Template for Raspberry Pi from: http://elinux.org/RPi_Low-level_peripherals#C_2

main.c

//
//  How to access GPIO registers from C-code on the Raspberry-Pi
//  Example program
//  15-January-2012
//  Dom and Gert
//  Revised: 15-Feb-2013


// Access from ARM Running Linux

#define BCM2708_PERI_BASE   0x20000000
#define GPIO_BASE           (BCM2708_PERI_BASE + 0x200000) /* GPIO controller */


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

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

int mem_fd;
void *gpio_map;

// I/O access
volatile unsigned *gpio;


// GPIO setup macros. Always use INP_GPIO(x) before using OUT_GPIO(x) or SET_GPIO_ALT(x,y)
#define INP_GPIO(g)         *(gpio+((g)/10)) &= ~(7<<(((g)%10)*3))
#define OUT_GPIO(g)         *(gpio+((g)/10)) |= (1<<(((g)%10)*3))
#define SET_GPIO_ALT(g,a)   *(gpio+((g)/10)) |= (((a)<=3?(a)+4:(a)==4?3:2)<<(((g)%10)*3))

#define GPIO_SET *(gpio+7)  // sets bits which are 1 ignores bits which are 0
#define GPIO_CLR *(gpio+10) // clears bits which are 1 ignores bits which are 0

void setup_io();

int main(int argc, char **argv)
{
    int g,rep;

    // Set up gpi pointer for direct register access
    setup_io();

    // Switch GPIO 7..11 to output mode

    /************************************************************************\
    * You are about to change the GPIO settings of your computer.           *
    * Mess this up and it will stop working!                                *
    * It might be a good idea to 'sync' before running this program         *
    * so at least you still have your code changes written to the SD-card!  *
    \************************************************************************/

    // Set GPIO pins 7-11 to output
    for (g=7; g<=11; g++)
    {
        INP_GPIO(g); // must use INP_GPIO before we can use OUT_GPIO
        OUT_GPIO(g);
    }

    for (rep=0; rep<10; rep++)
    {
        for (g=7; g<=11; g++)
        {
            GPIO_SET = 1<<g;
            sleep(1);
        }
        for (g=7; g<=11; g++)
        {
            GPIO_CLR = 1<<g;
            sleep(1);
        }
    }

    return 0;

} // main


//
// Set up a memory regions to access GPIO
//
void setup_io()
{
    /* open /dev/mem */
    if ((mem_fd = open("/dev/mem", O_RDWR|O_SYNC)) < 0) {
        printf("can't open /dev/mem \n");
        exit(-1);
    }

    /* mmap GPIO */
    gpio_map = mmap(
        NULL,                   // Any adddress in our space will do
        BLOCK_SIZE,             // Map length
        PROT_READ|PROT_WRITE,   // Enable reading & writting to mapped memory
        MAP_SHARED,             // Shared with other processes
        mem_fd,                 // File to map
        GPIO_BASE               // Offset to GPIO peripheral
    );

    close(mem_fd); //No need to keep mem_fd open after mmap

    if (gpio_map == MAP_FAILED) {
        printf("mmap error %d\n", (int)gpio_map); //errno also set!
        exit(-1);
    }

    // Always use volatile pointer!
    gpio = (volatile unsigned *)gpio_map;


} // setup_io