What happens when you try to free() already freed memory in c?

Question

For example:

char * myString = malloc(sizeof(char)*STRING_BUFFER_SIZE);
free(myString);
free(myString);

Are there any adverse side effects

Answer 1

In short: "Undefined Behavior".

(Now, what that can include and why that is the case the others have already said. I just though it was worth mentioning the term here as it is quite common).