Alignment of long double types

[thomasmoore-torc]

Bug report

Required Info:

• Operating System:
  • Ubuntu 22.04
• Installation type:
  • Source
• Version or commit hash:
  • rolling
• DDS implementation:
  • Fast-RTPS
• Client library (if applicable):
  • rclcpp

Steps to reproduce issue

While looking into how data is aligned by CDR, I found this issue which points out that long double types are 16 bytes in length but aligned on 8 byte boundaries. As such, I believe the following changes are required in order to properly compute the max serialized size for a type:

https://github.com/ros2/rosidl_typesupport_fastrtps/blob/rolling/rosidl_typesupport_fastrtps_cpp/resource/msg__type_support.cpp.em#L465-L468

     elif type_.typename == 'long double':
       strlist.append('  last_member_size = array_size * sizeof(long double);')
       strlist.append('  current_alignment += array_size * sizeof(long double) +')
-      strlist.append('    eprosima::fastcdr::Cdr::alignment(current_alignment, sizeof(long double));')
+      strlist.append('    eprosima::fastcdr::Cdr::alignment(current_alignment, 8);')

https://github.com/ros2/rosidl_typesupport_fastrtps/blob/rolling/rosidl_typesupport_fastrtps_c/resource/msg__type_support_c.cpp.em#L621-L624

     elif type_.typename == 'long double':
       strlist.append('  last_member_size = array_size * sizeof(long double);')
       strlist.append('  current_alignment += array_size * sizeof(long double) +')
-      strlist.append('    eprosima::fastcdr::Cdr::alignment(current_alignment, sizeof(long double));')
+      strlist.append('    eprosima::fastcdr::Cdr::alignment(current_alignment, 8);')

[clalancette]

We are considering removing long double completely, as it is underspecified in DDS. See ros2/rosidl_dynamic_typesupport#11 for the arguments.

Given that this is the first issue we've seen about long double in a long time, can you specify how you are using it?

[thomasmoore-torc]

Given that this is the first issue we've seen about long double in a long time, can you specify how you are using it?

We're not. I noticed the discrepancy and figured I'd point it out.

[clalancette]

We're not. I noticed the discrepancy and figured I'd point it out.

Got it, OK. In that case, I'm going to leave this open as a "will be fixed by removing the offending code".