#ifndef PINF_OPTS_H
#define PINF_OPTS_H
#endif //PINF_OPTS_H

struct _optL {
    struct opt avOpt[];
} optL;

struct _opt {
    char *option; // e.g. --group
    char *alias; // e.g. -G
    int reqArg; // Require Argument | 0: No 1: Yes
    int maxArgs; // -1: Undefined/ Unlimited
    int func; /* Run Function? 0: No 1: Yes
               * If No, it can be checked with function 'isOptEnabled'
               */
} opt;

struct _acOpt {
    struct opt *acOpt[];
} acOpt;

// == FUNC ==
void initOpts(void);

#include "opts.h"

#include <stdio.h>
#include <stdlib.h>

// == VAR ==
static struct optL *optList;
static struct acOpt *activeOpts;

// == CODE ==
void initOpt(void) {
    optList = (struct optL *)malloc(sizeof(struct optL *));
    activeOpts = (struct acOpt *)malloc(sizeof(struct acOpt *));
}

#include <stdio.h>
#include "../include/opts.h"

int main(void) {
    initOpts();
    return 0;
}

In file included from include/opts.c:5:0:
include/opts.h:14:16: error: array type has incomplete element type ‘struct opt’
     struct opt avOpt[];
                ^~~~~
include/opts.h:28:17: error: flexible array member in a struct with no named members
     struct opt *acOpt[];
                 ^~~~~