io.c:3974
static void
opt_i_set(val)
    VALUE val;
{
    if (!RTEST(val)) {
	if (ruby_inplace_mode) free(ruby_inplace_mode);
	ruby_inplace_mode = 0;
	return;
    }
    StringValue(val);
    if (ruby_inplace_mode) free(ruby_inplace_mode);
    ruby_inplace_mode = 0;
    ruby_inplace_mode = strdup(RSTRING(val)->ptr);
}
