[alsa-devel] S24_LE is 3 bytes ? in alsa-lib example pcm.c

[Takashi Iwai]

At Thu, 10 Jul 2008 12:19:05 +0200 (CEST),
Jaroslav Kysela wrote:
> 
> On Thu, 10 Jul 2008, Clemens Ladisch wrote:
> 
> > Norbert van Bolhuis wrote:
> > > as far as I know:
> > > sample format S24_LE is 24bit sample in 4 bytes
> > > sample format S24_3LE is 24bit sample in 3 bytes
> > > (why else have separate definitions), right ?
> > 
> > Yes.
> > 
> > > In the ALSA-LIb example /test/pcm.c however
> > > (http://www.alsa-project.org/alsa-doc/alsa-lib/_2test_2pcm_8c-example.html)
> > > there is no diff between the 2 sample formats.
> > 
> > That example program uses snd_pcm_format_width() although it should
> > have used snd_pcm_format_physical_width() when calculating memory
> > buffer sizes.
> 
> I think that the problem is in wrong area->step calculation in alsa-lib, 
> but I'm still investigating where the real culprit is. The
> snd_pcm_format_width() is used only for sample filling (which is OK).

I think Clemens is right.  The program uses snd_pcm_format_width()
wrongly.  Also, it won't work properly with strict aliasing...

Untested patch is below.


Takashi
--
diff --git a/test/pcm.c b/test/pcm.c
index cd29259..ecd0afa 100644
--- a/test/pcm.c
+++ b/test/pcm.c
@@ -38,7 +38,10 @@ static void generate_sine(const snd_pcm_channel_area_t *areas,
 	unsigned char *samples[channels], *tmp;
 	int steps[channels];
 	unsigned int chn, byte;
-	int ires;
+	union {
+		int i;
+		unsigned char c[4];
+	} ires;
 	unsigned int maxval = (1 << (snd_pcm_format_width(format) - 1)) - 1;
 	int bps = snd_pcm_format_width(format) / 8;  /* bytes per sample */
 	
@@ -59,8 +62,8 @@ static void generate_sine(const snd_pcm_channel_area_t *areas,
 	/* fill the channel areas */
 	while (count-- > 0) {
 		res = sin(phase) * maxval;
-		ires = res;
-		tmp = (unsigned char *)(&ires);
+		ires.i = res;
+		tmp = ires.c;
 		for (chn = 0; chn < channels; chn++) {
 			for (byte = 0; byte < (unsigned int)bps; byte++)
 				*(samples[chn] + byte) = tmp[byte];
@@ -868,7 +871,7 @@ int main(int argc, char *argv[])
 	if (verbose > 0)
 		snd_pcm_dump(handle, output);
 
-	samples = malloc((period_size * channels * snd_pcm_format_width(format)) / 8);
+	samples = malloc((period_size * channels * snd_pcm_format_physical_width(format)) / 8);
 	if (samples == NULL) {
 		printf("No enough memory\n");
 		exit(EXIT_FAILURE);
@@ -881,8 +884,8 @@ int main(int argc, char *argv[])
 	}
 	for (chn = 0; chn < channels; chn++) {
 		areas[chn].addr = samples;
-		areas[chn].first = chn * snd_pcm_format_width(format);
-		areas[chn].step = channels * snd_pcm_format_width(format);
+		areas[chn].first = chn * snd_pcm_format_physical_width(format);
+		areas[chn].step = channels * snd_pcm_format_physical_width(format);
 	}
 
 	err = transfer_methods[method].transfer_loop(handle, samples, areas);