String permutations

permut.c

/*
 * Print all permutations of a string,
 * using a tree-recursive routine. I
 * bet there are smart algorithms out
 * there that beat this. I did not look
 * up an algorithm when writing this.
 *
 * Bl0ckeduser
 * June 26, 2012
 */

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

#define N 16

void fail(char* s){
	puts(s); exit(1);
}

void permut(int n, char avail[], char prev[])
{
	int i, j;
	char child_avail[N];
	char chars[N];

	/* go through all available characters */
	for(i = 0; avail[i]; i++)
		if(avail[i] != '!') {
			/* found an available character;
			   cross it out from the child's
			   available characters list */
			strcpy(child_avail, avail);
			child_avail[i] = '!';

			/* add the character to this
			   permutation-string */
			strcpy(chars, prev);
			strncat(chars, &avail[i], 1);

			/* print out permutation-string
			   if a leaf is reached; otherwise,
			   spawn child */
			if(n == 1)
				puts(chars);
			else
				permut(n - 1, child_avail, chars);
		}
}

int main(int argc, char** argv)
{
	char set[N];

	if(scanf("%s", set) == EOF)
		fail("failed to input string");

	if(strlen(set) > N)
		fail("string bigger than hardcoded limit");

	if(strstr(set, "!"))
		fail("! is used internally");

	permut(strlen(set), set, "");

	return 0;
}