Programming

All of the code in the links this thread have pointed to wrote error messages if msgget() returned a value <= 0 even though an error is indicated only if the return value is strictly less than 0.

Nothing was shown indicating that there was any error from msgrcv() or msgsnd() in cases where msgget() returned 0.

Unless the man page explicitly states otherwise, the value of errno after a call to a function that completes successfully is meaningless. On function calls where the value returned to indicate an error can also be returned in a successful completion case, you'll usually see something like:
There is no statement like this on the msgget() page because the value returned when msgget() fails (-1) is never returned if msgget() succeeds.

Less forcefully than Don steated it, but I think is idea of success return code is the entire problem here, plus using msgid as a message and NOT as message queue id. Which it is. They are not the same thing. Period.

I do not understand why you make this statement, I have consistently used msgget() to get the ID of the queue, never as a message itself. the msgq_id is a parameter to msgsnd() and msgrcv() to specify the queue for the message.

Now, my confusion was over a msgq_id == 0 as a valid number. It is, in both practice and theory.

@Don is absolutely correct stating

Again, my confusion was that what seemed to be a successful return from msgget() (according to the man page), but not having success. I actually had two bugs that were subtle and misleading, but that is the life of a programmer. (Especially when dealing with *really* bad code.) My searches for information found scant help. The collected wisdom of the community have helped me many times (Thanks, Google!) in finding the obscure information on forums. (If one programs Atmel chips, I recommend avrfreaks.net.)

AND - my purpose in this post was to try to help provide some more information to the next person who had this issue at 3am. The helpful replies have added to that goal, and I thank the posters for their time and attention.

I am not trying to flame, but *please* point out *one* place I used msgq_id as a message. You keep saying the same thing but will not give one example of where I did so.

In the meantime, I am going to see my family for a week, so I will check for a reply when I get back.