Basic GPIO API just covering GPIO_DIGITAL_OUTPUT.

/* <rtems/gpio.h> */

/* a configured GPIO pin handle. this is filled in by rtems_gpio_configure_pin_*
 * and can then be passed to other api functions.
 */
typedef struct gpio_pin_handle;

/* fill in gpio_pin struct to use this pin in a digital out manner.
 * must be currently unused.
 * pin_number is a number that is interpreted by the BSP and will be defined
 * with device-specific names such as BBB_P9_33, indicating pin 33 on header P9
 * on the board.
 *
 * the gpio_pin struct is then filled in and can be passed to other functions.
 */
int rtems_gpio_configure_pin_digital_out(&gpio_pin_handle, pin_number);

/* for a digital_out pin, set it to logical high */
int rtems_gpio_digital_set(&gpio_pin_handle);

/* for a digital_out pin, set it to logical low */
int rtems_gpio_digital_clear(&gpio_pin_handle);

/* a currently configured pin is to be released and made unused, allowing repurposing. */
int rtems_gpio_release_pin(&gpio_pin_handle);

/* returned error codes */
#define GPIO_SUCCESSFUL        0 /* operation OK */
#define GPIO_UNKNOWN_PIN       1 /* pin not known by bsp */
#define GPIO_UNCONFIGURED_PIN  2 /* pin not configured with rtems_gpio_configure_* */
#define GPIO_MISCONFIGURED_PIN 3 /* pin configuration doesn't match operation */

/* <libcpu/am335x.h> */

/* gpio pin definitions for the AM335X SOC */
#define AM335X_V9     63  /* GPIO1[31], SOC pin  V9 */
#define AM335X_V15    53  /* GPIO1[21], SOC pin V15 */
#define AM335X_U15    86  /* GPIO2[22], SOC pin U15 */
#define AM335X_T15    87  /* GPIO2[23], SOC pin T15 */
#define AM335X_V16    88  /* GPIO2[24], SOC pin V16 */

/* <bsp/beagleboneblack.h> */

/* gpio pin definitions - same as above but with names
 * indicating how they are wired on the Beaglebone Black board
 */
#define BBB_P8_20 71  63  /* GPIO1[31], BBB P8 header pin 20 */
#define BBB_LED_USR0  53  /* GPIO1[21], BBB USR0 LED */
#define BBB_LED_USR1  86  /* GPIO2[22], BBB USR1 LED */
#define BBB_LED_USR2  87  /* GPIO2[23], BBB USR2 LED */
#define BBB_LED_USR3  88  /* GPIO2[24], BBB USR3 LED */

/* example usage */

#include <rtems/gpio.h>

struct gpio_pin_handle led_usr0_handle; /* opaque to the user */

if(rtems_gpio_configure_pin_digital_out(&handle, led_usr0_handle) == GPIO_SUCCESS) {
    rtems_gpio_digital_set(&led_usr0_handle);
    rtems_gpio_release_pin(&led_usr0_handle);
}