How can i declare and define shared variables between header file in C?

Question

I have three source file called supermercato.c cliente.c cassiere.c and the following header file cliente.h cassiere.h. supermercato.c is where is main(). In supermercato() there I create two thread which are located in the other two source files but I have a shared structure and I don't know how to share all of these. Please can someone help me ?

 '''
    #include "librerie.h"
    #include "cliente.h"
    #define K 5 //numero totale di casse del supermercato
    #define C 20 //numero totale clienti
    #define UNIX_PATH_MAX 108
    #define SOCKNAME "./mysock"
    #define ec_meno1(s,m) \
    if((s) == -1) { perror(m); exit(EXIT_FAILURE); }    
    #define ec_null(s,m) \
    if((s) == NULL) { perror(m); exit(EXIT_FAILURE); }
    #define ec_zero(s,m) \
    if((s) != 0) { perror(m); exit(EXIT_FAILURE); }
     //these two
    int casse_aperte = 0;
    casse c[K];
    int main(int argc,char* argv[]){
//lettura file parametri
pthread_t tid_cassiere[K];
pthread_t tid_cliente[C];
//inizializza casse
for(int i=0;i<K;i++){
    c[i].num_clienti=0;
    c[i].q=NULL;
    ec_zero((pthread_mutex_init(&c[i].mtx,NULL)),"pthread_mutex_init failed");
    ec_zero((pthread_cond_init(&c[i].empty,NULL)),"pthread_cond_init failed");
    ec_zero((pthread_cond_init(&c[i].full,NULL)),"pthread_cond_init failed");
}
//apro una cassa
ec_zero((pthread_create(&tid_cassiere[0],NULL,cassiere,&c[0])),"pthread_create failed");
casse_aperte++;
//genero un cliente
for(int i=0;i<C;i++){
    //generare numero random [0,K-1] rand
    ec_zero((pthread_create(&tid_cliente[i],NULL,cliente,&c[0])),"pthread_create failed");
}
for(int i=0;i<C;i++){
    ec_zero((pthread_join(tid_cliente[i],NULL)),"join failed");
}
ec_zero((pthread_join(tid_cassiere[0],NULL)),"join failed");

return 0;

}

The following is cliente.h

   /** 
  Thread cliente
*/
//se K non e' stata definita ==> la definisco
 #ifndef K
   #define K 5
  #endif

typedef struct coda{
    int val;
    struct coda *next;
 }coda; 

 typedef struct cassa{
     int num_clienti;
     coda *q;
    pthread_mutex_t mtx;
    pthread_cond_t empty;
    pthread_cond_t full;    
 }casse;

 extern int casse_aperte;
 extern casse c[K];



void* cliente(void *c);

the following is cliente.c

  #include "librerie.h"
  #include "cliente.h"


 //funzione thread cliente
  void* cliente(void *c){
      //all'atto dell'entrata genero numero casuale [0,P]
      printf("CLIENTE\n");
      if(c == NULL){
         perror("invalid argument");
         exit(EXIT_FAILURE);
       }
      casse* cassa = (casse*)c;
      pthread_mutex_lock(&cassa->mtx);
       while(cassa->num_clienti != 0){ //in questo caso condizione mai verificata perche c'e' 
    solo un cliente
            //il cliente valuta se cambiare coda (ogni volta che si sveglia dopo la signal)
           pthread_cond_wait(&cassa->empty,&cassa->mtx);
      }
       cassa->num_clienti++;
       printf("cliente si accoda e produce un valore\n");

      pthread_cond_signal(&cassa->full);
      pthread_mutex_unlock(&cassa->mtx);

        return NULL;
    }

And librerie.h is where there are all the standard libraries included