Prepending to a string

Question

What is the most efficient way to prepend to a C string, using as little memory as possible?

I am trying to reconstruct the path to a file in a large directory tree.

Answer 1

You could maintain the string starting from the end. Since you seem to know the maxSize already...

So basically if file initially was (foo.txt)

[] [] [] [] [] [f] [o] [o] [.] [t] [x] [t] [\0]
             ^
             |
          lastEmpty           

Now if you add a parent dir a/ it will look like

[] [] [] [a] [/] [f] [o] [o] [.] [t] [x] [t] [\0]
       ^      
       |      
    lastEmpty           

So the code will look something like (there might be bugs, but you get the idea).

char temp[LENGTH], file[LENGTH]; 
int lastEmpty = put_at_end(some_file_name, file);  
// lastEmpty points to right most empty slot

while (some_condition) { 
    parent_dir = some_calculation_that_yields_name_of_parent_dir; 

    int len = strlen(parent_dir);
    char *tmp = parent_dir + len -1;

    while (lastEmpty > 0) {
        file[lastEmpty] = *tmp;
        lastEmpty --;
        tmp--;
    }
} 

Since I suppose we could expect parent_dir to be small, going over it twice should be ok. If you want to pass around the file string, you can just use file+lastEmpty+1.