Usually, i2c devices are controlled by a kernel driver. But it is also possible to access all devices on an adapter from userspace, through the /dev interface. You need to load module i2c-dev for this. Each registered i2c adapter gets a number, counting from 0. You can examine /sys/class/i2c-dev/ to see what number corresponds to which adapter. I2C device files are character device files with major device number 89 and a minor device number corresponding to the number assigned as explained above. They should be called "i2c-%d" (i2c-0, i2c-1, ..., i2c-10, ...). All 256 minor device numbers are reserved for i2c. C example ========= So let's say you want to access an i2c adapter from a C program. The first thing to do is "#include <linux/i2c-dev.h>". Please note that there are two files named "i2c-dev.h" out there, one is distributed with the Linux kernel and is meant to be included from kernel driver code, the other one is distributed with lm_sensors and is meant to be included from user-space programs. You obviously want the second one here. Now, you have to decide which adapter you want to access. You should inspect /sys/class/i2c-dev/ to decide this. Adapter numbers are assigned somewhat dynamically, so you can not even assume /dev/i2c-0 is the first adapter. Next thing, open the device file, as follows: int file; int adapter_nr = 2; /* probably dynamically determined */ char filename[20]; sprintf(filename,"/dev/i2c-%d",adapter_nr); if ((file = open(filename,O_RDWR)) < 0) { /* ERROR HANDLING; you can check errno to see what went wrong */ exit(1); } When you have opened the device, you must specify with what device address you want to communicate: int addr = 0x40; /* The I2C address */ if (ioctl(file,I2C_SLAVE,addr) < 0) { /* ERROR HANDLING; you can check errno to see what went wrong */ exit(1); } Well, you are all set up now. You can now use SMBus commands or plain I2C to communicate with your device. SMBus commands are preferred if the device supports them. Both are illustrated below. __u8 register = 0x10; /* Device register to access */ __s32 res; char buf[10]; /* Using SMBus commands */ res = i2c_smbus_read_word_data(file,register); if (res < 0) { /* ERROR HANDLING: i2c transaction failed */ } else { /* res contains the read word */ } /* Using I2C Write, equivalent of i2c_smbus_write_word_data(file,register,0x6543) */ buf[0] = register; buf[1] = 0x43; buf[2] = 0x65; if ( write(file,buf,3) != 3) { /* ERROR HANDLING: i2c transaction failed */ } /* Using I2C Read, equivalent of i2c_smbus_read_byte(file) */ if (read(file,buf,1) != 1) { /* ERROR HANDLING: i2c transaction failed */ } else { /* buf[0] contains the read byte */ } IMPORTANT: because of the use of inline functions, you *have* to use '-O' or some variation when you compile your program! Full interface description ========================== The following IOCTLs are defined and fully supported (see also i2c-dev.h): ioctl(file,I2C_SLAVE,long addr) Change slave address. The address is passed in the 7 lower bits of the argument (except for 10 bit addresses, passed in the 10 lower bits in this case). ioctl(file,I2C_TENBIT,long select) Selects ten bit addresses if select not equals 0, selects normal 7 bit addresses if select equals 0. Default 0. This request is only valid if the adapter has I2C_FUNC_10BIT_ADDR. ioctl(file,I2C_PEC,long select) Selects SMBus PEC (packet error checking) generation and verification if select not equals 0, disables if select equals 0. Default 0. Used only for SMBus transactions. This request only has an effect if the the adapter has I2C_FUNC_SMBUS_PEC; it is still safe if not, it just doesn't have any effect. ioctl(file,I2C_FUNCS,unsigned long *funcs) Gets the adapter functionality and puts it in *funcs. ioctl(file,I2C_RDWR,struct i2c_rdwr_ioctl_data *msgset) Do combined read/write transaction without stop in between. Only valid if the adapter has I2C_FUNC_I2C. The argument is a pointer to a struct i2c_rdwr_ioctl_data { struct i2c_msg *msgs; /* ptr to array of simple messages */ int nmsgs; /* number of messages to exchange */ } The msgs[] themselves contain further pointers into data buffers. The function will write or read data to or from that buffers depending on whether the I2C_M_RD flag is set in a particular message or not. The slave address and whether to use ten bit address mode has to be set in each message, overriding the values set with the above ioctl's. Other values are NOT supported at this moment, except for I2C_SMBUS, which you should never directly call; instead, use the access functions below. You can do plain i2c transactions by using read(2) and write(2) calls. You do not need to pass the address byte; instead, set it through ioctl I2C_SLAVE before you try to access the device. You can do SMBus level transactions (see documentation file smbus-protocol for details) through the following functions: __s32 i2c_smbus_write_quick(int file, __u8 value); __s32 i2c_smbus_read_byte(int file); __s32 i2c_smbus_write_byte(int file, __u8 value); __s32 i2c_smbus_read_byte_data(int file, __u8 command); __s32 i2c_smbus_write_byte_data(int file, __u8 command, __u8 value); __s32 i2c_smbus_read_word_data(int file, __u8 command); __s32 i2c_smbus_write_word_data(int file, __u8 command, __u16 value); __s32 i2c_smbus_process_call(int file, __u8 command, __u16 value); __s32 i2c_smbus_read_block_data(int file, __u8 command, __u8 *values); __s32 i2c_smbus_write_block_data(int file, __u8 command, __u8 length, __u8 *values); All these transactions return -1 on failure; you can read errno to see what happened. The 'write' transactions return 0 on success; the 'read' transactions return the read value, except for read_block, which returns the number of values read. The block buffers need not be longer than 32 bytes. The above functions are all macros, that resolve to calls to the i2c_smbus_access function, that on its turn calls a specific ioctl with the data in a specific format. Read the source code if you want to know what happens behind the screens.