Question

I try to call a DllImport on the following Alsa function (from the documentation:

#define snd_seq_client_info_alloca( ptr )  __snd_alloca(ptr, snd_seq_client_info)

allocate a snd_seq_client_info_t
container on stack

Here is an implementation that I have found in C++ while reading the code of an application similar to what I want to accomplish in Mono:

snd_seq_client_info_t* cinfo;
snd_seq_client_info_alloca(&cinfo);

And here is what I have so far, but it is not working:

[DllImport(libasound.so.2)]
private static extern void snd_seq_client_info_alloca(out IntPtr ptr);

internal static void MyFunction ()
{
    IntPtr clientInfo = IntPtr.Zero;
    snd_seq_client_info_alloca(out clientInfo);
    // and then some more ...
}

But I get the following exception:

System.EntryPointNotFoundException: snd_seq_client_info_alloca
at (wrapper managed-to-native)   
MonoMultiJack.ConnectionWrapper.Alsa.LibAsoundWrapper:snd_seq_client_info_alloca (intptr&)

Answer 1

snd_seq_client_info_alloca is a pre-processor macro. You cannot P/Invoke pre-processor macros, only properly exported functions.

snd_seq_client_info_alloca expands to __snd_alloca which is a preprocessor macro as well.

__snd_alloca essentially expands to a call to alloca. You could try to rewrite this in C#. But I believe it doesn’t make a lot of sense to allocate memory in the stack frame of the caller if the caller is managed code.

Just allocate some memory somewhere else (for example, using Marshal.AllocHGlobal).