[alsa-devel] question about snd_ctl_card_info_free and valgrind

[Takashi Iwai]

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