Re: Control API, help

[Takashi Iwai <tiwai@suse.de>]

At Mon, 19 Jan 2004 08:31:01 -0400,
Manuel Jander wrote:
> 
> Hi,
> 
> I'm trying to familiarize myself with the Control API (userland
> programs). I'm somewhat scared about the huge amount of API functions...
> 
> I hammered some code together but i'm not very sure if i'm doing it
> right. If anyone can comment about this concept code, would be nice.
> 
> What i intend to do:
> 
> 1) Gain access to a control with numeric ID "CTRL_HRTF".
> 2) Read some info (value limits and such).
> 3) Write 6 (upto 112) integer values into that control.
> 
> The commented code (//) are things that i believe are not needed.
> 
> void test() {
>   int i;
>   snd_kcontrol_t *ctlp;
    ^^^^^^^^^^^^^^
    snd_ctl_t

> 	
>   /* Open the control. Is this really needed ? */
>   if ((err = snd_ctl_open(&ctlp, "3d_hrtf", SND_CTL_ASYNC)) != 0) {
>     fprintf (stderr, "3D CTL failed: (%s)\n", snd_strerror (err));
>     exit(1);
>   }

if you open "3d_hrtf", you have to define this in your asoundrc.
(i think it's intentional, right?)

>   
>   /* Retrieve some ifno to check if anything is as expected. */
>   snd_ctl_elem_info_t *info;
>   snd_ctl_card_info_alloca(info)
            ^^^^
            elem
> 	
>   /* Setup info struct. */
>   //snd_ctl_elem_info_set_id(info, const snd_ctl_elem_id_t *ptr);
>   snd_ctl_elem_info_set_numid(info, CTRL_HRTF);

you should use the name/index pair rather than the numid.
the number id may be changed e.g. if the driver is modified.

>   snd_ctl_elem_info_set_interface(info, snd_ctl_elem_iface_t val);
>   snd_ctl_elem_info_set_device(info, 0);
>   snd_ctl_elem_info_set_subdevice(info, 2);

subdevice 2 ??

>   //snd_ctl_elem_info_set_name(info, const char *val);
> 	
>   /* Get info field 0. */
>   snd_ctl_elem_info_set_index(info, 0);
>   if ((err = snd_ctl_elem_info(ctlp, info)) != 0) {
>     fprintf (stderr, "3D CTL failed: (%s)\n", snd_strerror (err));
>     exit(1);
>   }
>   snd_ctl_elem_info_free(info);

if you call alloca, you cannot (or don't have to) free it explicitly.

>   /* Declare and setup ctl_elem_value struct. */
>   snd_ctl_elem_value_t *ctl_val;
>   snd_ctl_elem_value_alloca(ctl_val);
> 	
>   //snd_ctl_elem_value_set_id(ctl_val, const snd_ctl_elem_id_t *ptr);
>   snd_ctl_elem_value_set_numid(ctl_val, CTRL_HRTF /* Some const defined
> elsewhere. */);
>   snd_ctl_elem_value_set_interface(ctl_val, SNDRV_CTL_ELEM_IFACE_PCM);
>   snd_ctl_elem_value_set_device(ctl_val, 0);
>   snd_ctl_elem_value_set_subdevice(ctl_val, 2);
>   //snd_ctl_elem_value_set_name(ctl_val, const char *val);
> 
>   /* Write values. */
>   for (i=0; i<6; i++) {
>     snd_ctl_elem_value_set_index(ctl_val, i);
>     snd_ctl_elem_value_alloca(ctl_val);

allocate twice??

>     snd_ctl_elem_value_set_integer(ctl_val, unsigned int idx, long val);

the second argument of set_integer is NOT the index of the element
itself.  it's the array index of that element.  (a control element is
an array of integer/boolean/etc.)

if you want to set different elements with different indices, call
snd_ctl_elem_write() in this loop.

>   }
>   /* Dispatch ctl write to soundcard. */
>   if ((err = snd_ctl_elem_write(ctlp, &control)) != 0) {
>     fprintf (stderr, "3D CTL failed: (%s)\n", snd_strerror (err));
>     exit(1);
>   }
>   snd_ctl_elem_value_free(ctl_val);

don't free the alloca'ed record.


> }



ciao,

Takashi


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