Error with two almost identical pieces of code - c

Question

**I am new to C so please keep that in mind when answering
I'm currently working on a larger program where I use classes called integer.c (which defines an integer and functions used on it), real.c (which defines a double and functions used on it) and comparator.c (which defines functions used to compare these different data types (each class does have a header as well).
When I try and compile the larger program, I get an error in my comparator.c file stating:
comparator.c:25:23: error: expected expression
return ((integer *) p)->value - ((integer *) q->value);
^
comparator.c:25:14: error: use of undeclared identifier 'integer'
return ((integer *) p)->value - ((integer *) q->value);
^
comparator.c:25:48: error: expected expression
return ((integer *) p)->value - ((integer *) q->value);
^
comparator.c:25:39: error: use of undeclared identifier 'integer'
return ((integer *) p)->value - ((integer *) q->value);
^
comparator.c:23:25: warning: unused parameter 'p' [-Wunused-parameter]
int intComparator(void *p, void *q)
^
comparator.c:23:34: warning: unused parameter 'q' [-Wunused-parameter]
int intComparator(void *p, void *q)
^
Here is my comparator.c class and I want to note how my intComparator functions and realComparator functions are almost identical so I cannot see why I do not get these errors for my realComparator:
#include <stdio.h>
#include <limits.h>
#include <assert.h>
#include <string.h>
#include "comparator.h"
#include "str.h"
#include "integer.h"
#include "real.h"
//returns >0 if p is > q, returns 0 if equal, returns <0 is q is > p
int intComparator(void *p, void *q)
{
return ((integer *) p)->value - ((integer *) q)->value;
}
////returns >0 if p is > q, returns 0 if equal, returns <0 is q is > p
double realComparator(void *p,void *q)
{
return ((real *) p)->value - ((real *) q)->value;
}
int stringComparator(void *p,void *q)
{
int r = strcmp((char *)p, (char *)q);
return r;
}
Here is my integer.c class and real.c class for comparison (please note that any programming style of this program that might seem weird is because it is for a class project where I need to follow style and program outline instructions):
integer.c:
#include <stdio.h>
#include <stdlib.h>
#include <limits.h>
#include <assert.h>
#include "integer.h"
extern void Fatal(char *,...);
integer *
newInteger(int x)
{
integer *p = malloc(sizeof(integer));
if (p == 0)
{
fprintf(stderr,"out of memory\n");
exit(-1);
}
p->value = x;
return p;
}
int
getInteger(integer *v)
{
assert(v!=0);
return v->value;
}
void
displayInteger(FILE *fp,void *v)
{
assert(v!=0);
fprintf(fp,"%d",getInteger((integer *) v));
}
void
freeInteger(integer *v)
{
free(v);
}
real.c:
#include <stdio.h>
#include <stdlib.h>
#include <limits.h>
#include <assert.h>
extern void Fatal(char *,...);
real *
newReal(double x)
{
real *p = malloc(sizeof(real));
if (p == 0)
{
fprintf(stderr,"out of memory\n");
exit(-1);
}
p->value = x;
return p;
}
double
getReal(real *v)
{
assert(v!=0);
return v->value;
}
void
displayReal(FILE *fp,void *v)
{
assert(v!=0);
fprintf(fp,"%f",getReal((real *) v));
}
void
freeReal(real *v)
{
free(v);
}
and here are my integer.h and real.h files:
integer.h:
#include <stdio.h>
#ifndef __INTEGER_INCLUDED__
#define __INTEGER_INCLUDED__
typedef struct integer
{
int value;
} integer;
extern integer *newInteger(int);
extern int getInteger(integer *);
extern void displayInteger(FILE *,void *);
extern void freeInteger(integer *);
#define PINFINITY IN_MAX
#define NINFINITY IN_MIN
#endif
real.h:
#include <stdio.h>
#ifndef __REAL_INCLUDED__
#define __REAL_INCLUDED__
typedef struct real
{
double value;
} real;
extern real *newReal(double);
extern double getReal(real *);
extern void displayReal(FILE *,void *);
extern void freeReal(real *);
#define PINFINITY IN_MAX
#define NINFINITY IN_MIN
#endif
Here is str.h:
#include <stdio.h>
#ifndef __STRING_INCLUDED__
#define __STRING_INCLUDED__
extern char * getString(char *);
extern void displayString(FILE *,void *);
extern void freeString(char *);
#define PINFINITY IN_MAX
#define NINFINITY IN_MIN
#endif
and comparator.h:
#ifndef comparator_h
#define comparator_h
typedef int (*Comparator)(void*,void*);
typedef void (*Printer)(FILE*,void*);
extern int intComparator(void *p, void *q);
extern int realComparator(void *p, void *q);
extern int stringComparator(void *p, void *q);
#endif /* comparator_h */
I've been staring at this for so long and have checked everything multiple times and can't figure out why I'm getting these errors. Any help would be great, thanks.

Answer 1

Look closely at the parenthesis:
return ((integer *) p)->value - ((integer *) q->value);
// ^^^^^^^^^^^^^^
return ((real *) p)->value - ((real *) q)->value;
// ^^^^^^^^^^^
In the first case, you're casting q->value to integer*.
In the second case, you're casting q to real*, then accessing ->value.
I assume that the first case is an error.

Answer 2

I see a couple of issues, but first, I want to suggest a code modification to assist in debugging. You wrote a lot of code in one line. I suggest that you break the code into parts until you understand it. Then when it is working, you can combine it together. Like this:
int intComparator(void *p, void *q)
{
int rval;
int p1;
int q1;
p1 = (int *)p -> value;
q1 = (int *)q -> value;
rval = p1 - q1;
return (rval);
}
Now, here are two issues that I see:
1. You are attempting to cast p and q as pointers to integers. I think you need to cast them as pointers to int, not integer.
2. In realcomparator, you are returning an int but you are working with real numbers, I think. Either you should change the return value to a real, or convert the result to an integer before returning it.