Re: [alsa-devel] Serious bug calling ALSA lib functions from .so vs. .o file

18 Jun 2019


      Fixed.
Again, apologies for my being dense there.
This is *great* it all works now. I don't think I would have figured out that link issue on my own.
Scott Franco
--------- Original Message --------- Subject: RE: Re: [alsa-devel] Serious bug calling ALSA lib functions from .so vs. .o file
From: "scott andrew franco" samiam@moorecad.com
Date: 6/18/19 2:10 pm
To: "Takashi Iwai" tiwai@suse.de
Cc: alsa-devel@alsa-project.org
Ok I see, apologies. Will try that.
Scott
--------- Original Message --------- Subject: Re: [alsa-devel] Serious bug calling ALSA lib functions from .so vs. .o file
From: "Takashi Iwai" tiwai@suse.de
Date: 6/18/19 1:49 pm
To: "scott andrew franco" samiam@moorecad.com
Cc: alsa-devel@alsa-project.org
On Tue, 18 Jun 2019 20:59:19 +0200,
 scott andrew franco wrote:
...
PS. I really appreciate the help, I have spent more than a week on this, it is a serious blocking issue.
I've gone over the example I posted quite carefully. I recommend you try the example out. You just need the
two source files below (test2.c and playwav.c). and execute the commands shown below.
The fact that it fails only in the .so file can explain why this bug hasn't affected more user code, since most
people don't call alsa from another library. However, it does not explain everything since I believe there must
be a few users who do put the calling code in a library.
What Jaroslav pointed is to pass -lasound2 at creating the *.so file,
 not at linking to the final binary. That is,
% gcc -shared test2.o -o test2.so -lasound
Otherwise the linker won't get the proper versioned symbol, hence an
 incorrect symbol may be taken instead of the expected versioned one.
Calling ALSA library from a shared object is commonly done,
 e.g. PulseAudio. The tricky part is the versioned symbol handling,
 and it's not specific to ALSA-lib but a generic problem.
Takashi
...
Thanks,
Scott Franco
--------- Original Message --------- Subject: RE: Re: [alsa-devel] Serious bug calling ALSA lib functions from .so vs. .o file
From: "scott andrew franco" samiam@moorecad.com
Date: 6/18/19 11:33 am
To: "Jaroslav Kysela" perex@perex.cz
Cc: alsa-devel@alsa-project.org
Hi, and thanks for the reply.
...
I think that you must link .so to -lasound, too.
This was done and shows in the command line example below, IE,
samiam@samiam-home-pc:~/projects/petit_ami$ gcc -fPIC -c test2.c -o test2.o
...
samiam@samiam-home-pc:~/projects/petit_ami$ gcc -g3 -Iinclude linux/playwav.c ./test2.so -lasound -o playwav
samiam@samiam-home-pc:~/projeclufthansa check reservationts/petit_ami$ ./playwav
alsaplaywave: rate: 1
alsaplaywave: rate: 0
samiam@samiam-home-pc:~/projects/petit_ami$ gcc -g3 -Iinclude linux/playwav.c ./test2.o -lasound -o playwav
samiam@samiam-home-pc:~/projects/petit_ami$ ./playwav
alsaplaywave: rate: 1
alsaplaywave: rate: 1
Note that in both the "succeeds" case and the "fails" case carry the -lasound option. It would not compile
if not.
Scott Franco
--------- Original Message --------- Subject: Re: [alsa-devel] Serious bug calling ALSA lib functions from .so vs. .o file
From: "Jaroslav Kysela" perex@perex.cz
Date: 6/18/19 11:18 am
To: "scott andrew franco" samiam@moorecad.com
Cc: alsa-devel@alsa-project.org
Dne 18. 06. 19 v 17:16 scott andrew franco napsal(a):
...
Hello,
The issue:
calling across .so (dynamic linking) produces different behavior in ALSA library calls than normal,
and causes ALSA to malfunction.
I have minimized my issue to as small a code section as possible. Note the code is from commonly available
internet examples, including Free Electrons "Audio in embedded linux systems".
It seems like a linker issue. I think that you must link .so to -lasound, too.
(add -lasound to the command producing your .so library to satisfy the
versioned symbols)
Jaroslav
...
The run is:
samiam@samiam-home-pc:~/projects/petit_ami$ gcc -fPIC -c test2.c -o test2.o
samiam@samiam-home-pc:~/projects/petit_ami$ gcc -g3 -Iinclude linux/playwav.c ./test2.so -lasound -o playwav
samiam@samiam-home-pc:~/projeclufthansa check reservationts/petit_ami$ ./playwav
alsaplaywave: rate: 1
alsaplaywave: rate: 0
samiam@samiam-home-pc:~/projects/petit_ami$ gcc -g3 -Iinclude linux/playwav.c ./test2.o -lasound -o playwav
samiam@samiam-home-pc:~/projects/petit_ami$ ./playwav
alsaplaywave: rate: 1
alsaplaywave: rate: 1
Note the only difference between the two runs of "playwave" is if the second module, test2, is linked as a .so or
linked as a .o.
The code is:
playwav.c
#include <alsa/asoundlib.h>
#include <stdio.h>
void alsaplaywave1(void)
{
snd_pcm_t *pcm_handle;
snd_pcm_hw_params_t *params;
unsigned int val;
unsigned int rate;
int r;
/* open pcm device */
r = snd_pcm_open(&pcm_handle, "default", SND_PCM_STREAM_PLAYBACK, 0);
if (r < 0) printf("Cannot open PCM output device");
snd_pcm_hw_params_alloca(&params); /* get hw parameter block */
snd_pcm_hw_params_any(pcm_handle, params);
r = snd_pcm_hw_params_set_access(pcm_handle, params, SND_PCM_ACCESS_RW_INTERLEAVED);
if (r < 0) printf("Cannot set interleaved mode");
r = snd_pcm_hw_params_set_format(pcm_handle, params, SND_PCM_FORMAT_S16_LE);
if (r < 0) printf("Cannot set format");
r = snd_pcm_hw_params_set_channels(pcm_handle, params, 2);
if (r < 0) printf("Cannot set channels number");
val = 44100;
r = snd_pcm_hw_params_set_rate_near(pcm_handle, params, &rate, 0);
if (r < 0) printf("Cannot set rate");
snd_pcm_hw_params_get_rate(params, &rate, 0);
printf("alsaplaywave: rate: %d\n", rate);
snd_pcm_close(pcm_handle);
}
extern void alsaplaywave2(void);
int main(int argc, char **argv)
{
alsaplaywave1();
alsaplaywave2();
return 0;
}
==================================================================================
test2.c
#include <alsa/asoundlib.h>
#include <stdio.h>
void alsaplaywave2(void)
{
snd_pcm_t *pcm_handle;
snd_pcm_hw_params_t *params;
unsigned int val;
unsigned int rate;
int r;
/* open pcm device */
r = snd_pcm_open(&pcm_handle, "default", SND_PCM_STREAM_PLAYBACK, 0);
if (r < 0) printf("Cannot open PCM output device");
snd_pcm_hw_params_alloca(&params); /* get hw parameter block */
snd_pcm_hw_params_any(pcm_handle, params);
r = snd_pcm_hw_params_set_access(pcm_handle, params, SND_PCM_ACCESS_RW_INTERLEAVED);
if (r < 0) printf("Cannot set interleaved mode");
r = snd_pcm_hw_params_set_format(pcm_handle, params, SND_PCM_FORMAT_S16_LE);
if (r < 0) printf("Cannot set format");
r = snd_pcm_hw_params_set_channels(pcm_handle, params, 2);
if (r < 0) printf("Cannot set channels number");
val = 44100;
r = snd_pcm_hw_params_set_rate_near(pcm_handle, params, &rate, 0);
if (r < 0) printf("Cannot set rate");
snd_pcm_hw_params_get_rate(params, &rate, 0);
printf("alsaplaywave: rate: %d\n", rate);
snd_pcm_close(pcm_handle);
}
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-- 
Jaroslav Kysela perex@perex.cz
Linux Sound Maintainer; ALSA Project; Red Hat, Inc.
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