text: // Copyright (c) 2014 Adafruit Industries
  // Author: Tony DiCola
  
  // Permission is hereby granted, free of charge, to any person obtaining a copy
  // of this software and associated documentation files (the "Software"), to deal
  // in the Software without restriction, including without limitation the rights
  // to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  // copies of the Software, and to permit persons to whom the Software is
  // furnished to do so, subject to the following conditions:
  
  // The above copyright notice and this permission notice shall be included in all
  // copies or substantial portions of the Software.
  
  // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  // IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  // FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  // AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  // LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  // SOFTWARE.
  #include <fcntl.h>
  #include <stdlib.h>
  #include <string.h>
  #include <sys/mman.h>
  #include <sys/stat.h>
  #include <sys/types.h>
  
  #include "bbb_mmio.h"
  
  #define GPIO_LENGTH 4096
  #define GPIO0_ADDR 0x44E07000
  #define GPIO1_ADDR 0x4804C000
  #define GPIO2_ADDR 0x481AC000
  #define GPIO3_ADDR 0x481AF000
  
  // Store mapping of GPIO base number to GPIO address.
  static uint32_t gpio_addresses[4] = { GPIO0_ADDR, GPIO1_ADDR, GPIO2_ADDR, GPIO3_ADDR };
  
  // Cache memory-mapped GPIO addresses.
  static volatile uint32_t *gpio_base[4] = { NULL };
  
  int bbb_mmio_get_gpio(int base, int number, gpio_t *gpio)
  {
      // Validate input parameters.
      if (gpio == NULL) {
          return MMIO_ERROR_ARGUMENT;
      }
      if (base < 0 || base > 3) {
          return MMIO_ERROR_ARGUMENT;
      }
      if (number < 0 || number > 31) {
          return MMIO_ERROR_ARGUMENT;
      }
      // Map GPIO memory if its hasn't been mapped already.
      if (gpio_base[base] == NULL) {
          int fd = open("/dev/mem", O_RDWR | O_SYNC);
          if (fd == -1) {
              // Error opening /dev/mem.  Probably not running as root.
              return MMIO_ERROR_DEVMEM;
          }
          // Map GPIO memory to location in process space.
          gpio_base[base] = (uint32_t *)mmap(NULL, GPIO_LENGTH, PROT_READ | PROT_WRITE, MAP_SHARED, fd,
                                             gpio_addresses[base]);
          if (gpio_base[base] == MAP_FAILED) {
              // Don't save the result if the memory mapping failed.
              gpio_base[base] = NULL;
              return MMIO_ERROR_MMAP;
          }
      }
      // Initialize and set GPIO fields.
      memset(gpio, 0, sizeof(gpio));
      gpio->base = gpio_base[base];
      gpio->number = number;
      return MMIO_SUCCESS;
  }
  