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SDL2: Implement SDL_GetDefaultAudioInfo() on the ALSA backend. #13333

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Merged
merged 2 commits into from
Jul 17, 2025
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SDL2: Implement SDL_GetDefaultAudioInfo() on the ALSA backend. #13333

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SDL2: Implement SDL_GetDefaultAudioInfo() on the ALSA backend.
vanfanel Jul 7, 2025
c9e7f19
SDL2: Add missing symbol loading for SDL_GetDefaultAudioInfo() implem…
vanfanel Jul 7, 2025
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65 changes: 65 additions & 0 deletions src/audio/alsa/SDL_alsa_audio.c
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Original file line number Diff line number Diff line change
Expand Up @@ -83,6 +83,12 @@ static int (*ALSA_snd_device_name_hint)(int, const char *, void ***);
static char *(*ALSA_snd_device_name_get_hint)(const void *, const char *);
static int (*ALSA_snd_device_name_free_hint)(void **);
static snd_pcm_sframes_t (*ALSA_snd_pcm_avail)(snd_pcm_t *);
static int (*ALSA_snd_pcm_info)(snd_pcm_t *, snd_pcm_info_t *);
static const char *(*ALSA_snd_pcm_info_get_name)(const snd_pcm_info_t *);
static int (*ALSA_snd_pcm_info_get_card)(const snd_pcm_info_t *);
static int (*ALSA_snd_card_get_name)(int, char **);
static int (*ALSA_snd_pcm_info_malloc)(snd_pcm_info_t **);
static int (*ALSA_snd_pcm_info_free)(snd_pcm_info_t *);
#ifdef SND_CHMAP_API_VERSION
static snd_pcm_chmap_t *(*ALSA_snd_pcm_get_chmap)(snd_pcm_t *);
static int (*ALSA_snd_pcm_chmap_print)(const snd_pcm_chmap_t *map, size_t maxlen, char *buf);
Expand Down Expand Up @@ -152,6 +158,12 @@ static int load_alsa_syms(void)
SDL_ALSA_SYM(snd_device_name_get_hint);
SDL_ALSA_SYM(snd_device_name_free_hint);
SDL_ALSA_SYM(snd_pcm_avail);
SDL_ALSA_SYM(snd_pcm_info);
SDL_ALSA_SYM(snd_pcm_info_get_card);
SDL_ALSA_SYM(snd_pcm_info_get_name);
SDL_ALSA_SYM(snd_card_get_name);
SDL_ALSA_SYM(snd_pcm_info_malloc);
SDL_ALSA_SYM(snd_pcm_info_free);
#ifdef SND_CHMAP_API_VERSION
SDL_ALSA_SYM(snd_pcm_get_chmap);
SDL_ALSA_SYM(snd_pcm_chmap_print);
Expand Down Expand Up @@ -466,6 +478,58 @@ static void ALSA_CloseDevice(_THIS)
SDL_free(this->hidden);
}

static int ALSA_GetDefaultAudioInfo(char **name, SDL_AudioSpec *spec, int iscapture)
{
const char *device = "default";
snd_pcm_t *pcm_handle;
snd_pcm_info_t *pcm_info;
snd_pcm_stream_t stream;
int card_index;
const char *dev_name;
char *card_name = NULL;
char final_name[256];

SDL_zero(final_name);
stream = iscapture ? SND_PCM_STREAM_CAPTURE : SND_PCM_STREAM_PLAYBACK;

if (ALSA_snd_pcm_open(&pcm_handle, device, stream, SND_PCM_NONBLOCK) < 0) {
return SDL_SetError("ALSA: Couldn't open default device");
}

if (ALSA_snd_pcm_info_malloc(&pcm_info) < 0) {
ALSA_snd_pcm_close(pcm_handle);
return SDL_SetError("ALSA: Couldn't allocate pcm_info");
}

if (ALSA_snd_pcm_info(pcm_handle, pcm_info) < 0) {
ALSA_snd_pcm_info_free(pcm_info);
ALSA_snd_pcm_close(pcm_handle);
return SDL_SetError("ALSA: Couldn't get PCM info");
}

card_index = ALSA_snd_pcm_info_get_card(pcm_info);
dev_name = ALSA_snd_pcm_info_get_name(pcm_info);

if (card_index >= 0 && ALSA_snd_card_get_name(card_index, &card_name) >= 0) {
SDL_snprintf(final_name, sizeof(final_name), "%s, %s", card_name, dev_name);
*name = SDL_strdup(final_name);
} else {
*name = SDL_strdup(dev_name ? dev_name : "Unknown ALSA Device");
}

if (spec) {
SDL_zero(*spec);
spec->freq = 48000;
spec->format = AUDIO_S16SYS;
spec->channels = 2;
spec->samples = 512;
Comment on lines +522 to +525
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Not sure what ALSA prefers by default if the hardware doesn't specify; the channel count is Good Enough(TM) but I wonder if there's a preference for format and samples in particular.

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To my knowledge, ALSA doesn't have any explicit preference, but we can rely on S16_LE and/or S16_BE support in ALSA for every device ALSA supports, and AUDIO_S16SYS choses the one supported by the system. That makes it a very sane default.
As for 512, it offers a good compromise between latency and buffer underrun protection, a good historic value. ALSA doesn't have any preference in this regard either to my knowledge, but other higher-level APIs that work on ALSA like OpenAL or Portaudio use 512 samples in ALSA too.

}

ALSA_snd_pcm_info_free(pcm_info);
ALSA_snd_pcm_close(pcm_handle);
return 0;
}

static int ALSA_set_buffer_size(_THIS, snd_pcm_hw_params_t *params)
{
int status;
Expand Down Expand Up @@ -975,6 +1039,7 @@ static SDL_bool ALSA_Init(SDL_AudioDriverImpl *impl)
impl->Deinitialize = ALSA_Deinitialize;
impl->CaptureFromDevice = ALSA_CaptureFromDevice;
impl->FlushCapture = ALSA_FlushCapture;
impl->GetDefaultAudioInfo = ALSA_GetDefaultAudioInfo;

impl->HasCaptureSupport = SDL_TRUE;
impl->SupportsNonPow2Samples = SDL_TRUE;
Expand Down
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