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Full Discussion: Msgget(2) returns 0 - a workaround fix
Top Forums Programming Msgget(2) returns 0 - a workaround fix Post 302906955 by mr_bandit on Tuesday 24th of June 2014 07:15:20 PM
Old 06-24-2014
errno after msgget() returns 0
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

Code:
------ Message Queues --------
key        msqid      owner      perms      used-bytes   messages
0x000000f0 0          gfi        666        0            0

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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LEARN ABOUT OPENSOLARIS

msgget

msgget(2)							   System Calls 							 msgget(2)

NAME

       msgget - get message queue

SYNOPSIS

       #include <sys/msg.h>

       int msgget(key_t key, int msgflg);

DESCRIPTION

       The msgget() argument returns the message queue identifier associated with key.

       A  message  queue identifier and associated message queue and data structure (see Intro(2)) are created for key if one of the following are
       true:

	   o	  key is IPC_PRIVATE.

	   o	  key does not already have a message queue identifier associated with it, and (msgflg&IPC_CREAT) is true.

       On creation, the data structure associated with the new message queue identifier is initialized as follows:

	   o	  msg_perm.cuid, msg_perm.uid, msg_perm.cgid, and msg_perm.gid are set to the effective user ID and effective  group  ID,  respec-
		  tively, of the calling process.

	   o	  The low-order 9 bits of msg_perm.mode are set to the low-order 9 bits of msgflg.

	   o	  msg_qnum, msg_lspid, msg_lrpid, msg_stime, and msg_rtime are set to 0.

	   o	  msg_ctime is set to the current time.

	   o	  msg_qbytes is set to the system limit. See NOTES.

RETURN VALUES

       Upon successful completion, a non-negative integer representing a message queue identifier is returned. Otherwise, -1 is returned and errno
       is set to indicate the error.

ERRORS

       The msgget() function will fail if:

       EACCES	 A message queue identifier exists for key, but operation permission (see Intro(2)) as specified by the low-order 9 bits of msgflg
		 would not be granted.

       EEXIST	 A message queue identifier exists for key but (msgflg&IPC_CREAT) and (msgflg&IPC_EXCL) are both true.

       ENOENT	 A message queue identifier does not exist for key and (msgflg&IPC_CREAT) is false.

       ENOSPC	 A  message  queue identifier is to be created but the system-imposed limit on the maximum number of allowed message queue identi-
		 fiers system wide would be exceeded. See NOTES.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Interface Stability	     |Standard			   |
       +-----------------------------+-----------------------------+

SEE ALSO

       rctladm(1M), Intro(2), msgctl(2), msgrcv(2), msgsnd(2), setrctl(2), ftok(3C), attributes(5), standards(5)

NOTES

       The system-defined limit used to initialize msg_qbytes is the minimum  enforced	value  of  the	calling  process's  process.max-msg-qbytes
       resource control.

       The  system-imposed  limit  on  the  number of message queue identifiers is maintained on a per-project basis using the project.max-msg-ids
       resource control. The zone.max-msg-ids resource control restricts the total amount of message queue identifiers that can be allocated by  a
       zone.

       See rctladm(1M) and setrctl(2) for information about using resource controls.

SunOS 5.11							    14 Aug 2006 							 msgget(2)
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