Handling errors

The IMS environments are sensitive to errors and error-handling issues. A failing IMS transaction or program can potentially corrupt an IMS database. IMS must know about the failure of a transaction or program that has been updating a database so that it can back out any updates made by that failing program.

z/OS® XL C/C++ provides extensive error-handling facilities for the programmer, but special steps are required to coordinate IMS and C or C++ error handling so that IMS can do its database rollbacks when a program fails.

When you are using IMS from C or C++:

• Run your C or C++ program with the TRAP(ON) option, and use IMS interfaces by calling the ctdli() library function. If your application programs also use SQL facilities provided by DB2, you must modify the user exit CEEBXITA to add the user abend codes 777 and 778 to prevent the error handler from trapping these abends. This will allow deadlocks to be successfully resolved by IMS. See z/OS Language Environment Programming Guide for more information on CEEBXITA.
• The ctdli() library function will keep track of calls to and returns from IMS. If an abend or program check occurs and the C or C++ error handler gets control, it can determine if the problem arose on the IMS side of the interface or on the C or C++ side.
• If a program check or abend occurs in IMS, when the C or C++ exception handler gets control, it immediately issues an ABEND. The IMS Region Controller gets control next and ensures that the integrity of the database is preserved.
• If a program check occurs in the C or C++ program rather than in IMS, all the facilities of C or C++ error handling apply, provided that you meet certain conditions when you code your program. For any error condition that arises, you must do one of the following:
  1. Resolve the error completely so that the application can continue.
  2. Have IMS back out the program's updates by issuing a rollback call to IMS, and then terminate the program.
  3. Make sure that the program terminates abnormally and provide an installation-modified runtime user exit that turns all abnormal terminations into operating system ABENDs to effect IMS rollbacks. See z/OS Language Environment Programming Guide for more information.

  The errors you most likely can fix in your program are arithmetic exception (SIGFPE) conditions. It is unlikely that you can resolve other types of program checks or system abends in your program.

Any program that invokes IMS by way of some other IMS interface should be executed with TRAP(OFF). You should be sure that the program contains code to issue a rollback call to IMS before terminating after an error. Refer to z/OS Language Environment Programming Reference for more information about the limitations of using TRAP(OFF).