pointer arithmetic and "generic" in c - c

Question

I tried to put program a generic method in C to identify the biggest element of an array.
At first, I programmed this:
int compare(const void* a, const void* b) {
if(a < b)
return 0;
return 1;
}
int main(void) {
int (*prt)(const void*, const void*);
prt=compare;
printf("%i",(*prt)(1,1));
return EXIT_SUCCESS;
}
This works fine, but if I try to put the function pointer prt
in a new method, I do not know how to handle it.
Addionally i dont know how to handle void* types.
void* maximum(int len, void* array, size_t size, int (*cmp)(const void*, const void*));
int compare(const void* a, const void* b) {
if(a < b)
return 0;
return 1;
}
int main(void) {
int (*prt)(const void*, const void*);
prt=compare;
printf("%i",(*prt)(1,1));
int array[6] = {3, 1, 0 , 4 , 3, 9};
maximum(len,array,0,prt);
return EXIT_SUCCESS;
}
void* maximum(int len, void* array, size_t size, int (*cmp)(const void*, const void*)) {
void* temp;
temp = array[0];
printf("%i",a);
int i;
for(i = 1; i < len; i++) {
if((*cmp)(temp,array[i]) == 0) {
temp = array[i];
}
}
return 0;
}
There are many errors... e.g. the variable temp or if((*cmp)(temp,array[i]) == 0).
Do you have an idea how to use not defined datatypes?

Answer 1

You are comparing addresses instead of values:
int compare(const void* a, const void* b) {
if(a < b)
return 0;
return 1;
}
Should be:
int compare(const void* a, const void* b) {
if(*(int *)a < *(int *)b)
return 0;
return 1;
}

Answer 2

Here is an example that can be taken as a base code.
#include <stdio.h>
int cmp( const void *a, const void *b )
{
return *( const int * )a < *( const int * )b;
}
void * maximum( const void *array, size_t size, size_t len,
int cmp( const void *, const void *) )
{
const void *max = array;
size_t i = 1;
for ( ; i < size; i++ )
{
if ( cmp( ( const char * )max, ( const char * )array + i * len ) )
{
max = ( const char * )array + i * len;
}
}
return ( void * )max;
}
int main(void)
{
int array[] = { 3, 1, 0 , 4 , 3, 9 };
int *max =
maximum( array, sizeof( array )/ sizeof( *array ), sizeof( int ), cmp );
printf( "Maximum = %d\n", *max );
return 0;
}
The output is
Maximum = 9

Answer 3

#include <stdio.h>
#include <stdlib.h>
void *maximum(int len, void* array, size_t size, int (*cmp)(const void*, const void*));
//comparison function must know about type.
//Because it is not known for functions like memcmp that type what is the layout.
int intcmp(const int *x, const int *y){
return *x < *y ? -1 : *x > *y;
}
int main(void) {
int array[6] = {3, 1, 0 , 4 , 3, 9};
int *p = maximum(sizeof(array)/sizeof(*array), array, sizeof(*array), (int (*)(const void*,const void*))intcmp);
printf("%d\n", *p);//9
return EXIT_SUCCESS;
}
void *maximum(int len, void *array, size_t size, int (*cmp)(const void*, const void*)) {
int i;
void *temp = array;
for(i = 1; i < len; i++) {
if(cmp((char*)array + size*i, temp)>0) {
temp = (char*)array + size*i;
}
}
return temp;
}