add

/*
 *  call-seq:
 *     thgrp.add(thread)   => thgrp
 *  
 *  Adds the given <em>thread</em> to this group, removing it from any other
 *  group to which it may have previously belonged.
 *     
 *     puts "Initial group is #{ThreadGroup::Default.list}"
 *     tg = ThreadGroup.new
 *     t1 = Thread.new { sleep }
 *     t2 = Thread.new { sleep }
 *     puts "t1 is #{t1}"
 *     puts "t2 is #{t2}"
 *     tg.add(t1)
 *     puts "Initial group now #{ThreadGroup::Default.list}"
 *     puts "tg group now #{tg.list}"
 *     
 *  <em>produces:</em>
 *     
 *     Initial group is #<Thread:0x401bdf4c>
 *     t1 is #<Thread:0x401b3c90>
 *     t2 is #<Thread:0x401b3c18>
 *     Initial group now #<Thread:0x401b3c18>#<Thread:0x401bdf4c>
 *     tg group now #<Thread:0x401b3c90>
 */

static VALUE
thgroup_add(group, thread)
    VALUE group, thread;
{
    rb_thread_t th;
    struct thgroup *data;

    rb_secure(4);
    th = rb_thread_check(thread);

    if (OBJ_FROZEN(group)) {
      rb_raise(rb_eThreadError, "can't move to the frozen thread group");
    }
    Data_Get_Struct(group, struct thgroup, data);
    if (data->enclosed) {
        rb_raise(rb_eThreadError, "can't move to the enclosed thread group");
    }

    if (!th->thgroup) {
        return Qnil;
    }
    if (OBJ_FROZEN(th->thgroup)) {
        rb_raise(rb_eThreadError, "can't move from the frozen thread group");
    }
    Data_Get_Struct(th->thgroup, struct thgroup, data);
    if (data->enclosed) {
        rb_raise(rb_eThreadError, "can't move from the enclosed thread group");
    }

    th->thgroup = group;
    return group;
}