Deal with permutations of a given string (with callback)

sperm.c

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

void swap(char *a, char *b);
void onperm(char *s);
void sperm(char *s, int len, int cur, void (*fn)(char *s));

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

void
sperm(char *s, int len, int cur, void (*fn)(char *s)) {
        int i;

        if(cur == len - 1) {
                fn(s);
                return;
        }
        for(i = cur; i < len; ++i) {
                swap(s + cur, s + i);
                sperm(s, len, cur + 1, fn);
                swap(s + cur, s + i);
        }
}

void
swap(char *a, char *b) {
        char t = *a;

        *a = *b;
        *b = t;
}

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