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

char* get_next_string(char* cur)
{
	size_t p=strlen(cur);
	int i=0;
	while(*(cur+p-1-i)=='z')
		*(cur+p-1-i++)='a';

	*(cur+p-1-i)+=1;
	return cur;
}

int view_all_combination(size_t p,FILE* out)
{
	char* start_s=(char*)malloc(sizeof(char)*(p+1));
	memset(start_s,'a',p);
	*(start_s+p)='\0';
	char* condition_s=(char*)malloc(sizeof(char)*(p+1));
	memset(condition_s,'z',p);
	*(condition_s+p)='\0';
	fprintf(out,"%s\n",start_s);
	while(strcmp(start_s,condition_s))
	{
		fprintf(out,"%s\n",get_next_string(start_s));
	}
	return 1;
}

int view_diap(int begin,int end,FILE* out)
{
	for(int i=begin;i<=end;++i)
		view_all_combination(i,out);
	return 1;
}

int main(int argc, char** argv)
{
	FILE* out=fopen("output.txt","w");
	view_diap(1,4,out);
	fclose(out);
	return 0;
}

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

char* get_next_string(char* cur,size_t p)
{
	int i = 0;
	while (*(cur + p - 1 - i) == 'z')
		*(cur + p - 1 - i++) = 'a';

	*(cur + p - 1 - i) += 1;
	return cur;
}

int view_all_combination(size_t p, FILE* out)
{
	char* start_s = (char*)malloc(sizeof(char)*(p + 1));
	memset(start_s, 'a', p);
	*(start_s + p) = '\0';
	char* condition_s = (char*)malloc(sizeof(char)*(p + 1));
	memset(condition_s, 'z', p);
	*(condition_s + p) = '\0';
	fprintf(out, "%s\n", start_s);
	while (strcmp(start_s, condition_s))
	{
		fprintf(out, "%s\n", get_next_string(start_s,p));
	}
	return 1;
}

int view_diap(int begin, int end, FILE* out)
{
	for (int i = begin; i <= end; ++i)
		view_all_combination(i, out);
	return 1;
}

int main(int argc, char** argv)
{
	FILE* out = fopen("output.txt", "w");
	view_diap(1, 4, out);
	fclose(out);
	return 0;
}

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

char* set_next_string(char* cur,size_t p)
{
	while (*(cur + p - 1) == 'z')	
		*(cur + p-- - 1) = 'a';

	*(cur + p - 1) += 1;
	return cur;
}

int view_all_combination(size_t p, FILE* out)
{
	char* start_s = (char*)malloc(sizeof(char)*(p + 1));
	memset(start_s, 'a', p);
	*(start_s + p) = '\0';
	char* condition_s = (char*)malloc(sizeof(char)*(p + 1));
	memset(condition_s, 'z', p);
	*(condition_s + p) = '\0';
	
	fprintf(out, "%s\n", start_s);
	while (strcmp(start_s, condition_s))
	{
		fprintf(out, "%s\n", set_next_string(start_s,p));
	}
	free(start_s);
	free(condition_s);
	return 1;
}

int view_diap(int begin, int end, FILE* out)
{
	for (int i = begin; i <= end; ++i)
		view_all_combination(i, out);
	return 1;
}

int main(int argc, char** argv)
{
	FILE* out = fopen("output.txt", "w");
	view_diap(1, 4, out);
	fclose(out);
	return 0;
}

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

char* set_next_string(char* cur,size_t p)
{
	while (*(cur + p - 1) == 'z')	
		*(cur + p-- - 1) = 'a';

	*(cur + p - 1) += 1;
	return cur;
}

int view_all_combination(size_t p, FILE* out)
{
	char* start_and_condition = (char*)malloc((p + 1)*2);
	memset(start_and_condition, 'a', p);//стартовое значение
	*(start_and_condition + p) = '\0';
	char* start_s = start_and_condition;
	memset(start_and_condition+p+1, 'z', p);//условие выхода
	*(start_and_condition + 2 *p + 1) = '\0';
	char* condition_s = start_and_condition + p + 1;
	
	fprintf(out, "%s\n",start_and_condition);
	while (strcmp(start_s, condition_s))
	{
		fprintf(out, "%s\n", set_next_string(start_s,p));
	}
	free(start_and_condition);
	return 1;
}

int view_diap(int begin, int end, FILE* out)
{
	for (int i = begin; i <= end; ++i)
		view_all_combination(i, out);
	return 1;
}

int main(int argc, char** argv)
{
	FILE* out = fopen("output.txt", "w");
	view_diap(1, 4, out);
	fclose(out);
	return 0;
}

void perebor(int N) // все комбинации N символов
{
	// Инициализанция
	char* buf = (char*)malloc(N + 1);
        char* last = buf + (N - 1);
	// Перебор
	char* cur = last;
	
...			
	if(cur < buf) 
}