array of structure pointers

Question

So I have 3 files: main.c,countries.h and countries.c

I declare pointer of the structure called "Country" in the countries.h

I have included the countries.h in countries.c and in main.c

and declared the structure its self in countries.c

countries.h

typedef struct Country* pCountry;

countries.c

struct Country {
    char *name;
    pCity cities;
    int numCities;
    pTerritory countryTerr;
};

now, I want to create array of pointers of the Country structure, using malloc

so I did that:

pCountry countries_array;
countries_array = (pCountry); 
malloc(num_of_countries*sizeof(countries_array));

and to assign pointers to each pointer,even though the malloc, does seems to work I cant

assign pointers to the elements in the array using []:

countries_array[0]= new_pointer;

I get "invalid use of undefine struct country" and "derefrecing pointer to incomplete",

what is the problem with the code?

thanks

malloc itselfs returns the pointer, NULL if there is no memory available like: countries_array = malloc(num_of_countries*sizeof countries_array); — Tom Kuschel
– Tom Kuschel, Commented Nov 21, 2018 at 22:48
You will want to review: Is it a good idea to typedef pointers?. — David C. Rankin
– David C. Rankin, Commented Nov 21, 2018 at 23:00
What is countries_array = (pCountry); supposed to mean? Was that intended to be a type cast of the result of the next line? — Barmar
– Barmar, Commented Nov 21, 2018 at 23:42
which file does the error point to, to which line? how declaration of the country_array looks like? — Serge
– Serge, Commented Nov 22, 2018 at 2:33

hellork · Accepted Answer · 2018-11-23 07:18:09Z

Looks good. Just assign it to something of the same type, struct Country. Also, as pointed out in the comments, it should be malloc num_of_countries * sizeof struct Country (not the pointer type), which is now correctly dereferenced below as sizeof (*countries_array) which also works.

pCountry countries_array;
countries_array = malloc(num_of_countries * sizeof (*countries_array));
struct Country Jefferson = {"Jefferson", 1,2,3 };
countries_array[0] = Jefferson;

// don't forget to free the memory when no longer needed.
free (countries_array);

If we must put a pointer into this array of structs, we can either dereference the pointer like countries_array[0] = *pointer, or... we could declare countries_array as an array of pointers, instead of an array of structs. Perhaps this is what you may want. Either way, the actual structures have to occupy memory somewhere...

pCountry *countries_array = malloc(num_of_countries*sizeof countries_array);
pCountry j = &Jefferson; // `&`, "address of" operator
countries_array[0] = j; // put a `pointer` into the array...

Checking right after malloc for out of memory condition is advisable. if (!countries_array) { fprintf(stderr, "out of memory"); exit 1; }
On casting malloc(). Since this is tagged "C" we don't (cast) the result of malloc, because it hides important compiler errors if we do. In C++, it is required, so it crops up quite a bit. c-faq.com/malloc/mallocnocast.html

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