The Mono runtime provides two mechanisms to expose C code to the CIL universe: internal calls and native C code. Internal calls are tightly integrated with the runtime, and have the least overhead, as they use the same data types that the runtime uses.
The other option is to use the Platform Invoke (P/Invoke) to call C code from the CIL universe, using the standard P/Invoke mechanisms.
To register an internal call, use this call you use the mono_add_internal_call routine.
Managed objects are represented as MonoObject* types. Those objects that the runtime consumes directly have more specific C definitions (for example strings are of type MonoString *, delegates are of type MonoDelegate* but they are still MonoObject *s).
As of Mono 1.2.x types defined in mscorlib.dll do not have their fields reordered in any way. But other libraries might have their fields reordered. In these cases, Managed structures and objects have the same layout in the C# code as they do in the unmanaged world.
Structures defined outside corlib must have a specific StructLayout definition, and have it set as sequential if you plan on accessing these fields directly from C code.
Important Internal calls do not provide support for marshalling structures. This means that any API calls that take a structure (excluding the system types like int32, int64, etc) must be passed as a pointer, in C# this means passing the value as a "ref" or "out" parameter.
Certain features of the Mono runtime, like DLL mapping, are available through a configuration file that is loaded at runtime. The default Mono implementation loads the configuration file from $sysconfig/mono/config (typically this is /etc/mono/config).
See the mono-config(5) man page for more details on what goes in this file.
The following APIs expose this functionality:
To wrap a function pointer into something that the Mono runtime can consume, you should use the mono_create_ftnptr. This is only important if you plan on running on the IA64 architecture. Otherwise you can just use the function pointer address.