C linear search error - c

Question

My lsearch function should find the value 11 in my array. It does not, and I don't know where the error is. Why does this code not find the value 11?
#include <stdio.h>
#include <string.h>
#define PF printf
int main() {
int intcmp(void *ip1, void * ip2);
void * lsearch(void *key, void *base, int n, int elemSize,
int(* cmpfun)(void *, void *));
int arr[] = {4, 6, 2, 3, 11, 22, 15};
int n = sizeof(arr) / sizeof(int);
int key = 11;
int *found = lsearch(&key, &arr, n, sizeof(int), intcmp);
PF("found=%p", found);
return 1;
}
int intcmp(void *ip1, void * ip2) {
int *p1 = ip1;
int *p2 = ip2;
return *p1 - *p2 == 0;
}
void * lsearch(void *key, void *base, int n, int elemSize,
int(* cmpfun)(void *, void *)) {
int i;
for(i = 0; i < n; i ++) {
void *elemArr = (char *)base + i * elemSize;
if(cmpfun(key, elemArr) == 0)
return elemArr;
}
return NULL;
}

Answer 1

There's a few oddities in your code (PF and the way the functions is declared in main yet defined globally), however, the problem is that your logic if inverted in a two places.
if(cmpfun(key, elemArr) == 0)
return elemArr;
And:
return *p1 - *p2 == 0;
Mentally run through that when the two elements are equal. The == expression will return 1 when the number does actually equal the other. 1 != 0 thus it's not considered found.
Either through a negation in there or just return *p1 - *p2; directly.

Answer 2

I have annotated the code below:
#include <stdio.h>
#include <string.h>
// This is bad practice. It makes your code less readable.
// I won't use it below.
#define PF printf
// Declare this first so a prototype is not needed.
// You violated a C pattern by using `cmp` in the name.
// Comparison functions in C return <0, 0, >0, not a binary value.
// To wit, later you used the comparison correctly. I've fixed the
// inconsistency.
int intcmp(void *vp1, void *vp2)
{ // Most C styles have the brace on its own line, unlike Java. Roll with it.
int *p1 = vp1, *p2 = vp2;
return *p1 - *p2;
}
// Search n elements of size elemSize in the array at
// base in sequence using cmpfun on key,
// to test for equality (cmpfun == 0). Return a pointer
// to the found element or NULL if none.
void *lsearch(void *key, void *base, int n,
int elemSize, int(* cmpfun)(void *, void *))
{
int i;
for (i = 0; i < n; i++) {
void *elemArr = (char*)base + i * elemSize;
if (cmpfun(key, elemArr) == 0)
return elemArr;
}
return NULL;
}
int main()
{
int arr[] = {4, 6, 2, 3, 11, 22, 15};
int n = sizeof(arr) / sizeof(int);
int key = 11;
int *found = lsearch(&key, &arr, n, sizeof(int), intcmp);
printf("found=%p (%d)", found, *(int*)found);
return 0; // By convention zero corresponds to no error.
}