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MSGGET(2)                  Linux Programmer's Manual                 MSGGET(2)



NAME
       msgget - get a message queue identifier

SYNOPSIS
       #include <sys/types.h>
       #include <sys/ipc.h>
       #include <sys/msg.h>

       int msgget(key_t key, int msgflg);

DESCRIPTION
       The msgget() system call returns the message queue identifier associated with the value of the key argument.  A
       new message queue is created if key has the value IPC_PRIVATE or key isn't IPC_PRIVATE, no message  queue  with
       the given key key exists, and IPC_CREAT is specified in msgflg.

       If msgflg specifies both IPC_CREAT and IPC_EXCL and a message queue already exists for key, then msgget() fails
       with errno set to EEXIST.  (This is analogous to the effect of the combination O_CREAT | O_EXCL for open(2).)

       Upon creation, the least significant bits of the argument msgflg define the permissions of the  message  queue.
       These  permission bits have the same format and semantics as the permissions specified for the mode argument of
       open(2).  (The execute permissions are not used.)

       If a new message queue is created, then its associated data structure msqid_ds (see msgctl(2))  is  initialized
       as follows:

              msg_perm.cuid and msg_perm.uid are set to the effective user ID of the calling process.

              msg_perm.cgid and msg_perm.gid are set to the effective group ID of the calling process.

              The least significant 9 bits of msg_perm.mode are set to the least significant 9 bits of msgflg.

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

              msg_ctime is set to the current time.

              msg_qbytes is set to the system limit MSGMNB.

       If  the  message  queue already exists the permissions are verified, and a check is made to see if it is marked
       for destruction.

RETURN VALUE
       If successful, the return value will be the message queue identifier (a  non-negative  integer),  otherwise  -1
       with errno indicating the error.

ERRORS
       On failure, errno is set to one of the following values:

       EACCES A  message  queue  exists for key, but the calling process does not have permission to access the queue,
              and does not have the CAP_IPC_OWNER capability.

       EEXIST A message queue exists for key and msgflg specified both IPC_CREAT and IPC_EXCL.

       ENOENT No message queue exists for key and msgflg did not specify IPC_CREAT.

       ENOMEM A message queue has to be created but the system does not have enough memory for the new data structure.

       ENOSPC A message queue has to be created but the system limit for the maximum number of message queues (MSGMNI)
              would be exceeded.

CONFORMING TO
       SVr4, POSIX.1-2001.

NOTES
       IPC_PRIVATE isn't a flag field but a key_t type.  If this special value  is  used  for  key,  the  system  call
       ignores everything but the least significant 9 bits of msgflg and creates a new message queue (on success).

       The following is a system limit on message queue resources affecting a msgget() call:

       MSGMNI System  wide  maximum  number  of message queues: policy dependent (on Linux, this limit can be read and
              modified via /proc/sys/kernel/msgmni).

   Linux Notes
       Until version 2.3.20 Linux would return EIDRM for a msgget() on a message queue scheduled for deletion.

BUGS
       The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more clearly show its function.

SEE ALSO
       msgctl(2), msgrcv(2), msgsnd(2), ftok(3), capabilities(7), mq_overview(7), svipc(7)

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2004-05-27                         MSGGET(2)