mkdir() question

I'm trying to make a user defined directory, so I get a string from the user, but then how do I correctly use the mkdir() function to use a string as an argument, and then handle the proper permissions.
oh, it's on a linux box...

right now I think (don't have the code with me right now) I have it like this;

mkdir(dirstring);

and that is making the proper directory now, but it is only accessbile from root. how do I tell it to give all permissions to users?
I have looked at the usage of chmod and such, and seem to have a basic understanding of how to handle permissions, but it's more a question of syntax, and how I use this in the mkdir function...

Thanks