As shown in the table in Section 2, most of the pins on modern parallel ports can be configured as either inputs or outputs. When Parapin is initialized, it is safest to assume that the state of all these switchable pins is undefined. That is, they must be explicitly configured as outputs before values are asserted, or configured as inputs before they are queried.
As mentioned earlier, Pins 2-9 can not be configured independently. They are always in the same mode: either all input or all output. Configuring any pin in that range has the effect of configuring all of them. The constant LP_DATA_PINS is an alias that refers to all the pins in this range (2-9).
The other switchable pins (Pins 1, 14, 16, and 17) may be independently configured. Pins 10, 11, 12, 13, and 15 are always inputs and can not be configured.
It is also worth reiterating that having two devices both try to assert (output) a value on the same pin at the same time can be dangerous. The results are undefined and are often dependent on the exact hardware implementation of your particular parallel port. This situation should be avoided. The protocol spoken between the PC and the external device you're controlling to should always agree who is asserting a value on a pin and who is reading that pin as an input.
Pins are configured using one of the following three functions:
void pin_input_mode(int pins);
void pin_output_mode(int pins);
void pin_mode(int pins, int mode);
The pins argument of all three functions accepts the LP_PINnn constants described in Section 3. The pin_mode function is just provided for convenience; it does the same thing as the pin_input and pin_output functions. Its mode argument takes one of the two constants LP_INPUT or LP_OUTPUT. Calling pin_mode(pins, LP_INPUT) is exactly the same as calling pin_input(pins).
Examples:
pin_input_mode(LP_PIN01);
/* Pin 1 is now an input */
pin_output_mode(LP_PIN14 | LP_PIN16);
/* Pins 14 and 16 are now outputs */
pin_mode(LP_PIN02, LP_INPUT);
/* Pins 2 through 9 are now ALL inputs */
pin_mode(LP_PIN01 | LP_PIN02, LP_OUTPUT);
/* Pin 1, and Pins 2-9 are ALL outputs */
pin_input_mode(LP_DATA_PINS);
/* The constant LP_DATA_PINS is an alias for Pins 2-9 */