test.c

/*
cc $(pkg-config --cflags --libs-only-L ruby-2.3) -Wl,-export-dynamic test.c -o test $(pkg-config --libs ruby-2.3)
*/

#include <stdio.h>
#include <dlfcn.h>
#include <pthread.h>

#include <ruby.h>
#include <ruby/thread.h>

const char script[] =
"require 'rubygems'\n"
"require 'ffi'\n"
"module LibWrap\n"
"  extend FFI::Library\n"
"  ffi_lib [FFI::CURRENT_PROCESS, '']\n"
"  callback :completion_function, [:string, :long, :uint8], :void\n"
"  attach_function :do_work, [:pointer, :completion_function], :int\n"
"  Callback = Proc.new do |buf_ptr, count, code|\n"
"    puts(\"ruby says #{buf_ptr}\")\n"
"  end\n"
"end\n"
"\n"
"LibWrap.do_work(\"test\", LibWrap::Callback)\n";

typedef void completion_function(char *buffer, long count, unsigned char code);

extern int do_work(char *buffer, completion_function *);

static completion_function *ruby_func;

int do_work(char *buffer, completion_function *fn)
{
    /* Calling fn directly here works */
    ruby_func = fn;

    return 0;
}

static void*
thread_call_func(void *arg)
{
    completion_function *fn = arg;

    fn("thread", 5, 0);
    return NULL;
}

static void *
join_pthread(void *arg)
{
    pthread_t tid = *(pthread_t *) arg;
    pthread_join(tid, NULL);
}

static void
nop(void *data)
{
}

int main()
{
    int state = 0;
    VALUE ret;
    /* construct the VM */
    ruby_init();
    ruby_init_loadpath();

    rb_eval_string_protect(script, &state);

    if (state)
    {
        VALUE e = rb_errinfo();
        ret = rb_funcall(e, rb_intern("message"), 0);
        fprintf(stderr, "exc %s\n", StringValueCStr(ret));
        rb_set_errinfo(Qnil);
    }
    else
    {
        pthread_t th;
        pthread_create(&th, NULL, thread_call_func, ruby_func);
        rb_thread_call_without_gvl(join_pthread, &th, nop, NULL);
    }

    /* destruct the VM */
    return ruby_cleanup(0);
}