Global struct in c (initializer element is not a compile-time constant)

Question

I am pretty new to C and I have some problems.

I have the following struct definition:

struct env {
  struct env *next;
  char varName[8];
  int n;
};

I want to create multiple of those structs eventually in some of my functions so I created a function to do that :

struct env *mkEnv(char c[8] , int value , struct env *toadd){
    struct env *enviroment = malloc(sizeof(struct env));
    enviroment->n = value;
    enviroment->next = toadd;
    strcpy(enviroment->varName , c);
    return enviroment;
}

I want to create a struct like this globally that is constant with some constant values and initially have the pointer to the next structure NULL.

So I did something like this :

    //not in a function
    struct env *list = mkEnv("pot" , 0 , NULL);

    //Beginning of a function
    int eval(struct expression *exp){
        ... // code here that might add a new structure to the pointer of list
    }

I get the following error however :

evalexp.c:116:20: error: initializer element is not a compile-time constant struct env *list = mkEnv("p" , 0 , NULL);

I understand what this error means after googling about the error message but is there a way to create a structure somewhere outside of a function without getting this compilation error?

To make it more clear : I want to create a structure as defined above (as if it was the head of a list). so that all my functions can access and add stuff to it. That is parsing it as a list and/or adding new elements in that list.

Thanks in advance!

Answer 1

A function cannot be called in file scope.

You must use constant values and define an actual struct variable:

struct env lists = { NULL , "pot" , 0 };
struct env *list = &lists;

Now the pointer list is initialized and can be used, but beware that it wasn't created with malloc, so it cannot be freed or reallocated.

Answer 2

Define the variable, and initialize it in main.

struct env *list;

int main() {
    list = mkEnv("pot" , 0 , NULL);
    ....
}

Sometimes libraries make you call a function to initialize the library before any of its functions should be used. Giving values to global variables that they use is one of the reasons why they do that.