Deal with permutations with repetitions of a given string (with callback)

srperm.c

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

void onperm(char *s, int len);
unsigned powu(unsigned base, unsigned exp);
int srperm(char *s, int len, void (*fn)(char *, int));

void
onperm(char *s, int len) {
	printf("%s\n", s);
}

unsigned
powu(unsigned base, unsigned exp) {
        unsigned int r = 1;

        while(exp > 0) {
                if(exp & 1)
                        r *= base;
                base = base * base;
                exp >>= 1;
        }
        return r;
}

int
srperm(char *s, int len, void (*fn)(char *, int)) {
        unsigned slen = strlen(s);
        char *gp = calloc(len + 1, sizeof(char));
        int total_n = powu(slen, len);
        int i, j, n;

        for(i = 0; i < total_n; ++i) {
                n = i;
                for(j = 0; j < len; ++j) {
                        gp[len -j -1] = s[n % slen];
                        n /= slen;
                }
                fn(gp, slen);
        }
        free(gp);
        return 0;
}

int
main(int argc, char *argv[]) {
        if(argc != 2) {
                printf("Usage: %s <string>\n", argv[0]);
                return 1;
        }
        srperm(argv[1], strlen(argv[1]), onperm);
        return 0;
}