallocates a block
    of memory using malloc, realloc and free */

    if (oldsize == 0 && ptr == NULL) {
        return malloc(newsize);
    }
    else if (oldsize == 0 && ptr != NULL) {
        free(ptr);
        return NULL;
    }
    return realloc(ptr, newsize);
}

char* reverse(char* str) {
    /* Reverses a string. Returns a copy of the
    string on success and a null pointer on
    failure*/

    int len = strlen(str);
    char* result = NEW(char, len);
    if (result == NULL) {
        return NULL;
    }
    memset(result, 0, len);
    for (int i = len - 1; i >= 0; i--) {
        result[len - i - 1] = str[i];
    }

    result[len] = '\0';
    return result;
}


char* input(char* prompt) {
    /* Reads from stdin until a newline is pressed.
    Returns null upon allocation failure, or a pointer
    to char otherwise.
    Prompt is an optional message to show to the user
    */

    char* buffer = NEW(char, INIT_SIZE);
    if (buffer == NULL) {
        return NULL;
    }
    memset(buffer, 0, INIT_SIZE * sizeof(char));
    char current;
    int len = 0;
    int buflen = INIT_SIZE * sizeof(char);
    printf("%s", prompt);
    while ((current = getchar()) != '\n') {
        if (len == buflen) {
            buflen = sizeof(char) * len * GROW_FACTOR;
            buffer = GROW(char, buffer, len, buflen);
            if (buffer == NULL) {
                return NULL;
            }
        }
        buffer[len] = current;
        len++;
    }
    // Ogni tanto non mette il demone finale della stringa
    // if (len >= buflen) {
    //     buffer[len] = '\0';
    // }
    buffer[len] = '\0';
    return buffer;
}


int main(int argc, char* argv[]) {
    /* I have no clue what I'm doing */

    char * buffer = input("> ");
    char* reversed = reverse(buffer);
    if (buffer == NULL) {
        printf("Error reading input");
        return -1;
    }
    if (reversed == NULL) {
        printf("Error reversing input");
        return -1;
    }
    printf("The reverse of '%s' is '%s'\n", buffer, reversed);
    printf("Length of strings is %lu bytes\n", strlen(buffer));
    free(buffer);
    free(reversed);
}