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I guess you're noticing this behavior due to line buffering. Try setting the environment variable RCUTILS_CONSOLE_STDOUT_LINE_BUFFERED=1.

Note, the default behavior was changed recently so that all log messages are flushed right away (so you shouldn't experience the same issue with ROS 2 Foxy or later). Related PR: https://github.com/ros2/rcutils/pull/196

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