[alsa-devel] question about snd_ctl_card_info_free and valgrind

[Ivica Bukvic]

Thank you very much!
On Sep 15, 2014 5:29 AM, "Takashi Iwai" <tiwai at suse.de> wrote:

> At Mon, 15 Sep 2014 01:15:24 -0400,
> Ivica Ico Bukvic wrote:
> >
> > Hi all, I have the following code snippet (excerpt from pd-l2ork's
> > g_audio_alsa.c that was inherited from pure-data--full version available
> > at https://github.com/pd-l2ork/pd/blob/master/pd/src/s_audio_alsa.c):
> >
> >          if (snd_ctl_open(&ctl, devname, 0) >= 0)
> >          {
> >              snd_ctl_card_info_malloc(&info);
> >              snd_ctl_card_info(ctl, info);
> >              desc = snd_ctl_card_info_get_name(info);
> >              snd_ctl_card_info_free(info);
> >          }
> >
> > With the said implementation are no crashes or noticeable problems other
> > than when ran through valgrind, I get the following error report:
> >
> > ==6884== Memcheck, a memory error detector
> > ==6884== Copyright (C) 2002-2011, and GNU GPL'd, by Julian Seward et al.
> > ==6884== Using Valgrind-3.7.0 and LibVEX; rerun with -h for copyright
> info
> > ==6884== Command: pd-l2ork
> > ==6884==
> > ==6884== Invalid read of size 1
> > ==6884==    at 0x719F061: vfprintf (vfprintf.c:1630)
> > ==6884==    by 0x725D263: __vsprintf_chk (vsprintf_chk.c:86)
> > ==6884==    by 0x725D1AC: __sprintf_chk (sprintf_chk.c:33)
> > ==6884==    by 0x51412D: alsa_getdevs (stdio2.h:34)
> > ==6884==    by 0x4D84DA: sys_set_audio_settings (s_audio.c:209)
> > ==6884==    by 0x4D156A: sys_loadpreferences (s_file.c:375)
> > ==6884==    by 0x4CD804: sys_main (s_main.c:288)
> > ==6884==    by 0x717576C: (below main) (libc-start.c:226)
> > ==6884==  Address 0xc1ac488 is 40 bytes inside a block of size 376 free'd
> > ==6884==    at 0x4C2A82E: free (in
> > /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
> > ==6884==    by 0x51410F: alsa_getdevs (s_audio_alsa.c:857)
> > ==6884==    by 0x4D84DA: sys_set_audio_settings (s_audio.c:209)
> > ==6884==    by 0x4D156A: sys_loadpreferences (s_file.c:375)
> > ==6884==    by 0x4CD804: sys_main (s_main.c:288)
> > ==6884==    by 0x717576C: (below main) (libc-start.c:226)
> > ==6884==
> > ==6884== Invalid read of size 1
> > ==6884==    at 0x71CFDB8: _IO_default_xsputn (genops.c:480)
> > ==6884==    by 0x719EC31: vfprintf (vfprintf.c:1630)
> > ==6884==    by 0x725D263: __vsprintf_chk (vsprintf_chk.c:86)
> > ==6884==    by 0x725D1AC: __sprintf_chk (sprintf_chk.c:33)
> > ==6884==    by 0x51412D: alsa_getdevs (stdio2.h:34)
> > ==6884==    by 0x4D84DA: sys_set_audio_settings (s_audio.c:209)
> > ==6884==    by 0x4D156A: sys_loadpreferences (s_file.c:375)
> > ==6884==    by 0x4CD804: sys_main (s_main.c:288)
> > ==6884==    by 0x717576C: (below main) (libc-start.c:226)
> > ==6884==  Address 0xc1ac488 is 40 bytes inside a block of size 376 free'd
> > ==6884==    at 0x4C2A82E: free (in
> > /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
> > ==6884==    by 0x51410F: alsa_getdevs (s_audio_alsa.c:857)
> > ==6884==    by 0x4D84DA: sys_set_audio_settings (s_audio.c:209)
> > ==6884==    by 0x4D156A: sys_loadpreferences (s_file.c:375)
> > ==6884==    by 0x4CD804: sys_main (s_main.c:288)
> > ==6884==    by 0x717576C: (below main) (libc-start.c:226)
> > ==6884==
> > ==6884== Invalid read of size 1
> > ==6884==    at 0x71CFDC7: _IO_default_xsputn (genops.c:479)
> > ==6884==    by 0x719EC31: vfprintf (vfprintf.c:1630)
> > ==6884==    by 0x725D263: __vsprintf_chk (vsprintf_chk.c:86)
> > ==6884==    by 0x725D1AC: __sprintf_chk (sprintf_chk.c:33)
> > ==6884==    by 0x51412D: alsa_getdevs (stdio2.h:34)
> > ==6884==    by 0x4D84DA: sys_set_audio_settings (s_audio.c:209)
> > ==6884==    by 0x4D156A: sys_loadpreferences (s_file.c:375)
> > ==6884==    by 0x4CD804: sys_main (s_main.c:288)
> > ==6884==    by 0x717576C: (below main) (libc-start.c:226)
> > ==6884==  Address 0xc1ac48a is 42 bytes inside a block of size 376 free'd
> > ==6884==    at 0x4C2A82E: free (in
> > /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
> > ==6884==    by 0x51410F: alsa_getdevs (s_audio_alsa.c:857)
> > ==6884==    by 0x4D84DA: sys_set_audio_settings (s_audio.c:209)
> > ==6884==    by 0x4D156A: sys_loadpreferences (s_file.c:375)
> > ==6884==    by 0x4CD804: sys_main (s_main.c:288)
> > ==6884==    by 0x717576C: (below main) (libc-start.c:226)
> > ==6884==
> > ==6884== Invalid read of size 1
> > ==6884==    at 0x719F061: vfprintf (vfprintf.c:1630)
> > ==6884==    by 0x725D263: __vsprintf_chk (vsprintf_chk.c:86)
> > ==6884==    by 0x725D1AC: __sprintf_chk (sprintf_chk.c:33)
> > ==6884==    by 0x51414B: alsa_getdevs (stdio2.h:34)
> > ==6884==    by 0x4D84DA: sys_set_audio_settings (s_audio.c:209)
> > ==6884==    by 0x4D156A: sys_loadpreferences (s_file.c:375)
> > ==6884==    by 0x4CD804: sys_main (s_main.c:288)
> > ==6884==    by 0x717576C: (below main) (libc-start.c:226)
> > ==6884==  Address 0xc1ac488 is 40 bytes inside a block of size 376 free'd
> > ==6884==    at 0x4C2A82E: free (in
> > /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
> > ==6884==    by 0x51410F: alsa_getdevs (s_audio_alsa.c:857)
> > ==6884==    by 0x4D84DA: sys_set_audio_settings (s_audio.c:209)
> > ==6884==    by 0x4D156A: sys_loadpreferences (s_file.c:375)
> > ==6884==    by 0x4CD804: sys_main (s_main.c:288)
> > ==6884==    by 0x717576C: (below main) (libc-start.c:226)
> > ==6884==
> > ==6884== Invalid read of size 1
> > ==6884==    at 0x719F061: vfprintf (vfprintf.c:1630)
> > ==6884==    by 0x725D263: __vsprintf_chk (vsprintf_chk.c:86)
> > ==6884==    by 0x725D1AC: __sprintf_chk (sprintf_chk.c:33)
> > ==6884==    by 0x51416A: alsa_getdevs (stdio2.h:34)
> > ==6884==    by 0x4D84DA: sys_set_audio_settings (s_audio.c:209)
> > ==6884==    by 0x4D156A: sys_loadpreferences (s_file.c:375)
> > ==6884==    by 0x4CD804: sys_main (s_main.c:288)
> > ==6884==    by 0x717576C: (below main) (libc-start.c:226)
> > ==6884==  Address 0xc1ac488 is 40 bytes inside a block of size 376 free'd
> > ==6884==    at 0x4C2A82E: free (in
> > /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
> > ==6884==    by 0x51410F: alsa_getdevs (s_audio_alsa.c:857)
> > ==6884==    by 0x4D84DA: sys_set_audio_settings (s_audio.c:209)
> > ==6884==    by 0x4D156A: sys_loadpreferences (s_file.c:375)
> > ==6884==    by 0x4CD804: sys_main (s_main.c:288)
> > ==6884==    by 0x717576C: (below main) (libc-start.c:226)
> > ==6884==
> > ==6884== Invalid read of size 1
> > ==6884==    at 0x719F061: vfprintf (vfprintf.c:1630)
> > ==6884==    by 0x725D263: __vsprintf_chk (vsprintf_chk.c:86)
> > ==6884==    by 0x725D1AC: __sprintf_chk (sprintf_chk.c:33)
> > ==6884==    by 0x514192: alsa_getdevs (stdio2.h:34)
> > ==6884==    by 0x4D84DA: sys_set_audio_settings (s_audio.c:209)
> > ==6884==    by 0x4D156A: sys_loadpreferences (s_file.c:375)
> > ==6884==    by 0x4CD804: sys_main (s_main.c:288)
> > ==6884==    by 0x717576C: (below main) (libc-start.c:226)
> > ==6884==  Address 0xc1ac488 is 40 bytes inside a block of size 376 free'd
> > ==6884==    at 0x4C2A82E: free (in
> > /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
> > ==6884==    by 0x51410F: alsa_getdevs (s_audio_alsa.c:857)
> > ==6884==    by 0x4D84DA: sys_set_audio_settings (s_audio.c:209)
> > ==6884==    by 0x4D156A: sys_loadpreferences (s_file.c:375)
> > ==6884==    by 0x4CD804: sys_main (s_main.c:288)
> > ==6884==    by 0x717576C: (below main) (libc-start.c:226)
> > ==6884==
> > ==6884== Invalid read of size 8
> > ==6884==    at 0x71CFD95: _IO_default_xsputn (genops.c:480)
> > ==6884==    by 0x719EC31: vfprintf (vfprintf.c:1630)
> > ==6884==    by 0x725D263: __vsprintf_chk (vsprintf_chk.c:86)
> > ==6884==    by 0x725D1AC: __sprintf_chk (sprintf_chk.c:33)
> > ==6884==    by 0x51412D: alsa_getdevs (stdio2.h:34)
> > ==6884==    by 0x4D84DA: sys_set_audio_settings (s_audio.c:209)
> > ==6884==    by 0x4D156A: sys_loadpreferences (s_file.c:375)
> > ==6884==    by 0x4CD804: sys_main (s_main.c:288)
> > ==6884==    by 0x717576C: (below main) (libc-start.c:226)
> > ==6884==  Address 0xc1ada58 is 40 bytes inside a block of size 376 free'd
> > ==6884==    at 0x4C2A82E: free (in
> > /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
> > ==6884==    by 0x51410F: alsa_getdevs (s_audio_alsa.c:857)
> > ==6884==    by 0x4D84DA: sys_set_audio_settings (s_audio.c:209)
> > ==6884==    by 0x4D156A: sys_loadpreferences (s_file.c:375)
> > ==6884==    by 0x4CD804: sys_main (s_main.c:288)
> > ==6884==    by 0x717576C: (below main) (libc-start.c:226)
> > ==6884==
> >
> > The repeating error refers to the line 857 which is
> >
> > snd_ctl_card_info_free(info);
> >
> > If I read valgrind error report, it appears there is a double free (???)
> > error taking place even though there is no other place in that code that
> > the snd_ctl_card_info_free is being called. Now, the loop does cycle
> > through all the available alsa devices. Could this be beccause some of
> > them are invalid and if so, how would one go about detecting that before
> > calling snd_ctl_card_info_free?
> >
> > If I comment that line (as it is currently in the linked source file on
> > git), valgrind does not complain any more but I wonder if this means
> > there is now a memory leak instead?
>
> Yes, it's a memory leak there if you comment it out.
>
> The problem is that you're obtaining the pointer of the freed data in
> the code snippet above.  snd_ctl_card_info_get_name() gives the
> pointer of the data embedded inside the object info.  It doesn't give
> the newly allocated string.  So, the content of the returned pointer
> will be freed together with snd_ctl_card_info_free().
>
>
> Takashi
>