[LAD] mixing music and sound effects using libao

dave at 661.org dave at 661.org
Thu Apr 30 08:41:55 UTC 2015

I've been playing around with libsndfile, libvorbisfile, libmodplug, and 
libao to provide background music and sound effects for a game engine.  I 
have a solid handle on making one noise at a time, but I don't understand 
mixing.  At first I thought I would have to write my own mixer and handle 
the channels, sample rate, and sample size issues myself.  I really don't 
want to have to do that.  Then I noticed in the libao documentation that 
nothing says that I cannot spawn two threads, each of which calls 
ao_open_live() and ao_play() on different sound files.  Indeed, the docs 
for ao_initialize() seem to suggest this can be done with the statement 
that it be called in the main thread.

I tried this and it works so erratically that I'm not sure libao was 
written with this in mind.  I get these results in order of likelihood:

1) "Segmentation fault"

2) "*** glibc detected *** ./threadtest7: malloc(): memory corruption
     (fast): 0x0000000001bc5390 *** Bus error"

3) "*** glibc detected *** ./threadtest7: double free or corruption (out):
     0x00000000018ba2e0 ***"

4) A bit of the first file, then the second file.

5) Only the second file.

6) Both files messily mixed with a zipping noise. (very rare)

Is libao in fact intentionally capable of doing this?  If not, what are my 
options for mixing music and sound effects?  I really don't want to use 
SDL_mixer for a variety of reasons, chiefly because my program is 
terminal-only.  Virtual beer if you can guess what the program is.

David Griffith
dave at 661.org

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