RaspberryPi gpio sample

gpio.c

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/mman.h>

#define PERI_BASE  0x3F000000
#define GPIO_BASE  (PERI_BASE + 0x200000) /* GPIO controller */

#define PAGE_SIZE (4*1024)
#define BLOCK_SIZE (4*1024)

// I/O access
volatile unsigned *gpio;
static   int      mem_fd = 0;

void check( int condition, char* msg )
{
  if( condition ){ printf("%s", msg); exit (-1);}
}

volatile unsigned * io_mapping(int base_addr)
{
    char *gpio_mem, *gpio_map;
    
    if (!mem_fd) {
      mem_fd = open("/dev/mem", O_RDWR|O_SYNC);
      check((mem_fd < 0), "can't open /dev/mem \n");
    }
    
    gpio_mem = malloc(BLOCK_SIZE + (PAGE_SIZE-1));
    check( ( gpio_mem == NULL), "allocation error \n");

    // Make sure pointer is on 4K boundary
    if ((unsigned long) gpio_mem % PAGE_SIZE)
      gpio_mem += PAGE_SIZE - ((unsigned long)gpio_mem % PAGE_SIZE);
    
    // Now map it
    gpio_map = (char *)mmap((caddr_t)gpio_mem, 
                            BLOCK_SIZE, 
                            PROT_READ|PROT_WRITE, 
                            MAP_SHARED|MAP_FIXED, 
                            mem_fd, 
                            base_addr );

    check( ((long)gpio_map < 0), "mmap error \n");

    return (volatile unsigned *)gpio_map;
}

void main()
{
  int count;
  volatile unsigned *addr;

  gpio = io_mapping(GPIO_BASE);

  // set GPIO20 to output mode ( bit[2:0] ). offset is 0x8( byte address ) .
  *( gpio + 0x8/4 ) = (*( gpio + 0x8/4 ) & ~(0x7)) | (0x1 << 0);

  for( count = 0 ; ; count++){
    usleep(1000); 
    addr = ( count % 2 ) ? gpio + 0x1C/4 :  gpio + 0x28/4 ;
    *addr = 0x1 << 20;
  }
}