Jouke has asked for the wisdom of the Perl Monks concerning the following question:

Hello, I'm trying to complete my Audio::BASSXS module by wrapping more of the functions supported by the BASS library.

One of the things I want to do is add support for callbacks. This turns out to be not so simple. Here's what I'm trying to do:

In bass.h there's the following function:
HSTREAM BASSDEF(BASS_StreamCreate)(DWORD freq, DWORD chans, DWORD flag +s, void *proc, DWORD user);
This proc parameter takes a STREAMPROC callback, which is defined like this:
typedef DWORD (CALLBACK STREAMPROC)(HSTREAM handle, void *buffer, DWOR +D length, DWORD user);
Obviously I want to achieve that the BASS_StreamCreate can be called from perl, taking a coderef as a parameter.
There's an example how to call this in C supplied with the BASS package, which looks like this (I stripped it down to the most important piece):
DWORD CALLBACK stream(HSTREAM handle, char *buffer, int length, DWORD +user) { int c; // check how much recorded data is buffered c=BASS_ChannelGetData(rchan,0,BASS_DATA_AVAILABLE); c-=length; if (c>2*chunk+1764) { // buffer has gotten pretty large so remove +some c-=chunk; // leave a single 'chunk' BASS_ChannelGetData(rchan,0,c); // remove it } // fetch recorded data into stream c=BASS_ChannelGetData(rchan,buffer,length); if (c<length) memset(buffer+c,0,length-c); // short of data return length; } // create a stream to play the recording data, and start it chan=BASS_StreamCreate(44100,2,0,(STREAMPROC*)stream,0); BASS_StreamPlay(chan,0,BASS_SAMPLE_LOOP);

My XS code (also stripped down to show relevant parts) looks like this:
#include "EXTERN.h" #include "perl.h" #include "XSUB.h" #include "ppport.h" #include <bass.h> #include "callback.c" #include "" MODULE = Audio::BASSXS PACKAGE = Audio::BASSXS INCLUDE: PROTOTYPES: DISABLE HSTREAM BASS_StreamCreate(freq, chans, flags, proc, user) DWORD freq DWORD chans DWORD flags SV * proc DWORD user CODE: if (streamproccallback == (SV*)NULL) /* First time, so create a new SV */ streamproccallback = newSVsv(proc) ; else /* Been here before, so overwrite */ SvSetSV(streamproccallback, proc) ; RETVAL = BASS_StreamCreate(freq,chans,flags,(STREAMPROC*)MyStreamP +roc, user); OUTPUT: RETVAL
And the callback.c looks like this (not stripped down...this is all there is):
static SV * streamproccallback = (SV*)NULL; DWORD CALLBACK MyStreamProc(handle,buffer,length,user) HSTREAM handle; char *buffer; int length; DWORD user; { if (streamproccallback == (SV*)NULL) return 0; dSP ; int ret; SV * shandle; SV * sbuffer; SV * slength; SV * suser; ENTER; SAVETMPS; shandle = sv_2mortal(newSViv(handle)); sbuffer = sv_2mortal(newSVpv(buffer,length)); slength = sv_2mortal(newSViv(length)); suser = sv_2mortal(newSViv(user)); PUSHMARK(SP); XPUSHs(shandle); XPUSHs(sbuffer); XPUSHs(slength); XPUSHs(suser); PUTBACK; ret = call_sv(streamproccallback, G_SCALAR); buffer = SvPV(sbuffer,length); FREETMPS; LEAVE; return ret; }
This all compiles without a problem. However, when I run the following script:
#!/usr/bin/perl use strict; use warnings; use Audio::BASSXS; our $audio = 'test.mp3'; open(FH, "<", $audio) || die "Can't open $audio: $!"; binmode(FH); sub testsub { # We get 4 parameters in the @_: # ($handle,$buffer,$length,$user) = @_; my $read = sysread(FH, $_[1], $_[2]); unless ($read) { $read = $read|BASS_STREAMPROC_END; } warn "$read\n"; return $read; } BASS_Init(1,44100,0,0,0); my $stream = BASS_StreamCreate(44100,2,0,\&testsub, 0); warn BASS_ErrorString(BASS_ErrorGetCode) unless $stream; BASS_StreamPlay($stream,0,BASS_SAMPLE_LOOP); sleep(10); close(FH);
This however prints '35280' (so my callback is called once at least), and then ends with 'Unhandled exception in perl.exe (BASSXS.dll): 0xC0000005: Access violation' Then the MSVC debugger starts and shows me it has ended at the line     if (streamproccallback == (SV*)NULL) return 0; in callback.c. I've tried many, many different things (that if statement where it ends now wasn't there yesterday, and then it crashed on the dSP statement), but can't find anything that works. Does anyone have an idea?

Jouke Visser
Speaking at the 2004 O'Reilly Open Source Convention about pVoice