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

#define I2C_DEVICE "/dev/i2c-1"
#define I2C_DEV_ADDR 0x69
#define I2C_DEV_SELF 0x0
#define I2C_DEV_INT 0x1a
#define I2C_DEV_REG_START_ADDR 0x1b
#define I2C_DEV_REG_END_ADDR 0x22

main ()
{ int file,buffer;
  int i,j,k;
  int raw_data[4],data[4];
  char buf[256] = {0};

  if ((file = open(I2C_DEVICE, O_RDWR)) < 0)
  { /* ERROR HANDLING: you can check errno to see what went wrong */
    perror("Failed to open the i2c bus");
    exit(1);
  }

  if (ioctl(file, I2C_SLAVE, I2C_DEV_ADDR) < 0)
  { printf("Failed to acquire bus access and/or talk to slave.\n");
    /* ERROR HANDLING; you can check errno to see what went wrong */
    exit(1);
  }

/*For some unexpected reason I'm getting responses from the other I2C devices
  on the same bus so I'm ignoring data that does not match the WHO AM I reg
  and also data that has not set the interrupt register (meaning that data is
  not really avalible)
*/
  while (buf[0] != 0x69 || buf[26] == 0)
  { if (read(file,buf,sizeof(buf)) != sizeof(buf))
    { /* ERROR HANDLING: i2c transaction failed */
      printf("Failed to read from the i2c bus.\n");
      exit(1);
    }
  }

  j=0;
  for(i=I2C_DEV_REG_START_ADDR;i<=I2C_DEV_REG_END_ADDR;i++)
  { k=0;
    k= (buf[i] << 8) + buf[i+1];
    i++;
    raw_data[j]=k;
    printf("%4x \n",raw_data[j]);
    j++;
  }

  close(file);
}

/*
 This software uses a BSD license.

Copyright (c) 2010, Sean Cross / chumby industries
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>


#define I2C_FILE_NAME "/dev/i2c-0"
#define USAGE_MESSAGE \
    "Usage:\n" \
    "  %s r [addr] [register]   " \
        "to read value from [register]\n" \
    "  %s w [addr] [register] [value]   " \
        "to write a value [value] to register [register]\n" \
    ""

static int set_i2c_register(int file,
                            unsigned char addr,
                            unsigned char reg,
                            unsigned char value) {

    unsigned char outbuf[2];
    struct i2c_rdwr_ioctl_data packets;
    struct i2c_msg messages[1];

    messages[0].addr  = addr;
    messages[0].flags = 0;
    messages[0].len   = sizeof(outbuf);
    messages[0].buf   = outbuf;

    /* The first byte indicates which register we'll write */
    outbuf[0] = reg;

    /* 
     * The second byte indicates the value to write.  Note that for many
     * devices, we can write multiple, sequential registers at once by
     * simply making outbuf bigger.
     */
    outbuf[1] = value;

    /* Transfer the i2c packets to the kernel and verify it worked */
    packets.msgs  = messages;
    packets.nmsgs = 1;
    if(ioctl(file, I2C_RDWR, &packets) < 0) {
        perror("Unable to send data");
        return 1;
    }

    return 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;

    // 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);
    }


    if(argc > 3 && !strcmp(argv[1], "r")) {
        int addr = strtol(argv[2], NULL, 0);
        int reg = strtol(argv[3], NULL, 0);
        unsigned char value;
        if(get_i2c_register(i2c_file, addr, reg, &value)) {
            printf("Unable to get register!\n");
        }
        else {
            printf("Register %d: %d (%x)\n", reg, (int)value, (int)value);
        }
    }
    else if(argc > 4 && !strcmp(argv[1], "w")) {
        int addr = strtol(argv[2], NULL, 0);
        int reg = strtol(argv[3], NULL, 0);
        int value = strtol(argv[4], NULL, 0);
        if(set_i2c_register(i2c_file, addr, reg, value)) {
            printf("Unable to get register!\n");
        }
        else {
            printf("Set register %x: %d (%x)\n", reg, value, value);
        }
    }
    else {
        fprintf(stderr, USAGE_MESSAGE, argv[0], argv[0]);
    }


    close(i2c_file);


    return 0;
}

open("/dev/i2c-1", O_RDWR)              = 3
ioctl(3, 0x705, 0xbee842f0)             = 0
ioctl(3, 0x703, 0x69)                   = 0
ioctl(3, 0x720, 0xbee842f4)             = 0
close(3)                                = 0

ioctl(3, 0x705, 0xbee842f0)= 0 -> I2C_FUNCS Get the adapter functionality mask
ioctl(3, 0x703, 0x69)      = 0 -> I2C_SLAVE Use this slave address
ioctl(3, 0x720, 0xbee842f4)= 0 -> I2C_SMBUS SMBus transfer

root@pi:~# cc -O -o testme i2c_example.c
In file included from i2c_example.c:39:0:
/usr/local/include/linux/i2c-dev.h:37:8: error: redefinition of 'struct i2c_msg'
In file included from i2c_example.c:38:0:
/usr/include/linux/i2c.h:68:8: note: originally defined here
In file included from i2c_example.c:39:0:
/usr/local/include/linux/i2c-dev.h:89:7: error: redefinition of 'union i2c_smbus_data'
In file included from i2c_example.c:38:0:
/usr/include/linux/i2c.h:126:7: note: originally defined here
root@pi:~#