#include <linux/i2c-dev.h>
#include <linux/i2c.h>
#include <sys/ioctl.h>
#include <linux/ioctl.h>
#include <fcntl.h>

int zhongshan_i2c_transfer(unsigned char slave_address, unsigned char reg_part1, unsigned char reg_part2, unsigned char *result, int result_len) {
    struct i2c_msg msgs[2];
    unsigned char *buf;
    struct i2c_rdwr_ioctl_data rdwr;
    int file, nmsgs_sent;

    file = open("/dev/i2c-0", O_RDWR);
    if (file < 0)
        printf("can not open /dev/i2c-0.");

    //construct write message.
    msgs[0].addr = slave_address;
    msgs[0].flags = 0;
    msgs[0].len = 2;
    buf = (unsigned char *)malloc(msgs[0].len);
    if (!buf)
        printf("Error: No memory for buffer\n");
    memset(buf, 0, msgs[0].len);
    msgs[0].buf = buf;
    msgs[0].buf[0] = reg_part1;
    msgs[0].buf[1] = reg_part2;

    //construct read message.
    msgs[1].addr = slave_address;
    msgs[1].flags = I2C_M_RD;   // #define I2C_M_RD        0x0001
    msgs[1].len = result_len;
    buf = (unsigned char *)malloc(msgs[1].len);
    if (!buf)
        printf("Error: No memory for buffer\n");
    memset(buf, 0, msgs[1].len);
    msgs[1].buf = buf;

    rdwr.msgs = msgs;
    rdwr.nmsgs = 2;
    nmsgs_sent = ioctl(file, I2C_RDWR, &rdwr);
    if (nmsgs_sent < 0) {
        printf("Error: Sending messages failed\n");
        return 1;
    } else if (nmsgs_sent < rdwr.nmsgs) {
        printf("Error: only %d/%d messages were sent\n", nmsgs_sent, rdwr.nmsgs);
        return 1;
    }
    
    result[0] = msgs[1].buf[0];
    if (result_len > 1)
        result[1] = msgs[1].buf[1];

    close(file);

    for (int i = 0; i < 2; i++)
        free(msgs[i].buf);

    return 0;
}