grazie, il problema era molto semplice...

#include <stdio.h>

int main(void);
void help(void);
void get_time(int* ore_ptr,int* min_ptr);
void ore2min(int ora, int min, int* tot_min);
void get_min(int tot_min_i, int tot_min_f);

int main(void)
{
int ora_i;
int min_i;
int tot_min_i;

int ora_f;
int min_f;
int tot_min_f;

int exit;

help();

while(exit!=-1)
switch(getchar())
{
case 'c':
get_time(&ora_i,&min_i);
get_time(&ora_f,&min_f);

ore2min(ora_i,min_i,&tot_min_i);
ore2min(ora_f,min_f,&tot_min_f);

get_min(tot_min_i,tot_min_f);
break;

case 'q':
exit=-1;
break;
}

return 0;
}

void help(void)
{
printf("c - Calcola\n"
"q - quit\n");
}

void get_time(int* ore_ptr,int* min_ptr)
{
printf("\nInsert h>");
scanf("%d",ore_ptr);

printf("Insert m>");
scanf("%d",min_ptr);
}

void ore2min(int ora, int min, int* tot_min)
{
*tot_min=ora*60+min;
}

void get_min(int tot_min_i, int tot_min_f)
{
printf("minuti = %d\n",tot_min_f-tot_min_i);
}