Possibles Combinations of Alphabet in C

Hi people I'm programming in C like a month. I just made this code yesterday, but I want the same result with a better code without using recursion for optimization, i think mine is sloppy. Also im using linux distribution... Any ideas to change this code in 1 or 2 functions?

Anything will help! Thanks.

Code:

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

#define Plain "plain.txt" 


int length ;
int i;
char alphalower[] = "abcdefghijklmnopqrstuvwxyz"; 


int main(){

    printf("\tEnter Maximum Char Length: ");
    scanf("%d",&length);
    for(i=1;i<=length;i++){
        combinations(alphalower,i);
    }
   return 0;

}

int possibles (const void * a, const void * b)
{
    return ( *(char *)a - *(char *)b );
}
void back2 (char *str, char* info, int last, int index)
{
    FILE *fp;
    if ((fp = fopen(Plain, "a+")) == NULL)
    {
        perror (Plain);
        exit (EXIT_FAILURE);
    }
    int i, length = strlen(str);
    for ( i=0; i < length; i++ )
    {
        info[index] = str[i] ;
        if (index == last)
            fprintf(fp,"%s\n", info);
        else 
            back2 (str, info, last, index + 1);
    }
    fclose(fp); 

}

int combinations(char *str, int length)
{

    char *info = (char *) malloc (sizeof(char) * (length + 1)) ;
    info[length] = '\0';
    qsort(str, length, sizeof(char), possibles);
    back2 (str, info, length-1, 0);
    free(info);

    return 0;
}