The error message "Changing data structure forced command termination" is typically encountered when you are using the onstat command to view session information in an Informix database and there is a change in the data structure of the database while the command is running.

This can happen if there is a concurrent DDL (Data Definition Language) statement being executed while the onstat command is running. DDL statements include commands such as ALTER, CREATE, and DROP, which can modify the structure of the database and cause the onstat command to terminate.

To avoid this error, you can try running the onstat command at a time when there is less activity on the database or when no DDL statements are being executed.

Another solution is to use the -U option, this will allow you to run onstat command even when there is a change in the database structure.

onstat -g ses -U

This will run the onstat command and will not interrupt even if there is a change in database structure.
Please keep in mind that the performance of the onstat command might be impacted when using the -U option, so use it only if it is really needed.