Xamarin binding C library params not working (variadic functions)

Question

I need to use a C library and I got it to work on the emulator easily, but on an arm64 device only with some strange trickery. The issue is that C functions with … (variadic functions) do not pass values correctly from C# to the library.

This is the C function, with ...

cmd_ln_t *
cmd_ln_init(cmd_ln_t *inout_cmdln, const arg_t *defn, int32 strict, ...)
{
va_list args;
const char *arg, *val;
char **f_argv;
int32 f_argc;

va_start(args, strict);
f_argc = 0;
while ((arg = va_arg(args, const char *))) {
    ++f_argc;
    E_INFO("name: %s   ", arg);
    E_INFO(" retrieving value...");
    val = va_arg(args, const char*);
    E_INFO("value retrieved. \n");
    E_INFO("value: %s \n", val);
    if (val == NULL) {
        E_ERROR("Number of arguments must be even!\n");
        return NULL;
    }
    ++f_argc;
}
va_end(args);
.....................................

I check if the values are correct with the E_INFO()

Approach 1 - The default PARAMS doesn't work:

When I use the following default params expression approuch for c bindings, the ‘arg’ printed in the function shows unknown characters and when ‘val’ is used the function crashes.

[DllImport("__Internal")] public static extern unsafe cmd_ln_t*
cmd_ln_init(cmd_ln_t* inout_cmdln, arg_t* defn, int strict, params string[] arguments);

Approach 2 - a more elaborate approach works:

When I use the a more elaborate approach everything works, on x86_64 architecture normally but for arm64 with a strange work-around. the binding expression in a more elaborate approach.

[DllImport("__Internal")]
public static extern unsafe cmd_ln_t* cmd_ln_init(cmd_ln_t* inout_cmdln, arg_t* defn, int strict, string arg1, string arg2);

[DllImport("__Internal")]
public static extern unsafe cmd_ln_t* cmd_ln_init(cmd_ln_t* inout_cmdln, arg_t* defn, int strict, string arg1, string arg2, string arg3);

[DllImport("__Internal")]
public static extern unsafe cmd_ln_t* cmd_ln_init(cmd_ln_t* inout_cmdln, arg_t* defn, int strict, string arg1, string arg2, string arg3, string arg4);
//etc etc… for x numbers of arguments

The binding works works the following code

    // works for x86_64
    var cmdPointer = MyBindingLib.cmd_ln_init(null, psArgsPointer, 1,
    "-hmm", hmmFolder,
        "-dict", dictFile,
        "-mmap", "no",
        "-kws_threshold", "1e-80", 
        "-lw", "2.0", 
        null);

    // works for arm64
    var cmdPointer = MyBindingLib.cmd_ln_init(null, psArgsPointer, 1,
        null, null,
        null, null, null,
        "-hmm", hmmFolder,
        "-dict", dictFile,
        "-mmap", "no",
        "-kws_threshold", "1e-80",
        "-lw", "2.0", 
        null);

As you see, the x86_64 works normally to get the values to the C library. But the arm64 version needs to have 5 null values, others half of the values won't make it to the C library (I can check that with the E_INFO function in the C function).

Anyone any idea how to get this Xamarin C binding correct with params or without the 5 prefix null values?

Source is on github uses c library at sphinxbase