extconf.rb

require "mkmf"

create_makefile("null")

null.c

#include <stdlib.h>
#include <ruby.h>

VALUE string_with_null(VALUE self) {
  char str[] = { 102, 111, 111, 0, 98, 97, 114 };

  return rb_str_new(str, 7);
}


void Init_null() {
  rb_define_global_function("string_with_null", string_with_null, 0);
}

out.log

$ ruby extconf.rb 
creating Makefile
$ make
gcc -I. -I/opt/ruby-enterprise-1.8.6-20090520/lib/ruby/1.8/i686-linux -I/opt/ruby-enterprise-1.8.6-20090520/lib/ruby/1.8/i686-linux -I. -D_FILE_OFFSET_BITS=64 -fPIC -g -O2  -c null.c
gcc -shared -o null.so null.o -L. -L/opt/ruby-enterprise-1.8.6-20090520/lib -Wl,-R/opt/ruby-enterprise-1.8.6-20090520/lib -L.  -rdynamic -Wl,-export-dynamic    -ldl -lcrypt -lm   -lc
$ ruby test.rb
foobar
7

test.rb

require "null"

puts string_with_null
puts string_with_null.size