Procedural native audio synthesis with OnAudioFilterRead

I’m trying to use the Unity3.5 audio buffer access to integrate a synthesis engine in C to produce audio.
When I try to do it in the most efficient way, i.e. passing the audio buffer pointer, I get crashes in what looks like FMOD memory management.

Maybe I’m not allowed to pass arrays by reference like that, since the (hoped for) memory pinning seems to invoke some runtime code that is not allowed in the audio thread or so?
I only have shaky knowledge about the Mono C# to C interface, p/invoke, marshaling, and all that.

the C# audio filter script on the (empty) audio source:

[DllImport ("synth-lib")]
private static extern void synth (IntPtr self, int nsamp, float[] data, int channels);
    
void OnAudioFilterRead (float[] data, int channels)  {
        synth(impl_, data.Length, data, channels); 
}

the C code prototype:

 void synth (mysynth_t *self, int nsamp, float *data, int channels); 

the crash log:

Thread 0:  Dispatch queue: com.apple.main-thread
4   libmono.0.dylib               	0x02845278 mono_handle_native_sigsegv + 327
5   libSystem.B.dylib             	0x93c6505b _sigtramp + 43
6   ???                           	0xffffffff 0 + 4294967295
7   fmod_editor.dylib             	0x02ac904e 0x2ac7000 + 8270
8   fmod_editor.dylib             	0x02b2b249 FMOD::SystemI::getMemoryUsedImpl(FMOD::MemoryTracker*) + 2181
9   fmod_editor.dylib             	0x02b28db1 FMOD::System::lockDSP() + 39
10  com.unity3d.UnityEditor3.x    	0x006514a7 AudioManager::ProcessScheduledSources() + 23
11  com.unity3d.UnityEditor3.x    	0x00659ea3 AudioManager::Update() + 51

Thread 16 Crashed:
0   libSystem.B.dylib             	0x93c6c0ee __semwait_signal_nocancel + 10
6   libmono.0.dylib               	0x027be019 mono_sigsegv_signal_handler + 498
7   libSystem.B.dylib             	0x93c6505b _sigtramp + 43
8   ???                           	0xffffffff 0 + 4294967295
9   libmono.0.dylib               	0x02986483 g_log + 41
10  libmono.0.dylib               	0x028d5946 mono_array_to_lparray + 164
11  ???                           	0x08bd400c 0 + 146620428
12  ???                           	0x38641a78 0 + 946084472
13  ???                           	0x3864161f 0 + 946083359
14  ???                           	0x3864193e 0 + 946084158
15  libmono.0.dylib               	0x027c5140 mono_jit_runtime_invoke + 1149
16  libmono.0.dylib               	0x0290d170 mono_runtime_invoke + 133
17  com.unity3d.UnityEditor3.x    	0x00678933 AudioCustomFilter::readCallback(FMOD_DSP_STATE*, float*, float*, unsigned int, int, int) + 291
18  fmod_editor.dylib             	0x02b0483b FMOD::DSPConnection::getInput(FMOD::DSP**) + 18131

Have to pass the onaudiofilterread function a float array of same length as audiobuffer settings.

See reference for example of the loop that has to be in the function, i dont think that above code would work on it. it’s a tricky code it needs just 1-2 loops and if functions.