How do you cast a string as an integer?

I have a number in a string (actually its a character array, c style string) and i want to put it in an integer variable. The character array simply contains one a digit in each index and then, of course, a terminating character (\0). I tried:

integervariable = int(string);
integervariable = (int)string;

Niether of these works, they just put the ascii codes for the digits in there. How do i do it?

integer = atoi(yourString);