Why am i getting EXC_BAD_ACCESS (code=1, address=0x0)? - c

Question

I am trying to write a program that, when given a certain price, will break it into 20s, 10s, 5s and 1s.
However, I am getting the following error: EXC_BAD_ACCESS (code=1, address=0x0)
The error shows up on the line: *tens=diff in the function.
I do not understand what is wrong!
Here is the code:
#include<stdio.h>
#include<stdlib.h>
void pay_month(int , int *, int *, int *, int *);
int main(){
int dollars, *twenties=0, *tens=0, *fives=0, *ones=0;
printf("enter amount to pay: ");
scanf("%d",&dollars);
pay_month(dollars, twenties, tens, fives, ones);
return 0;
}
void pay_month(int dollars, int *twenties, int *tens, int *fives, int
*ones){
int diff=0;
int odolls;
odolls=dollars;
while(dollars%20!=0){
dollars=dollars-1;
diff++;
}
*tens=diff;
*twenties=dollars/20;
diff=0;
while(*tens%10!=0){
*tens=*tens-1;
diff++;
}
*tens=*tens/10;
*fives=diff;
diff=0;
while(*fives%5!=0){
*fives=*fives-1;
diff++;
}
*ones=diff;
diff=0;
while(*ones%1!=0){
*ones=*ones-1;
diff++;
}
printf("in %d there is %d twentis %d tens %d fives and %d ones\n",odolls, *twenties, *tens, *fives, *ones);
}

Answer 1

You are dereferencing a NULL pointer. That is, you are asking the computer to write into memory location 0. You are not allowed to do that.
I think what you've done is used pointers without quite understanding how to obtain them. Possibly just to stop the compiler from throwing errors.
This simple change should solve the problem:
int dollars, twenties=0, tens=0, fives=0, ones=0;
printf("enter amount to pay: ");
scanf("%d",&dollars);
pay_month(dollars, &twenties, &tens, &fives, &ones);
Notice that none of the values in the calling function are pointers anymore. They're just integers. You prefix with & when passing them to pay_month. That's the reference operator, which obtains a pointer to where the value is stored.
Now, when you dereference the tens pointer inside pay_month, you're actually accessing the same part of memory where the value tens is stored on the stack in main.

Answer 2

a classic case of pointers and variables
essentially when you are declaring pointer as
int *twenties=0, *tens=0, *fives=0, *ones=0;
you are essentially making these pointers to point to NULL memory which means no memory is allocated to those.
one way to solve this is by declaring these as variables and passing the address of these variables
int dollars, twenties=0, tens=0, fives=0, ones=0;
and
pay_month(dollars, &twenties, &tens, &fives, &ones);