permutations.c

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

static const size_t LETTERS = 26;
void permutations(char *str, size_t idx, size_t size, char *letters)
{
	if (idx == size) {
		printf("%s\n", str);
		return;
	}
	for (size_t i = 0; i < LETTERS; i++) {
		if (!letters[i])
			continue;
		str[idx] = i + 'a';
		letters[i]--;
		permutations(str, idx + 1, size, letters);
		letters[i]++;
	}
}

void permutation(const char *str)
{
	size_t size = strlen(str);
	char result[size + 1];
	result[size] = '\0';
	static char letters[LETTERS];
	//memset(letters, 0, sizeof(letters));
	for (size_t i = 0; i < size; i++) {
		letters[str[i] - 'a']++;
	}
	permutations(result, 0, size, letters);
}

int main()
{
	const char *str = "abc";
	permutation(str);
}