Mixer音量调节

const char *sound_device_names[] = SOUND_DEVICE_NAMES;
int fd;
int devmask, stereodevs;
char *name;

void usage()
{
    int i;
    fprintf(stderr, "usage: %s \n"
      "       %s \n\n"
      "Where is one of:\n", name, name);

    for (i = 0 ; i < SOUND_MIXER_NRDEVICES ; i++)
        if ((1 << i) & devmask)
            fprintf(stderr, "%s ", sound_device_names[i]);
        fprintf(stderr, "\n");
    exit(1);
}

int main(int argc, char *argv[])
{
    int left, right, level;
    int status;
    int device;
    char *dev;
    int i;

    name = argv[0];
    fd = open("/dev/mixer", O_RDONLY);
    if (fd == -1) {
        perror("unable to open /dev/mixer");
        exit(1);
    }

    status = ioctl(fd, SOUND_MIXER_READ_DEVMASK, &devmask);
    if (status == -1)
        perror("SOUND_MIXER_READ_DEVMASK ioctl failed");
        status = ioctl(fd, SOUND_MIXER_READ_STEREODEVS, &stereodevs);
    if (status == -1)
        perror("SOUND_MIXER_READ_STEREODEVS ioctl failed");

    if (argc != 3 && argc != 4)
        usage();
    dev = argv[1];

    for (i = 0 ; i < SOUND_MIXER_NRDEVICES ; i++)
        if (((1 << i) & devmask) && !strcmp(dev, sound_device_names[i]))
        break;

    if (i == SOUND_MIXER_NRDEVICES) {
        fprintf(stderr, "%s is not a valid mixer device\n", dev);
        usage();
    }

    device = i;
    if (argc == 4) {
        left  = atoi(argv[2]);
        right = atoi(argv[3]);
    } else {
        left  = atoi(argv[2]);

        right = atoi(argv[2]);
    }

    if ((left != right) && !((1 << i) & stereodevs)) {
        fprintf(stderr, "warning: %s is not a stereo device\n", dev);
    }

    level = (right << 8) + left;
    status = ioctl(fd, MIXER_WRITE(device), &level);
    if (status == -1) {
        perror("MIXER_WRITE ioctl failed");
        exit(1);
    }
    left  = level & 0xff;
    right = (level & 0xff00) >> 8;
    fprintf(stderr, "%s gain set to %d%% / %d%%\n", dev, left, right);
    close(fd);
    return 0;
}