QCAPCMD has several important advantages over QCMDEXC. For the purposes of this tip, I am going to focus on the error handling abilities of QCAPCMD. However, it would be worth your time to check out the IBM documentation on the QCAPCMD and find out what the other differences are.
To understand the difference between the way QCMDEXC handles errors and the way QCAPCMD handles errors it is necessary to understand the way most system APIs handle exceptions. Most system APIs use an error structure called QUSEC (or Qus_EC_t if you are using C) to return information regarding exception conditions and error data. You can find the layout of this structure in the QSYSINC library (QUSEC in QRPGLESRC). This structure is what nearly all APIs use to provide exception data back to the calling program. It is interesting to note that QCMDEXC is somewhat of an exception (no pun intended) in the way it handles exception conditions because it does not use the QUSEC structure.
What you gain by using the QUSEC structure is that if an exception condition occurs when the QCAPCMD command is executed, then the information regarding the exception is returned via the QUSEC structure. More importantly, the exception condition is "trapped" and not passed up the stack to the RPG program that ran it (i.e., a program exception condition will not be triggered and no *PSSR). This gives you much more flexibility in the way that you handle these types of conditions.
On a separate note, if you are going to be using system APIs and have access to C, then I would highly recommend creating simple C "wrappers" for commonly used APIs. By using "wrappers," regardless of language, you can usually reduce the number of parameters that you need to pass and create a simpler interface to the API. Also, when you can use C, the program code is simpler (and usually shorter) and you have access to structure field names that makes sense. Maybe it's just me, but "Err->Exception_Id" is much clearer than "QUSEI." In the downloadable code, I have included both a C wrapper and an RPG wrapper for the QCAPCMD so you can be the judge as to which is simpler to read and use.
Use CRTRPGMOD to create both the TESTCMD module and the RPG version of the RUNCMD module. Use CRTCMOD to create the RUNCMD module from the C source. The TESTCMD program will function the same regardless of which RUNCMD module you bind it to.
Jeff Olen is a member of the AS/400 development team at Gauss Interprise, a content management software company located in Irvine, California. He can be reached by email at
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