Why do I get warnings when I do this?

dirtest.c

#include <stdarg.h>
#include <string.h>
#include <dirent.h>
#include <stdlib.h>
#include <stdio.h>

int min_example(const struct dirent *ent, char *string)
{
	static const char *s = NULL;
	if(string) {
		s = string;
		return 0;
	} else if(!s) {
		return 0;
	}
	return !strcmp(ent->d_name, s);
}

int shim(const struct dirent *ent)
{
	return min_example(ent, NULL);
}

void main(void)
{
	struct dirent **ent;
	min_example(NULL, "afile");
	int n = scandir(".", &ent, shim, NULL); /* Error on function min_example not matching. */
	while(n--){
		printf("%s\n", ent[n]->d_name);
		free(ent[n]);
	}
	free(ent);
}