I'm getting undefined reference to `i2c_smbus_read_word_data and 'extern "C"' doesn't fix it

Question

I'm getting the error undefined reference to i2c_smbus_read_word_data(int, unsigned char)`

I've tried wrapping a few of my libraries in extern "C" but I get the same error. I tried this after seeing this answer to a similar problem.

Regardless of whether I wrap some or all of these include #include <linux/i2c-dev.h>, #include <i2c/smbus.h>, #include <linux/i2c.h>, #include <sys/ioctl.h> statements I get the same error.

The error is i2c_read.cpp:(.text+0xf8): undefined reference to i2c_smbus_read_word_data(int, unsigned char)' collect2: error: ld returned 1 exit status`

I am running my command $g++ i2c_read.cpp -li2c with -li2c as you can see.

extern "C" {
    #include <linux/i2c-dev.h>
    #include <i2c/smbus.h>
}

#include <linux/i2c.h>
#include <sys/ioctl.h>

#include <fcntl.h>    /* For O_RDWR */
#include <unistd.h> 

#include <iostream>

using namespace std;

int file;
int adapter_nr = 2;
char filename[20];

int main() {
        cout << filename << 19 << "/dev/i2c-1" << adapter_nr;
        file = open(filename, O_RDWR);
        if (file < 0) {
            exit(1);
        }
    int addr = 0x74;

    if (ioctl(file, I2C_SLAVE, addr) < 0) {
         exit(1);
    }

    __u8 reg = 0x40;
    __s32 res;
    char buf[10];
    res = i2c_smbus_read_word_data(file, reg);
    if (res < 0) {
      /* ERROR HANDLING: i2c transaction failed */
    } else {
      /* res contains the read word */
    }
    buf[0] = reg;
    buf[1] = 0x42;
    buf[2] = 0x43;
    if (write(file, buf, 3) != 3) {
      /* ERROR HANDLING: i2c transaction failed */
    }
}

Answer 1

I did a sudo apt-get update and the problem went away.
I also ran a few commands to update the compiler, though it still says my g++ version is 7.5, so that might have also contributed.