Arguments by reference in Objective-C

I'm trying to pass an NSString by reference but it doesn't work.

This is the function:

+(void)fileName:(NSString *) file
{
    file = @"folder_b";
}

and this is the call:

NSString *file;

[function fileName:file];

nslog(@"%@",file);    // and there is nothing in the string....

What I must do to pass my string by reference?