// file: i2c.c
#include <stdio.h>
#include <stdlib.h>
#include <linux/i2c.h>
#include <linux/i2c-dev.h>
#include <sys/ioctl.h>
#include <sys/un.h>

/*************************************************************************/
int I2C_Read_nbyte(
   int i2c_file,           // A file descriptor pointing to the device node
   unsigned char address,  // The address on the I2C bus
   unsigned char reg,      // The register on the device
   unsigned char *inbuf,   // Pointer to a buffer where the data read is put
   int length              // Number of characters to read
   ) {

   unsigned char outbuf[1]; // A buffer containing data written to the bus
   struct i2c_rdwr_ioctl_data packets;
   struct i2c_msg messages[2];

   outbuf[0] = reg;

   // This first message is for data written to the I2C device. It specifies
   // the address on the I2C bus and the register to read.
   messages[0].addr    = address;
   messages[0].flags   = 0;
   messages[0].len     = sizeof(outbuf);
   messages[0].buf     = outbuf;

   // This second message is for data read from the I2C device. It specifies
   // how many bytes to read and the pointer to the buffer to store the
   // data that was read.
   messages[1].addr    = address;
   messages[1].flags   = I2C_M_RD;
   messages[1].len     = length;
   messages[1].buf     = inbuf;

   packets.msgs = messages;
   packets.nmsgs = 2;

   if(ioctl(i2c_file, I2C_RDWR, &packets) < 0) {
      perror("Unable to write/read data");
      return 1;
   }

   return 0;
}

/*************************************************************************/
int I2C_Write_nbyte(
   int i2c_file,          // A file descriptor pointing to the device node
   unsigned char address, // The address on the I2C bus
   unsigned char reg,     // The register on the device
   unsigned char *outbuf, // Pointer to a buffer containg data to write
   int length             // Number of characters to write
   ) {

   char buf[length+1];

   if (ioctl(i2c_file, I2C_SLAVE, address) < 0) {
      perror("Unable to assign slave address");
      return 1;
   }

   // Create buffer containing both the register and the data to write
   // to the register.
   buf[0] = reg;
   memcpy(&(buf[1]), outbuf, length);
   
   if(write(i2c_file, buf, length+1) != length+1) {
      perror("Unable to write value");
      return 1;
   }

   return 0;
}