In the past, I have written a couple of TechTips about some of the APIs I use frequently (TechTip: Using the QTMMSENDMAIL API and TechTip: Getting the IP Address of a Workstation Device). But what about the others? With that thought, I decided to compile my own "Top 10" list of APIs. It turned out not be as simple as I had expected.
To start with, when I started researching to find the "best" APIs, I ended up learning some new ones. Did you know that there is not just one but several random-number-generator APIs? I didn't. Or did you know that there are functions to return the day of the week for a given Lilian date? What the heck is a Lilian date? The good news is that there is also an API to convert standard dates to Lilian dates. Have you ever wondered how the file editor programs (e.g., DBM, DFU, etc.) managed to create those displays without knowing the file structure ahead of time? Here's your answer: There is a whole group of APIs for handling display input and output without using a display file (Dynamic Screen Manager APIs ).
Once I was able to reel myself in from reading about all the APIs I had never known about, I was able to get down to making the list. I wanted to limit my list to just 10. The problem was that some of the most useful APIs come in groups. All of the List APIs, for example, are really handy. These include some really useful stuff: list spool files, list objects, list jobs, etc. And what about the User Space APIs? Heck, you can't even use the List APIs without using a user space. Speaking of the User Space APIs, now that we have the ability to use pointers in RPG, some of the User Space APIs are pretty much obsolete--the Retrieve User Space API (QUSRTVUS) for example.
With that said, I'll cut to the chase. Here is the Top 10 API list (IMHO, of course).
1. QCMDEXC or QCAPCMD---Execute a CL command. This is the first one I learned to use and perhaps still the most widely used API out there. QCAPCMD extends the abilities of QCMDEXC but accomplishes essentially the same thing.
2. User Space APIs--Yes, I know I said some of these were obsolete, but the others are necessities. I couldn't separate them because you really need to be able to create one, get the pointer to it, and eventually delete it. Also, as I mentioned, many of the other useful APIs depend on user spaces.
3. QRTVOBJD--I use the Retrieve Object Description API in almost every single command I ever create.
4. List APIs--Again, all these are together because I use them so frequently. I have used the List Spool API to create automatic spool file cleanup programs, programs to find out which spool files or objects are using up the most disk space, and spool file archiving programs. (You'll need to access the List APIs from the alphabetic API list. Scroll down to "L.")
5. QTMMSENDMAIL--If you need to send an email to more than one recipient or with more than one attachment, SNDDST will not help you. This API will.
6. HTTP APIs--I almost forgot about these. Since I started using C for all my CGI-type applications, I don't use them anymore. But if you are using the AS/400 as a Web server and using RPG, then this group of APIs should be near the top of your list.
7. QUSCMDLN--I was so happy to discover the Display Command Window API. After that, I didn't have to rig up my own "command window" using QCMDEXC.
8. QMHRTVM--I almost forgot about this one, too. But anyone who uses subfile messaging and/or has to support multiple languages will appreciate the Retrieve Message API.
9. QGYGTLE--I couldn't very well leave the Get List Entries API off the list (no pun intended). After all, creating a list of jobs or objects is all well and good, but it's not worth much unless you can retrieve the entries. You'll also need the associated Open List API.
10. Debug APIs--I nearly left these off the list because I haven't actually used them much. But I once deleted a program source member (thinking I had a copy somewhere else) and then realized that I was left with a program object and no source code. These APIs enabled me to recover the ILE source code from the object...and probably saved me from having to find a new line of work.
Well, that's it. As you can probably tell, I sort of fudged the list because I couldn't narrow it down to just 10. I'll bet you can't either. I would encourage everyone to do the exercise that I did and just look through all the available APIs, either by browsing IBM's API page or by reading iSeries and AS/400 APIs at Work. You may find some new tools. I certainly did.
By the way, a Lilian date is the number of days elapsed since October 14, 1582 (of course!?).