errno


Reading data from /dev/i2c-%d

The following code will read a byte from position 0x10, of the register at 0x3f of the device /dev/i2c-2.

To compile this code, you need the helper library i2c-dev.h which can be found in the download package here: Reading data from /dev/i2c-2 Full C Example (Compressed) (23 downloads)

main.c

//Based on https://www.kernel.org/doc/Documentation/i2c/dev-interface

#include "linux/i2c-dev.h"
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <errno.h>

int main() {
    const int adapter_nr = 2;
    char filename[20];
    snprintf(filename, 19, "/dev/i2c-%d", adapter_nr);
    const int file = open(filename, O_RDWR);
    if (file < 0) {
        printf("Oh dear, something went wrong with open()! %s\n", strerror(errno));
        exit(EXIT_FAILURE);
    }

    // The I2C address. Got it from `i2cdetect -y 2;`
    const int addr = 0x3f;

    if (ioctl(file, I2C_SLAVE, addr) < 0) {
        printf("Oh dear, something went wrong with ioctl()! %s\n", strerror(errno));
        exit(EXIT_FAILURE);
    }

    // Device register to access.
    const __u8 reg = 0x10;
    // Using SMBus commands
    const __s32 result = i2c_smbus_read_byte_data(file, reg);
    if (result < 0) {
         // ERROR HANDLING: i2c transaction failed
         printf("Oh dear, something went wrong with i2c_smbus_read_byte_data()>i2c_smbus_access()>ioctl()! %s\n", strerror(errno));
        exit(EXIT_FAILURE);
    } else {
        // res contains the read word
        printf("0x%+02x\n", result);
    }

    close(file);

    return(EXIT_SUCCESS);
}

linux/i2c-dev.h

/*
    i2c-dev.h - i2c-bus driver, char device interface

    Copyright (C) 1995-97 Simon G. Vogl
    Copyright (C) 1998-99 Frodo Looijaard <[email protected]>

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
    MA 02110-1301 USA.
*/

#ifndef _LINUX_I2C_DEV_H
#define _LINUX_I2C_DEV_H

#include <linux/types.h>
#include <sys/ioctl.h>
#include <stddef.h>


/* -- i2c.h -- */


/*
 * I2C Message - used for pure i2c transaction, also from /dev interface
 */
struct i2c_msg {
    __u16 addr;    /* slave address            */
    unsigned short flags;
#define I2C_M_TEN    0x10    /* we have a ten bit chip address    */
#define I2C_M_RD    0x01
#define I2C_M_NOSTART    0x4000
#define I2C_M_REV_DIR_ADDR    0x2000
#define I2C_M_IGNORE_NAK    0x1000
#define I2C_M_NO_RD_ACK        0x0800
    short len;        /* msg length                */
    char *buf;        /* pointer to msg data            */
};

/* To determine what functionality is present */

#define I2C_FUNC_I2C            0x00000001
#define I2C_FUNC_10BIT_ADDR        0x00000002
#define I2C_FUNC_PROTOCOL_MANGLING    0x00000004 /* I2C_M_{REV_DIR_ADDR,NOSTART,..} */
#define I2C_FUNC_SMBUS_PEC        0x00000008
#define I2C_FUNC_SMBUS_BLOCK_PROC_CALL    0x00008000 /* SMBus 2.0 */
#define I2C_FUNC_SMBUS_QUICK        0x00010000
#define I2C_FUNC_SMBUS_READ_BYTE    0x00020000
#define I2C_FUNC_SMBUS_WRITE_BYTE    0x00040000
#define I2C_FUNC_SMBUS_READ_BYTE_DATA    0x00080000
#define I2C_FUNC_SMBUS_WRITE_BYTE_DATA    0x00100000
#define I2C_FUNC_SMBUS_READ_WORD_DATA    0x00200000
#define I2C_FUNC_SMBUS_WRITE_WORD_DATA    0x00400000
#define I2C_FUNC_SMBUS_PROC_CALL    0x00800000
#define I2C_FUNC_SMBUS_READ_BLOCK_DATA    0x01000000
#define I2C_FUNC_SMBUS_WRITE_BLOCK_DATA 0x02000000
#define I2C_FUNC_SMBUS_READ_I2C_BLOCK    0x04000000 /* I2C-like block xfer  */
#define I2C_FUNC_SMBUS_WRITE_I2C_BLOCK    0x08000000 /* w/ 1-byte reg. addr. */

#define I2C_FUNC_SMBUS_BYTE (I2C_FUNC_SMBUS_READ_BYTE | \
                             I2C_FUNC_SMBUS_WRITE_BYTE)
#define I2C_FUNC_SMBUS_BYTE_DATA (I2C_FUNC_SMBUS_READ_BYTE_DATA | \
                                  I2C_FUNC_SMBUS_WRITE_BYTE_DATA)
#define I2C_FUNC_SMBUS_WORD_DATA (I2C_FUNC_SMBUS_READ_WORD_DATA | \
                                  I2C_FUNC_SMBUS_WRITE_WORD_DATA)
#define I2C_FUNC_SMBUS_BLOCK_DATA (I2C_FUNC_SMBUS_READ_BLOCK_DATA | \
                                   I2C_FUNC_SMBUS_WRITE_BLOCK_DATA)
#define I2C_FUNC_SMBUS_I2C_BLOCK (I2C_FUNC_SMBUS_READ_I2C_BLOCK | \
                                  I2C_FUNC_SMBUS_WRITE_I2C_BLOCK)

/* Old name, for compatibility */
#define I2C_FUNC_SMBUS_HWPEC_CALC    I2C_FUNC_SMBUS_PEC

/*
 * Data for SMBus Messages
 */
#define I2C_SMBUS_BLOCK_MAX    32    /* As specified in SMBus standard */
#define I2C_SMBUS_I2C_BLOCK_MAX    32    /* Not specified but we use same structure */
union i2c_smbus_data {
    __u8 byte;
    __u16 word;
    __u8 block[I2C_SMBUS_BLOCK_MAX + 2]; /* block[0] is used for length */
                                                /* and one more for PEC */
};

/* smbus_access read or write markers */
#define I2C_SMBUS_READ    1
#define I2C_SMBUS_WRITE    0

/* SMBus transaction types (size parameter in the above functions)
   Note: these no longer correspond to the (arbitrary) PIIX4 internal codes! */
#define I2C_SMBUS_QUICK            0
#define I2C_SMBUS_BYTE            1
#define I2C_SMBUS_BYTE_DATA        2
#define I2C_SMBUS_WORD_DATA        3
#define I2C_SMBUS_PROC_CALL        4
#define I2C_SMBUS_BLOCK_DATA        5
#define I2C_SMBUS_I2C_BLOCK_BROKEN  6
#define I2C_SMBUS_BLOCK_PROC_CALL   7        /* SMBus 2.0 */
#define I2C_SMBUS_I2C_BLOCK_DATA    8


/* /dev/i2c-X ioctl commands.  The ioctl's parameter is always an
 * unsigned long, except for:
 *    - I2C_FUNCS, takes pointer to an unsigned long
 *    - I2C_RDWR, takes pointer to struct i2c_rdwr_ioctl_data
 *    - I2C_SMBUS, takes pointer to struct i2c_smbus_ioctl_data
 */
#define I2C_RETRIES    0x0701    /* number of times a device address should
                   be polled when not acknowledging */
#define I2C_TIMEOUT    0x0702    /* set timeout in units of 10 ms */

/* NOTE: Slave address is 7 or 10 bits, but 10-bit addresses
 * are NOT supported! (due to code brokenness)
 */
#define I2C_SLAVE    0x0703    /* Use this slave address */
#define I2C_SLAVE_FORCE    0x0706    /* Use this slave address, even if it
                   is already in use by a driver! */
#define I2C_TENBIT    0x0704    /* 0 for 7 bit addrs, != 0 for 10 bit */

#define I2C_FUNCS    0x0705    /* Get the adapter functionality mask */

#define I2C_RDWR    0x0707    /* Combined R/W transfer (one STOP only) */

#define I2C_PEC        0x0708    /* != 0 to use PEC with SMBus */
#define I2C_SMBUS    0x0720    /* SMBus transfer */


/* This is the structure as used in the I2C_SMBUS ioctl call */
struct i2c_smbus_ioctl_data {
    __u8 read_write;
    __u8 command;
    __u32 size;
    union i2c_smbus_data *data;
};

/* This is the structure as used in the I2C_RDWR ioctl call */
struct i2c_rdwr_ioctl_data {
    struct i2c_msg *msgs;    /* pointers to i2c_msgs */
    __u32 nmsgs;            /* number of i2c_msgs */
};

#define  I2C_RDRW_IOCTL_MAX_MSGS    42


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_write_quick(int file, __u8 value)
{
    return i2c_smbus_access(file,value,0,I2C_SMBUS_QUICK,NULL);
}

static inline __s32 i2c_smbus_read_byte(int file)
{
    union i2c_smbus_data data;
    if (i2c_smbus_access(file,I2C_SMBUS_READ,0,I2C_SMBUS_BYTE,&data))
        return -1;
    else
        return 0x0FF & data.byte;
}

static inline __s32 i2c_smbus_write_byte(int file, __u8 value)
{
    return i2c_smbus_access(file,I2C_SMBUS_WRITE,value,
                            I2C_SMBUS_BYTE,NULL);
}

static inline __s32 i2c_smbus_read_byte_data(int file, __u8 command)
{
    union i2c_smbus_data data;
    if (i2c_smbus_access(file,I2C_SMBUS_READ,command,
                         I2C_SMBUS_BYTE_DATA,&data))
        return -1;
    else
        return 0x0FF & data.byte;
}

static inline __s32 i2c_smbus_write_byte_data(int file, __u8 command,
                                              __u8 value)
{
    union i2c_smbus_data data;
    data.byte = value;
    return i2c_smbus_access(file,I2C_SMBUS_WRITE,command,
                            I2C_SMBUS_BYTE_DATA, &data);
}

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;
}

static inline __s32 i2c_smbus_write_word_data(int file, __u8 command,
                                              __u16 value)
{
    union i2c_smbus_data data;
    data.word = value;
    return i2c_smbus_access(file,I2C_SMBUS_WRITE,command,
                            I2C_SMBUS_WORD_DATA, &data);
}

static inline __s32 i2c_smbus_process_call(int file, __u8 command, __u16 value)
{
    union i2c_smbus_data data;
    data.word = value;
    if (i2c_smbus_access(file,I2C_SMBUS_WRITE,command,
                         I2C_SMBUS_PROC_CALL,&data))
        return -1;
    else
        return 0x0FFFF & data.word;
}


/* Returns the number of read bytes */
static inline __s32 i2c_smbus_read_block_data(int file, __u8 command,
                                              __u8 *values)
{
    union i2c_smbus_data data;
    int i;
    if (i2c_smbus_access(file,I2C_SMBUS_READ,command,
                         I2C_SMBUS_BLOCK_DATA,&data))
        return -1;
    else {
        for (i = 1; i <= data.block[0]; i++)
            values[i-1] = data.block[i];
        return data.block[0];
    }
}

static inline __s32 i2c_smbus_write_block_data(int file, __u8 command,
                                               __u8 length, const __u8 *values)
{
    union i2c_smbus_data data;
    int i;
    if (length > 32)
        length = 32;
    for (i = 1; i <= length; i++)
        data.block[i] = values[i-1];
    data.block[0] = length;
    return i2c_smbus_access(file,I2C_SMBUS_WRITE,command,
                            I2C_SMBUS_BLOCK_DATA, &data);
}

/* Returns the number of read bytes */
/* Until kernel 2.6.22, the length is hardcoded to 32 bytes. If you
   ask for less than 32 bytes, your code will only work with kernels
   2.6.23 and later. */
static inline __s32 i2c_smbus_read_i2c_block_data(int file, __u8 command,
                                                  __u8 length, __u8 *values)
{
    union i2c_smbus_data data;
    int i;

    if (length > 32)
        length = 32;
    data.block[0] = length;
    if (i2c_smbus_access(file,I2C_SMBUS_READ,command,
                         length == 32 ? I2C_SMBUS_I2C_BLOCK_BROKEN :
                          I2C_SMBUS_I2C_BLOCK_DATA,&data))
        return -1;
    else {
        for (i = 1; i <= data.block[0]; i++)
            values[i-1] = data.block[i];
        return data.block[0];
    }
}

static inline __s32 i2c_smbus_write_i2c_block_data(int file, __u8 command,
                                                   __u8 length,
                                                   const __u8 *values)
{
    union i2c_smbus_data data;
    int i;
    if (length > 32)
        length = 32;
    for (i = 1; i <= length; i++)
        data.block[i] = values[i-1];
    data.block[0] = length;
    return i2c_smbus_access(file,I2C_SMBUS_WRITE,command,
                            I2C_SMBUS_I2C_BLOCK_BROKEN, &data);
}

/* Returns the number of read bytes */
static inline __s32 i2c_smbus_block_process_call(int file, __u8 command,
                                                 __u8 length, __u8 *values)
{
    union i2c_smbus_data data;
    int i;
    if (length > 32)
        length = 32;
    for (i = 1; i <= length; i++)
        data.block[i] = values[i-1];
    data.block[0] = length;
    if (i2c_smbus_access(file,I2C_SMBUS_WRITE,command,
                         I2C_SMBUS_BLOCK_PROC_CALL,&data))
        return -1;
    else {
        for (i = 1; i <= data.block[0]; i++)
            values[i-1] = data.block[i];
        return data.block[0];
    }
}


#endif /* _LINUX_I2C_DEV_H */