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Scott Klement wrote:
Peter, this bit of the system's Java documentation: http://publib.boulder.ibm.com/infocenter/iseries/v5r4/topic/rzaha/invapiex.htm mentions that three file descriptors should be opened in the job for Java I/O to work when the JVM is started by the invocation API (which is how RPG starts the JVM). It shows some sample C code that opens the three descriptors.I'm struggling to understand when/where this should be used. Are you suggesting that RPG should always close descriptors 0-2 and open them as /dev/null whenever it uses Java?
No, not at all; it should definitely not close those descriptors. It may coincidentally open those descriptors, but it is just as likely to open descriptors 14-16, or maybe 14, 25, 34. It just seems to be necessary to have at least 3 descriptors open for some aspects of Java I/O to work correctly. I don't believe the descriptors are actually used for the 3 I/O streams by Java. At least, all the evidence I have seen suggests that they aren't used. The RPG program isn't closing descriptors 0-2. It's closing the descriptors in an array that it earlier filled by calling open(). for i = 1 to 3; closeFile(fd(i));
How does this interact with QIBM_USE_DESCRIPTOR_STDIO? Or C programs? or QShell scripts? Will it cause havoc for them, or is it only affecting stdin/stdout/stderr for the RPG program?
If you use QIBM_USE_DESCRIPTOR_STDIO, the data doesn't get written out to your files unless the descriptors were opened. There's no apparent Java exception in this case, but the output doesn't seem to go anywhere. I don't think it has anything to do with C's stdin stdout and stderr. The normal Java output screen isn't shared with the C stdout screen. I believe QShell already opens 3 descriptors when it starts up, so if your RPG program opens 3 more, it won't affect anything.
An RPG version of this code is below; you may notice that the C version allocates to /dev/null1, null2 and null3. Apparently that is not correct; it should be /dev/null as the RPG version does.Correct. /dev/null is a special file (of type *CHRSF) that simply discards any data sent to it. The idea is that an existing program can work with these streams as normal without knowing that the input/output is being discarded.It is not really necessary (and maybe not advisable, I'm not sure) to close the file descriptors since they will be closed when the job ends, but the program can be called with parameter 'N' if you want to close the files after the Java has completed.See, what's part of what I'm confused about. If I close these descriptors and re-open them with /dev/null, what happens when I subsequently call a C program? Will it's input/output (assuming it's using stdin/stdout/stderr) go to/from /dev/null? Won't that break things?
You shouldn't close descriptors 0 - 2; those might be active for something completely unrelated. You should only close the three descriptors that you opened yourself, if any.
If I close them later, won't that cause an earlier C program in my call stack to fail when control gets back to it? What about subsequently called programs?
Nothing would be using those descriptors, since your RPG program is the only thing that has access to them.
I suppose I should test this for myself (and see what happens) rather than asking you.
I only have empirical evidence myself; I have never seen anything bad happen as a result of doing this. I have seen it correct problems with QIBM_USE_DESCRIPTOR_IO and with Java I/O exceptions. I admit that I had pretty much the same questions as you are having when I first read about it in the system's documentation on using the invocation API.
If you assume that the RPG program is the only program that's run in the job, or you don't call anything else that works with descriptors 0-2, then I guess you'd have no problems. I'm just a little leery of doing this unconditionally...
I would be leery too, if it was explicitly closing the descriptors with the actual handle 0 1 and 2.
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