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Error-Checking C APIs

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Using APIs is always an exciting opportunity with RPG IV. Apparently, IBM believes that providing prototypes and accurate data structures with meaningful names is something that only Windows or Linux developers are worthy of; and yet, life goes on.

I often use C APIs (including the so-called UNIX-style APIs) and MI APIs in my code. While the C APIs from the C runtime library aren't strictly considered APIs, they do work on all i5 systems, so I consider them APIs. MI has a set of instructions that have been interfaced with the C runtime library, presumably to make it easier for C programs to use MI. Since the MI instructions are provided as C runtime functions, they too make great APIs for RPG IV programmers.

One of the things that bothers me about the C runtime APIs is that they do not strictly produce traditional CPF exception/error messages. Instead they produce a C error code, referred to as errno.

For example, if I call the open64() API and it fails, the value of errno is normally set to either 3021 or 3408. When the open64() API returns a -1, it is considered to have failed, and the errno variable can be tested for 3021 or any of a dozen or more other errors.

But these number by themselves are primarily for developers who want to do something specific when they occur. This is similar to what we do when we use CL commands such as CHKOBJ and then test for a CPF9801 being issued.

The error number (errno) is returned to your program by calling the __errno API. This API returns a pointer to an integer, making it a bit complex for non-C programming languages. But RPG IV is pretty good at doing complex tasks today, so here's how it can be used in RPG IV:

     D nErrNo          S             10I 0 BASED(pErrNo)
     D c_ErrNo         PR              *   extProc('__errno')

          nRtn = open64('/home/cozzi/notfound.txt':O_TEXTDATA);
          if (nRtn = -1);
             pErrNo = c_errno();

For the open64() API, I purposely specified a file that does not exist. In addition, I specified incorrect flags for parameter 2. Upon failing, open64() sets errno to 3021.

But again, that error number is meaningless to us RPG IV programmers. We'd really like to see the text associated with the error.

As you might expect, yet another C runtime API can be used to materialize the text associated with an error code. The strerror() API does this.

The strerror() API converts a C runtime error number into a text string that describes the message. To enhance the routine above so that it retrieves, the strerror() API is introduced, as follows:

         nRtn = 

         if (nRtn = -1);                  
             pErrNo = c_errno();                 
             szErrText = %str( c_strerror(nErrNo));           
             joblog('%s - %s':%char(nErrNo):%TrimR(szErrText));

The output of this change writes the following line of text to the joblog:

3021 - The value specified for the argument is not correct.

As you can see, the message text is somewhat vague, but it's certainly no more vague than some of the compiler messages we've been seeing introduced lately.

Using C and other APIs can be beneficial to RPG IV applications. As more and more Web and Web services applications are being developed, along with more-sophisticated general-purpose business applications, the need to use C functions will become standard practice in RPG IV programs.

Bob Cozzi is host of iSeriesTV.com, an audio and video podcast/netcast Web site dedicated to the iSeries/System i world. Bob is also the author of several books, including The Modern RPG IV Language and RPG TNT: 101 Tips 'n Techniques for RPG IV. He is also the producer of RPG World, an annual conference for RPG IV programmers.


Bob Cozzi is a programmer/consultant, writer/author, and software developer. His popular RPG xTools add-on subprocedure library for RPG IV is fast becoming a standard with RPG developers. His book The Modern RPG Language has been the most widely used RPG programming book for more than a decade. He, along with others, speaks at and produces the highly popular RPG World conference for RPG programmers.

MC Press books written by Robert Cozzi available now on the MC Press Bookstore.

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