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 */
    }
}

Okay. `file` isn't actually a filename? I don't quite understand every line of this code. I'm new to C++ and [this is where I got the code](https://www.kernel.org/doc/Documentation/i2c/dev-interface). I'm not sure what `O_RDWR` is or what `char filename[20]` is supposed to be. Sorry. — Ant, Jan 18 '22 at 21:14
The buffer filename is an empty string. You need to put the name of the device you want to open in it! Sending multiple things to cout prints each of them on the terminal, not puts them in the buffer. — Tom V, Jan 18 '22 at 21:37
Okay. In the original code that line was `snprintf(filename, 19, "/dev/i2c-%d", adapter_nr);` I thought it was an outdated way of printing. The [code is here](https://www.kernel.org/doc/Documentation/i2c/dev-interface). — Ant, Jan 18 '22 at 22:53
The second argument to snprintf shouldn't be 19 it should be `sizeof filename` ie. 20. — Tom V, Jan 18 '22 at 23:25

score -1 · Accepted Answer · answered Jan 21 '22 at 19:33

-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.

answered Jan 21 '22 at 19:33

Ant

933
2
17
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Answers are usually reserved for answers to the original question, not updates to the problem after trying fixes. – Kukanani Jan 22 '22 at 02:13
These updates fixed it. What should I do? Delete the original question? – Ant Jan 22 '22 at 02:19

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

1 Answers1