Because you are using a wrong header file for your application.
If you see an extern on the function i2c_smbus_read_word_data() in your header, it's a header file for your kernel, but not for your application. The Linux kernel has i2c_smbus_read_word_data() and other i2c smbus functions for its internal use. But they are a) not system calls, or b) not accessible from your application.
Instead, get i2c-tools from Linux Kernel Wiki and install it. If you are using Debian, just
sudo apt-get install libi2c-dev
and use i2c_smbus_read_word_data() or any other interfaces they offer.
Version Notes
i2c-dev, untill version 3.x, used be a header only package, meaning that there was no library to link to. All functions were inline functions defined using ioctl().
e.g.)
static inline __s32 i2c_smbus_access(int file, char read_write, __u8 command,
int size, union i2c_smbus_data *data)
{
struct i2c_smbus_ioctl_data args;
args.read_write = read_write;
args.command = command;
args.size = size;
args.data = data;
return ioctl(file,I2C_SMBUS,&args);
}
:
static inline __s32 i2c_smbus_read_word_data(int file, __u8 command)
{
union i2c_smbus_data data;
if (i2c_smbus_access(file,I2C_SMBUS_READ,command,
I2C_SMBUS_WORD_DATA,&data))
return -1;
else
return 0x0FFFF & data.word;
}
But since v4.0, it start to be a standard shared library with libi2c.so.0 and i2c/smbus.h. You have to include the header file in your source code
#include <i2c/smbus.h>
And link libi2c.so.0 with -li2c
gcc -o a.out main.o -li2c
