void digitalWrite(uint8_t pin, uint8_t val);
The digitalWrite() function sets the logic level being output on a pin set to OUTPUT mode. The pin is the numeric (or symbolic when used with preprocessor macros) identified for the pin on the board. A val of 0 (or the symbolic name LOW) with output a LOW value (near 0V or ground) on the pin. Any other positive 8-bit value (or the symbolic name HIGH) will output a HIGH logic value (near Vdd).
To support legacy code digitalWrite() can also be used to manipulate built-in pullup resistors (when available) on a pin which is set to INPUT mode. This was the primary method used in the early (pre-1.x.x) Arduino API. Setting a pin, which is in INPUT mode, to HIGH will enable the built-in pullup resistor. Setting it to LOW will disable the pullup resistor.
The pinMode() function conforms to the Arduino 1.6.x specification.
To set a pin to an output and set it HIGH:
pinMode(13, OUTPUT); digitalWrite(13, HIGH);
Input with a pullup manually enabled:
pinMode(2, INPUT); digitalWrite(2, HIGH);