[iOS] How to read byte in NSData?

如何從NSData讀Byte? 

NSData * theData = [NSData dataWithContentsOfFile: path];
const char* theBytes = (const char*)[theData bytes];

那讀幾個Byte?

NSRange range = NSMakeRange(location, length);

NSData *someData = [theData subdataWithRange:range]; 
const char* theBytes = (const char*)[someData bytes];