c - I2C Read returns incorrect value -

i trying read in i2c value i2cget, returning wrong value in 1 of cases.

i2cget -y 0 0x57 0x40 returns 0x57
i2cget -y 0 0x3b 0x09 returns 0x86

when run program #define i2c_addr 0x57 , buffer[0] = 0x40 program returns 0x57.

but when run program #define i2c_addr 0x3b , buffer[0] = 0x09 program returns 0x00.

here code:

// #define i2c_addr 0x57 #define i2c_addr 0x3b  // #define i2c_reg 0x40 #define i2c_reg 0x09  int main(int argc, char **argv) {      char buffer[1];     buffer[0] = i2c_reg;      int fd;      // i2c file descriptor     if((fd = open("/dev/i2c-0", o_rdwr)) >= 0){          // set i2c block address         if((ioctl(fd, i2c_slave, i2c_addr)) >= 0) {              // set i2c register address             write(fd, buffer, 1);              // read data @ register buffer             read(fd, buffer, 1);              // close fd             close(fd);              // print result             printf("0x%02x\n", buffer[0]);          } else {             // ioctl error             printf("ioctl error: %s\n", strerror(errno));         }     } else {         // file error         printf("error opening file: %s\n", strerror(errno));     }      return 0; } 

i ran strace on i2cget -y 0x3b 0x09 , program. here piece of output shows how 2 reads differ.

i2cget:

open("/dev/i2c-0", o_rdwr|o_largefile)  = 3 ioctl(3, 0x705, 0xbeae5bf8)             = 0 ioctl(3, 0x703, 0x3b)                   = 0 ioctl(3, 0x720, 0xbeae5b4c)             = 0 close(3)                                = 0 

my program:

open("/dev/i2c-0", o_rdwr|o_largefile)  = 3 ioctl(3, 0x703, 0x3b)                   = 0 write(3, "\t", 1)                       = 1 read(3, "\0", 1)                        = 1 close(3)  

from strace of i2cget researched 0x720 , found out value of i2c_smbus. found source code i2cget in buildroot.

there no use of in c code, there use of function i2c_smbus_read_byte_data . added code , ran it, wouldn't run due function being undefined. copied i2c-dev.h file buildroot folder , changed #include include local file , ran, , output right data.

here code:

#include <stdio.h> #include <stdlib.h> #include <fcntl.h> #include <sys/ioctl.h> #include <errno.h> #include "i2c-dev.h"  #define i2c_addr 0x3b #define i2c_reg 0x09  // prints value of /dev/i2c-0 @ block 0x3b register 0x09 int main(int argc, char **argv) {     int fd, res;      if ((fd = open("/dev/i2c-0", o_rdwr)) >= 0) {          ioctl(fd, i2c_slave, i2c_addr);          res = i2c_smbus_read_byte_data(fd, i2c_reg);          close(fd);          printf("value - 0x%02x\n", res);     } else {         printf("error opening file: %s\n", strerror(errno));     }      return 0; }