Question

I want to port same function from oss mixer to alsa but I don’t now how to check if device
channel have capture mask.
Code for OSS:

These functions handle the mixer device

static int mixer_fd = -1, mixer_src = -1;

int mixer_init(char *mixer_device, char *mixer_source)
{
    int i;  
    mixer_src = -1; 
    for (i=0;i<SOUND_MIXER_NRDEVICES;i++)
        if (strcmp(mixer_source, devices[i]) == 0) 
            mixer_src = i;

    mixer_fd = open(mixer_device, O_RDWR);
    if (mixer_src < 0)  
        return -1;          
    if (mixer_fd < 0)
        return 0;
    return 1;
}

char** mixer_get_rec_devices(void)
{
    int i, o, devmask, res;
    char** result;

    if ((ioctl(mixer_fd, SOUND_MIXER_READ_RECMASK, &devmask)) == -1)
    return NULL;
    else
    {
        result = malloc(sizeof(char*)*SOUND_MIXER_NRDEVICES);
        o = 0;
        for (i=0;i<SOUND_MIXER_NRDEVICES;i++)
            {
            res = (devmask >> i)%2;
                if (res)
            {
                result[o] = malloc(strlen(devices[i])+1);
                sprintf(result[o], "%s", devices[i]);
                o++;
            }
            result[o] = NULL;   
        }
    }
    return result;
}

int mixer_set_rec_device(void)
{
    int devmask, recmask;

    if (mixer_fd <= 0)
        return 0;

    if (mixer_src < 0)
        return 0;

    if ((ioctl(mixer_fd, SOUND_MIXER_READ_RECMASK, &devmask)) == -1)
        return 0;

    recmask = 1 << mixer_src;
    if (!(recmask & devmask))
        return 0;

    if ((ioctl(mixer_fd, SOUND_MIXER_WRITE_RECSRC, &recmask)) == -1)
        return 0;
    return 1;
}

I need help to port this code to ALSA mixer.

function “mixer_set_rec_device()” is used to check if channel mixer can record if user activate recording, where settings.mixer is channel choose by user.

if (!mixer_set_rec_device()) {
        fprintf(stderr, "Could not set \"%s\" as recording Source", settings.mixer);
        return -1;
    }

Answer 1

Some devices can capture from multiple sources; these devices typically have several boolean mixer controls named xxxxx Capture Switch.

Some devices can capture from only one source; these devices typically have an enumerated mixer control named Capture Source.

In the general case, the device’s mixer might work entirely different.

In any case, general-purpose applications should never modify the user’s mixer configuration, but just record from some PCM device; configuring the mixer should be done by the user, or by some specialized mixer application that knows the hardware.