the 3rd argument is the size of the array to insure that it doesn't overflow.
if you're receiving a ascii string, you want to receive the complete string using the terminator, 1st argument, to know when it is complete
if you're receiving an ascii string representing an integer, such as 600, there are 3 ascii bytes. a 16-bit integer can represent a value of 65535, which is 5 ascii characters. atoi() will translate the c-string to a 2 byte integer
it's returning the # of characters received. the received string in the array passed as the 2nd argument.
maybe there's a misunderstanding about sending the data as an ascii c-string. is there?