Accelerometer code from http://www.ladyada.net/learn/chumby/i2c.html

mma7455.c

/*
 This software uses a BSD license.

Copyright (c) 2010, Sean Cross / chumby industries &  Limor Fried / adafruit industries (we are both industrious people, eh?)

All rights reserved.
 
 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions
 are met:

 * Redistributions of source code must retain the above copyright
   notice, this list of conditions and the following disclaimer.
 * Redistributions in binary form must reproduce the above copyright
   notice, this list of conditions and the following disclaimer in the
   documentation and/or other materials provided with the
   distribution.  
 * Neither the name of Sean Cross / chumby industries nor the names
   of its contributors may be used to endorse or promote products
   derived from this software without specific prior written
   permission.

 THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
 BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
 OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY
 WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 POSSIBILITY OF SUCH DAMAGE.
 */

#include <stdio.h>
#include <linux/i2c.h>
#include <linux/i2c-dev.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <string.h>

// The 'raw' 7 bit address shifted up 
#define MMA7455_I2CADDR (0x1D * 2)

// The registers to read!
#define MMA7455_XOUT8 6
#define MMA7455_YOUT8 7
#define MMA7455_ZOUT8 8


#define I2C_FILE_NAME "/dev/i2c-0"

static int get_i2c_register(int file,
                            unsigned char addr,
                            unsigned char reg,
                            unsigned char *val) {
    unsigned char inbuf, outbuf;
    struct i2c_rdwr_ioctl_data packets;
    struct i2c_msg messages[2];

    /*
     * In order to read a register, we first do a "dummy write" by writing
     * 0 bytes to the register we want to read from.  This is similar to
     * the packet in set_i2c_register, except it's 1 byte rather than 2.
     */
    outbuf = reg;
    messages[0].addr  = addr;
    messages[0].flags = 0;
    messages[0].len   = sizeof(outbuf);
    messages[0].buf   = &outbuf;

    /* The data will get returned in this structure */
    messages[1].addr  = addr;
    messages[1].flags = I2C_M_RD/* | I2C_M_NOSTART*/;
    messages[1].len   = sizeof(inbuf);
    messages[1].buf   = &inbuf;

    /* Send the request to the kernel and get the result back */
    packets.msgs      = messages;
    packets.nmsgs     = 2;
    if(ioctl(file, I2C_RDWR, &packets) < 0) {
        perror("Unable to send data");
        return 1;
    }
    *val = inbuf;

    return 0;
}


int main(int argc, char **argv) {
    int i2c_file;
    int8_t x, y, z;  // the readings are 8 bits and signed!

    // Open a connection to the I2C userspace control file.
    if ((i2c_file = open(I2C_FILE_NAME, O_RDWR)) < 0) {
        perror("Unable to open i2c control file");
        exit(1);
    }

    // ignore arguments!

    while (1) {
      
      // read X and Y and Z from the register
      if( get_i2c_register(i2c_file, MMA7455_I2CADDR, MMA7455_XOUT8, &x) ||
	  get_i2c_register(i2c_file, MMA7455_I2CADDR, MMA7455_YOUT8, &y) ||
	  get_i2c_register(i2c_file, MMA7455_I2CADDR, MMA7455_ZOUT8, &z) ) {

	printf("Unable to read register!\n");
	return -1;
      }

      printf("X = %d\tY = %d\tZ = %d\n", x, y, z); 
    }

    close(i2c_file);
    return 0;
}