How to declare a global array of structs and use it in different functions?

Question

Switching from a OO language (C#), I would like to know what is the best way to declare a struct array that has application lifetime in C. After 1h struggle (and research for ex. about why not to use typedef, why to repeat struct later, etc.), this code is working:

// declaration
struct server {
    char* name;
    char* ip_address;
    int port;
} server;

struct server *servers; // declaring struct server[] servers; does not work

Then using like this in a function, working as well (after multiple experiments with & and *...):

// nb_servers is known from previous calculation
servers = malloc(nb_servers * sizeof(struct server));

// later in the same function
free(servers);

Questions

Why does declaring the struct array with [] not work? Question actually is, is it also possible to declare an array with '[]' (unknown size) and then dynamically initialize it later with malloc and if yes, what is the syntax to do it? Pure syntax question independent of differences in how memory is managed.
If I free(servers) I can no longer use the values after that. But if I don't, if this function gets called multiple times, will the value of this variable simply be overwritten by the result of the new call? Will not freeing servers cause a memory leak?

Hope it is clear, I'm 100% new to C.

did you try `struct server servers[100];` or something like that, depending on what you want the size of the array to be? — Wyck, Sep 05 '19 at 18:08
@Wyck That works, but as mentioned, the size of the array is unknown at declaration time. — evilmandarine, Sep 05 '19 at 19:00
The "why" part of Question 1 is a duplicate of [Why aren't variable-length arrays part of the C++ standard?](https://stackoverflow.com/questions/1887097/why-arent-variable-length-arrays-part-of-the-c-standard) — Wyck, Sep 05 '19 at 19:45
I didn't even think about looking at VLAs because there is a size initialiser there ("n"), even if it is not known at compilation time. My question is: can we declare an empty array with [] then decide about the number of elements later? — evilmandarine, Sep 05 '19 at 22:07

score 0 · Accepted Answer · answered Sep 05 '19 at 18:06

is it also possible to declare an array with '[]' (unknown size) and then dynamically initialize it later with malloc?

No. An array must have a knowable size when it is declared. This can be done by explicitly giving it a size in the brackets, or it can be inferred by its initializer. Once its size is set, it will remain that size for its lifetime. malloc, in the strictest sense, doesn't create an "array", but rather returns a pointer to a segment of memory that can be indexed similarly to an array syntactically, but is not truly an array in the same sense as an object declared with [].

Will not freeing servers cause a memory leak?

Yes, you will have a memory leak if you do not free servers before mallocing more memory for it without freeing. If you simply need to resize the array without ditching the data, you can use realloc.

How to declare a global array of structs and use it in different functions?

1 Answers1