given a number of characters. how do I make every possible combination into a string. C

first this is part of a much bigger program. so I have a vector of strings. all the strings are of the same size. but their common size can be different. and I have a bunch of letters. and a dash. '-'. its a game that if I explain i'd get [ON HOLD] for being too broad. so I will keep it simple. I input 1 letter. and I make combinations of strings of that letter depending on the common size of strings. say we have strings with size 4 and the first letter is 'a'. the combinations could be a---, aa---,aaa-,aaaa,-a--. etc. you get the point. again the size of the string can be different. lets say now I input a second letter.'b'. the strings could be ab--, abab. a---b. and even a---. the letter doesn't have to be included. even first time. ---- is valid. how do I do that. generate every possible key. current letter doesn't have to be included. and put everyone in a vector of strings.