write() returns -1 when writing to I2C_SLAVE device

Question

I've read through the Linux kernel documents on i2c and written a code to try to replicate the command i2cset -y 0 0x60 0x05 0xff

The code that I've written is here:

#include <stdio.h>
#include <linux/i2c.h>
#include <linux/i2c-dev.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <stdint.h>
#include <string.h>

int main(){

 int file;    
 file = open("/dev/i2c-0", O_RDWR);
 if (file < 0) {
  exit(1);
 }

 int addr = 0x60;

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

__u8 reg = 0x05;
__u8 res;
__u8 data = 0xff;

int written = write(file, &reg, 1); 
printf("write returned %d\n", written);

written = write(file, &data, 1); 
printf("write returned %d\n", written);

}

When I compile and run this code I get: write returned -1
write returned -1

I've tried to follow exactly what the docs tell me, my understanding is that the address is set first with the call to ioctl, then I need to write() the register and then the data that I want sent to the register.

I've also tried to use use SMbus, but I can't get my code to compile using this, it complains at the linking stage that it can't find the functions.

Have I made any mistakes in this code? I'm a beginner to i2c and don't have a lot of experience with c either.

EDIT: errno give the following message: Operation not supported. I am logged in as root on this machine though, so I don't think it can be a permissions thing, although I may be wrong.

Answer 1

The way I got around this problem was to use SMBus, in particular the functions i2c_smbus_write_byte_data and i2c_smbus_read_byte_data. I was able to use these functions to successfully read and write to the device.

I did have a little trouble finding these functions, I kept trying to download libraries using apt-get to install the appropriate header files. In the end I simply downloaded the files smbus.c and smbus.h.

Then the code I needed was:

#include <stdio.h>
#include <linux/i2c.h>
#include <linux/i2c-dev.h>
#include "smbus.h"
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <stdint.h>
#include <string.h>
#include <errno.h>


int main(){

int file;     
file = open("/dev/i2c-0", O_RDWR);
if (file < 0) {
    exit(1);
}

int addr = 0x60;

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

__u8 reg = 0x05; /* Device register to access */
__s32 res;

res = i2c_smbus_write_byte_data(file, reg, 0xff);
close(file);
}

Then if I compile the smbus.c file: gcc -c smbus.c and myfile: gcc -c myfile.c, then link them: gcc smbus.o myfile.o -o myexe I get a working executable that runs my I2C command. Ofcourse, I have smbus.c and smbus.h in the same directory as myfile.c.

Answer 2

In C, you can check the content of the errno variable to get more details into what went wrong. It is automatically declared when including errno.h and you can get a more descriptive text by calling strerror(errno).

Have you checked that you had write access to /dev/i2c-0 ?