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

[Takashi Iwai]

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 at moorecad.com>
> Date: 6/18/19 11:33 am
> To: "Jaroslav Kysela" <perex at perex.cz>
> Cc: alsa-devel at 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 at samiam-home-pc:~/projects/petit_ami$ gcc -fPIC -c test2.c -o test2.o
> > samiam at samiam-home-pc:~/projects/petit_ami$ gcc -g3 -Iinclude linux/playwav.c ./test2.so -lasound -o playwav
> > samiam at samiam-home-pc:~/projeclufthansa check reservationts/petit_ami$ ./playwav
> > alsaplaywave: rate: 1
> > alsaplaywave: rate: 0
> > samiam at samiam-home-pc:~/projects/petit_ami$ gcc -g3 -Iinclude linux/playwav.c ./test2.o -lasound -o playwav
> > samiam at 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 at perex.cz>
> Date: 6/18/19 11:18 am
> To: "scott andrew franco" <samiam at moorecad.com>
> Cc: alsa-devel at 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 at samiam-home-pc:~/projects/petit_ami$ gcc -fPIC -c test2.c -o test2.o
>  > samiam at samiam-home-pc:~/projects/petit_ami$ gcc -g3 -Iinclude linux/playwav.c ./test2.so -lasound -o playwav
>  > samiam at samiam-home-pc:~/projeclufthansa check reservationts/petit_ami$ ./playwav
>  > alsaplaywave: rate: 1
>  > alsaplaywave: rate: 0
>  > samiam at samiam-home-pc:~/projects/petit_ami$ gcc -g3 -Iinclude linux/playwav.c ./test2.o -lasound -o playwav
>  > samiam at 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 at perex.cz>
>  Linux Sound Maintainer; ALSA Project; Red Hat, Inc.
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