I am posting this because my searches for this problem only came up with two posts and no helpful suggestions. I have a "solution" (read work-around hack) and have not tried yet to find a root cause, and may never because I am busy doing other things (read working to pay the bills).
However, I post this with two goals:
1. For the poor shmuck at 3am
2. document in case someone really has a wild hair (hare?) up their butt
Simply put, msgget(2) will return 0 for some reason, which the msgsnd(2) and msgrcv(2) do not like. My notes indicate msgsnd() was OK, and msgrcv() complained, but this was 12 hours into a debugging session....
There are two threads I have found in the interwebs:
forums.codeguru (dot) com/showthread.php?403036-strange-problem-in-using-msgget%28%29-in-Linux
and
unix (dot) com/programming/3755-about-msgget-troble.html
Both of these threads are "old" and closed, otherwise I would have responded to one of them.
NOTE: The codeguru.com has the best code example. The unix.com code has what may be a fatal flaw: it uses IPC_EXCL as part of the permissions - so the second time it is run it should complain, unless he first removed the message queue. However, he should have gotten errno == EEXIST and it appears he did not - he does print errno.
The Linux distro is Ubunto 8, not patched. Because the other posts are from 2006 and 2005, the CPU does not seem to be an issue.
The interesting thing is:
Running ipcs gives (in addition to various semaphores and shared memory):
The original key was 0xF0 which returned 0x8000 when it was working. The hex for the decimal 163840 = 0x28000. I arbitrarily tried a key of 0x7B (well, decimal 123) and got a msgqid = 0x8001 (which == 32769 decimal).
I also see cases in my slime trail that when msgget() was returning non-zero, for a while it returned 0x10001. In all cases I am using an int to hold the msgQ_id. The key = 0xF0 returns 0, not 0x8000, so truncation is not an issue. I have not tried switching back to a key = 0xF0. I will try looking on another system running the same code (ie using 0xF0) to see what ipcs shows.
Another thing: 0 is supposed to be a legal return:
Quote:
Upon successful completion, msgget() returns a non-negative integer, namely a message queue identifier. Otherwise, it returns -1 and errno is set to indicate the error.
So - I don't know why msgget() will start returning 0. Honestly, I had another bug which (for a while) masked what msgsnd() was doing - a "(u)" instead of a "(%lu")" printf was throwing SIGSEGV (sigh) and I fixed both at the same time (ie new key) - this is a non-trivial system to run a code build on && one wants to do as much as one can between runs.
The only suggestion I can make is have the system come up with a unique key using ftok() every time, and remove old message queues. A good start on a key would be the parent process PID.
(please forgive the chopped links - apparently I am not yet blessed to give raw links yet :^)
I would gently point out I provided a link to a post WITH the code Here it is. The only two changes from the codeguru example and mine is I changed the key. (Obiously, one should ALSO check for < 0 ... and I added that)
Also, I was able to look on another development machine. It uses the same key as above (0xF0). ipcs reports the queue Id = 0x8000
Also - please note I am mainly posting this so that some poor programmer in the future with this problem can find this post.
The only folks who could provide any real answers, no offense, are the maintainers of msgget() and family. A link or contact point with them would be most appreciated.
msgid is NOT a message it is a message queue id. (A shared (IPC) memory object, not an individual message) A return of msgid ==0 means success. Any number >-1 == success.
Since you really did not post much code --
You should be calling msgrcv like this (note infinite loop is NOT required):
The above snippet works correctly, I use it in other code....
Last edited by jim mcnamara; 06-24-2014 at 11:25 PM..
This User Gave Thanks to jim mcnamara For This Post:
To be honest, I did not look at errno after msgget() returned 0 (It was in the wee early hours and I had bigger fish..). I will try that and get back with the results. This will also tell me if the original key returns 0.
The interesting thing is we loaded a *way* earlier version of the code to test a completely different thing. I jut did a ipcs and got
where you can see msgqid == 0 and the system seems to be performing happily (at least this part of it). This would seem to enforce the observation / theory that once the msgqid becomes 0, it stays 0. No complaints from msgsnd() or msgrcv().
So - this may be a brainfart on my part. After all, I did have another bug throwing a SIGSEGV at the same time. And it was very late/early. If it is a brainfart, my apologies.
Again, this an unpatched Ubuntu 8
@achenlehas an interesting suggestion. I had not thought of setting errno to a value before making the call. The question then becomes: why not set it to (-1)? errno values are positive, at least on Linux. (I seem to remember them being negative numbers on BSD 4.1, but that was a *long* time ago...)
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