Q:

Why are i2c_smbus function not available? (I2C – Embedded Linux)

There are many references to using i2c_smbus_ functions when developing embedded Linux software to communicate on the I2C bus. When i2c_smbus functions such as i2c_smbus_read_word_data are referenced in software project for ARM8 processor errors such as ‘i2c_smbus_read_word_data’ was not declared in this scope are generated at compile. 
Investigation of the following header files indicate the absence of most i2c_smbus function definition. 

/usr/arm-linux-gnueabi/include/linux/i2c.h
/usr/arm-linux-gnueabi/include/linux/i2c-dev.h

Also in that following reference i2c.h file has all the i2c_smbus defined.
How can this problem be resolved?
Research references 

Using I2C from userspace in Linux
I2C Communication from Linux Userspace – Part II
I2C dev interface 

A:

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 lm-sensors 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.  i2c-dev is a header only package, meaning that there is no library to link to.  All functions are 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;
}

A:

I ran into this today.  The i2c_smbus_* functions are defined in:
/usr/include/linux/i2c-dev.h

...but when I would try to cross-compile for ARM on an older version of Ubuntu, I was running into errors such:
i2c_smbus_read_block_data was not declared in this scope

Turns out the functions are not defined in the equivalent ARM-specific location:
/usr/arm-linux-gnueabi/include/linux/i2c-dev.h

When cross-compiling, this 2nd older header file is the one used.  Had to re-declare locally a few of the inline i2c_smbus_... functions to get around the problem.

