I2C Bus - Raspberry Pi Hardware Reference - page 178

To work with an I2C bus controller, your application must open the driver, made
available at the device node:

int fd;

fd = open("/dev/i2c−1",O_RDWR);
if ( fd < 0 ) {
perror("Opening /dev/i2c−1");

Note that the device node (/dev/i2c-1 ) is owned by root, so you'll need elevated
privileges to open it or have your program use setuid(2) .

ioctl(2,I2C_FUNC)
In I2C code, a check is normally performed to make sure that the driver has the right
support. The I2C_FUNC ioctl(2) call allows the calling program to query the I2C
capabilities. The capability flags returned are documented in Table 12-1 .

long funcs;
int rc;

rc = ioctl(fd,I2C_FUNCS,&funcs);
if ( rc < 0 ) {
perror("ioctl(2,I2C_FUNCS)");
abort();
}

/ Check that we have plain I2C support /

∗ ∗

assert(funcs & I2C_FUNC_I2C);

Table 12-1. I2C_FUNC bits

Bit Mask Description

I2C_FUNC_I2C Plain I2C is supported (non SMBus)

I2C_FUNC_10BIT_ADDR Supports 10-bit addresses

I2C_FUNC_PROTOCOL_MANGLING Supports:

I2C_M_IGNORE_NAK

I2C_M_REV_DIR_ADDR

I2C_M_NOSTART

I2C_M_NO_RD_ACK

The assert() macro used here checks that at least plain I2C support exists.
Otherwise, the program aborts.