HSTREAM BASSDEF(BASS_StreamCreate)(DWORD freq, DWORD chans, DWORD flag
+s, void *proc, DWORD user);
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typedef DWORD (CALLBACK STREAMPROC)(HSTREAM handle, void *buffer, DWOR
+D length, DWORD user);
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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);
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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 "const-c.inc"

MODULE = Audio::BASSXS        PACKAGE = Audio::BASSXS        

INCLUDE: const-xs.inc

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
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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;
}
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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);
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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?