char i2c_write_single_byte_at(char slave, unsigned int address, char value);
char i2c_read_single_byte_at(char slave, unsigned int address);
...
if (i2c_write_single_byte_at(0xA0, 0x1234, 0x56) != OK) { // an error
    ...
value = i2c_write_single_byte_at(0xA0, 0x1234);           // read value