summarizing array of void * values - c

Question

#include <stdio.h>
#include <stdlib.h>
#define LEN 5
#define sum_mac(func,type)\
void* func (void* arr, int size)\
{\
int i;\
type *sum = (type*)malloc(sizeof(type));\
if(sum == NULL)\
{\
printf("Error\n");\
exit(1);\
}\
*sum = 0;\
for(i = 0; i<size ; i++)\
{\
*sum = *sum + *((type*)arr[i]);\
}\
return sum;\
}\
sum_mac(int_sum,int);
void *summary(void* arr, int size, void *(*func)(void*, int))
{
if (func == NULL)
return NULL;
return (*func)(arr, size);
}
int main()
{
int arr[] = { 1,2,3,4,5 };
int *sum = summary(arr, LEN, int_sum(arr, LEN));
printf("the sum is: %d ", *sum);
free(sum);
return 0;
}
I get the following error when I try to compile this code:
Error LNK2019 unresolved external symbol _int_sum referenced in function _main
When I searched possible causes for this problem I got that "a reference to a function or variable that the linker can't resolve, or find a definition for".
Can someone help me find the problem.

Answer 1

There is (at least) three problems:
You included the macro instanciation in the macro itself, change:
}\
sum_mac(int_sum,int);
to:
}
sum_mac(int_sum,int);
Thus sum_mac is not part of the macro.
In the macro definition, change:
*sum = *sum + *((type*)arr[i]);
to:
*sum = *sum + ((type*)arr)[i];
In the first case, you try to use indexing on void pointer type, which is not possible (void has no size). So convert arr to pointer of the right type and use arithmetic on it.
--------------EDIT-----------
Change:
int *sum = summary(arr, LEN, int_sum(arr, LEN));
to
int *sum = summary(arr, LEN, int_sum);
In the first case you call summary with the third parameter value being the result of a call to int_sum, and that result if not a function pointer but the pointer to some int. You need to pass the function pointer.
Most of your problems are due to macro usage. This bypass the type system and fool the compiler, which is never a good idea.

Answer 2

Problem is here:
return sum;\
}\ <---------- Remove the \
sum_mac(int_sum,int); <---- also remove the ; because this is not a statement
void *summary(void* arr, int size, void *(*func)(void*, int))
Compiler thinks that sum_mac(int_sum,int); is part of the macro definition because \ at the end of line concatenates lines. That is why sum_mac(int_sum,int); is never called.
However, this reveals another problem with operator precendence on line:
*sum = *sum + *((type*)arr[i]);\
Array access [i] has higher precedence than cast (type*), so you are trying to access void array which won't work. Also the last dereference is pointless. Line should be changed to:
*sum = *sum + ((type*)arr)[i];\
There is also third problem: You are passing calling int_sum too early. Function expects function pointer so you should only pass the pointer:
int *sum = summary(arr, LEN, int_sum); // Only pass int_sum
You should make sure that you have enabled all warnings on the compiler, since this is an error that compiler could warn you about.

Answer 3

The other problem is here:
int *sum = summary(arr, LEN, int_sum(arr, LEN));
The third argument of summary should be a function pointer, but int_sum(arr, LEN) is not a function pointer but it is the result of the int_sum function.
You need to write this:
int *sum = summary(arr, LEN, int_sum);

Answer 4

You don't declare the function you call anywhere, so the linker doesn't know what to do.
That being said, putting whole functions inside macros is not a good idea, nor to malloc single items. And the code *sum = *sum + *((type*)arr[i]); goes wildly out of bounds, because you didn't allocate an array, but use it as if it was one.
I would rewrite this from scratch. This is what you can do instead:
#include <stdio.h>
#include <stdlib.h>
int int_sum (size_t size, int arr[size])
{
int sum = 0;
for(size_t i=0; i<size; i++)
{
sum += arr[i];
}
return sum;
}
float float_sum (size_t size, float arr[size])
{
float sum = 0.0f;
for(size_t i=0; i<size; i++)
{
sum += arr[i];
}
return sum;
}
#define sum_array(arr) _Generic((arr[0]), \
int: int_sum, \
float: float_sum) (sizeof(arr)/sizeof(arr[0]), arr)
int main (void)
{
int arr[] = { 1,2,3,4,5 };
int sum = sum_array(arr);
printf("int, the sum is: %d\n", sum);
float farr[] = {1.0f, 2.0f, 3.0f};
printf("float, the sum is: %f\n", sum_array(farr));
return 0;
}
Which can be made even more generic, even without involving function pointers:
#define do_stuff(arr, stuff) _Generic((arr[0]), \
int: int_##sum, \
float: float_##sum) (sizeof(arr)/sizeof(arr[0]), arr)
int main (void)
{
int arr[] = { 1,2,3,4,5 };
int sum = do_stuff(arr, sum);
printf("int, the sum is: %d\n", sum);
float farr[] = {1.0f, 2.0f, 3.0f};
printf("float, the sum is: %f\n", do_stuff(farr, sum));
return 0;
}