Disabling class_loader_hide_library_symbols()

[anilsripadarao]

We need to call the CompressRVL() and DecompressRVL() functions directly. We need this as we would like to do the compression and publishing at different times/from different threads for performance issues. Disabling _class_loader_hide_librarysymbols() in the CMakeLists.txt would enable us to do this.

[bmegli]

No.

This is transport plugin, not general purpose library.

It should be loadable/unloadable at runtime.

See for example this commit of class loader

• https://github.com/ros/class_loader/commit/bfaed321b3abab134062851420054ee733281bf1

---

Hiding symbols also protects from falling into incorrect implementation at runtime

See for example here:

• https://labjack.com/blogs/news/simple-c-symbol-visibility-demo

This should in general not happen with correctly written code

• e.g. C would use static to restrict internal function visibility

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See also here:

• https://github.com/ros-perception/perception_pcl/issues/9#issuecomment-17361157

[bmegli]

If you need to use CompressRVL() / DecompressRVL() the correct solution would be to:

• split it to separate package
• consume this package in your package (and possibly in depth image_transport)

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But I also doubt you are diagnosing your problem correctly (or close enough)

[bmegli]

The problem you are facing likely is:

• transport plugin
  • runs in camera driver (publisher)
• camera driver
  • was written without understanding this
  • or taking into consideration

Now when driver gets data and processes it the total time it has available for frame is also limited by:

• compression time
• publishing time

---

We need this as we would like to do the compression and publishing at different times/from different threads for performance issues.

The problem is typically

• not between publishing and compression
• but between camera driver internals and compression

[bmegli]

We need this as we would like to do the compression and publishing at different times/from different threads for performance issues

You may achieve similar effect by using

• image_transport republish
• from raw data and subscribing compressed data from republished

Pros:

• decouples compression to separate thread

Cons:

• you add extra publish/subscribe in the workflow (running on local machine)

[bmegli]

If the camera driver is written as nodelet:

• it is possible to decouple compression from driver without any extra (processing/communication/time) cost
• by using zero-copy nodelet message passing

I typically use image transport republish nodelet for that

• similar to image transport republish node
• but there is no public implementation available in ROS currently

[anilsripadarao]

OK, got it. Thanks for the clarification. Let me think about the other options.

[ijnek]

Closing, as problem explained above.