stringhe.h
#if !defined STRINGHE_H
#define STRINGHE_H
#define _CRT_SECURE_NO_WARNINGS
#include <stdlib.h>
#include <string.h>
extern char *center(const char *str, size_t n, char c);
#endif // !defined STRINGHE_H

stringhe.c
#include "stringhe.h"
void riempi(char *s, char c, size_t pos, size_t n) {
	for (unsigned int i = pos; i < pos + n; ++i) {
		s[i] = c;
	}
}
void stampa_stringa(char *dest, char *origin, size_t pos, size_t n) {
	size_t c = 0;
	for (unsigned int i = pos; i <pos+n; i++){
		dest[i] = origin[c];
		c++;
	}
}
char *center(const char *str, size_t n, char c) {
	if (str == 0) return NULL;
	size_t dim = strlen(str), nmeta, count = 0;
	nmeta = n - dim;
	char *tmp = malloc((dim + 1) * sizeof(char));
	strcpy(tmp, str);
	char *p = malloc((n+1) * sizeof(char));
	if ((dim % 2 == 0 && n % 2 == 0 && count == 0) || (n%2!=0 && (n-dim)%2==0 && count == 0) ) {
		riempi(p, c, 0, (nmeta / 2));
		stampa_stringa(p, tmp, (nmeta / 2), dim);
		riempi(p, c, (nmeta / 2 + dim), (nmeta / 2));
		p[n] = 0;
	}
	if (dim >= n && count == 0) {
		p = realloc(p, dim + 1);
		stampa_stringa(p, tmp, 0, dim);
		p[dim] = 0;
		count++;
	}
	if (dim % 2 == 0 && n % 2 != 0 && count == 0){
		riempi(p, c, 0, (nmeta / 2+1));
		stampa_stringa(p, tmp, (nmeta / 2+1), dim);
		riempi(p, c, (nmeta / 2 + dim+1), (nmeta / 2));
		p[n] = 0;
		count++;
	}
	if (dim % 2 != 0 && n % 2 == 0 && count == 0) {
		riempi(p, c, 0, (nmeta / 2));
		stampa_stringa(p, tmp, (nmeta / 2), dim);
		riempi(p, c, (nmeta / 2 + dim), (nmeta / 2+1));
		p[n] = 0;
		count++;
	}
	free(tmp);
	return p;
}