Exploring Expect

Parity

Parity refers to the process of error detection by modification and inspection of a single bit in each byte. There are two basic types of parity. Odd parity means that the number of 1 bits in the byte is odd. If a letter is not naturally represented by an odd number of 1 bits, the high-order bit is forced to be 1. Even parity is just the opposite.

Parity was never much good, being very susceptible to transmission noise. In this day and age, parity is totally useless. Nonetheless, some old computer peripherals generate it anyway. And worse, they provide no way of disabling it. Locally spawned processes do not add parity in the first place. You only have to worry about parity when communicating with other peripherals, such as modems or telecommunication switches.

By default, expect respects parity. expect passes characters with their parity on to the standard output (or log file) and also does pattern matching with the original parity. The reason this is useful, of course, is that many programs use all the bits in a byte to represent data. Eight-bit character sets (prevalent in Europe) do not work if one of the bits is used for parity.

Usually, parity is not a consideration. Indeed, if your Expect dialogues are working just fine, then you can skip this section. However, you may occasionally find that some of your characters are unreadable or just plain wrong. For example, suppose that you use tip to dial up another computer and the following gibberish appears instead of a prompt ...