string_alloc_test.c

#include <stdio.h>
#include <ruby.h>

static VALUE alloc(VALUE self)
{
  return rb_str_new_cstr("abcdef");
}

static struct st_table* string_table;

static VALUE reuse(VALUE self)
{
  const char *str = "abcdef";
  VALUE rb_str;
  if( !st_lookup(string_table, (st_data_t) str, (st_data_t *) &rb_str) ){
    rb_str = rb_str_new_cstr(str);
    rb_gc_register_mark_object(rb_str);
    st_insert(string_table, (st_data_t) str, rb_str);
  }
  return rb_str;
}

void Init_string_alloc_test(void)
{
  string_table = st_init_strtable();
  VALUE mod = rb_define_module("StringAllocTest");
  rb_define_module_function(mod, "c_alloc", alloc, 0);
  rb_define_module_function(mod, "c_reuse", reuse, 0);
}

See https://gist.github.com/larskanis/c1ef84c3e058e819786f5fd20bc7c207