why does realloc crash my program when i am trying to allocate new memory?

Question

i have some issue with my realloc function. i have the struct Country and inside i have struct "City" wich include point to cities array that every city is have 3 fiels:

typedef struct Country {
    char *name;
    int numberOfCities;
    City* cities;
    cordinate cordinateOfCountryLeft;
    cordinate cordinateOfCountryRight;
}Country;

typedef struct City
{
    char *name;
    char * popluarFood;
    int numberOfPeople;

}City;

i need to delete city from the cities array so i free the city with function that i built called freeCity:

void freeCity(City *pCity)
{
    free(pCity->name);
    free(pCity->popluarFood);
    free(pCity);
}

but after delelte when i am trying to realloc i get error in this function when realloc

status freeCityFromCountry(Country *country, char *cityName)
{
    for (int i = 0; i < country->numberOfCities; i++) {//for
        if (strcmp(country->cities[i].name, cityName)==0)
        {
           freeCity(country->cities+i);
            country->cities[i] = country->cities[country->numberOfCities - 1];
          //  free(country->cities[country->numberOfCities - 1]);
            country->cities = (City*)realloc(country->cities,(country->numberOfCities-1));
            country->numberOfCities--;
            return success;
        }
    }//for
    return failure;
}

i malloc country->cities in other function. where can be the problem? thank you

Answer 1

You cannot call free on a pointer to the middle of an allocated chunk. country->cities points to a contiguous block in memory ( N x sizeof(City) ). A good rule of thumb is, if you have malloc(something) , you must have (and can only have) free(something) somewhere else ( malloc(country->cities) -> free(country->cities) ).

The code crashes because the first call to freeCity frees country->cities . Perhaps you want .cities to be an array of pointers, ie City** , in that case you would allocate each City separately, and the cities array would then point to chunks which you can free individually.

Answer 2

Assuming that you allocate country->cities in another function.

You are calling freeCity(country->cities+i);

But in the freeCity function you are also freeing the city free(pCity);

So, for the cities array, you are calling free on city[i] . This has two issues

1. For the first loop, when i is 0 , you are freeing the array and then reallocating it.
2. For the other iterations, when i is non zero, you are freeing at the wrong place. You should free the base of the array and not inside the array.