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



NAME
       msgget - get a message queue identifier

SYNOPSIS
       #include <&lt;sys/types.h>&gt;
       #include <&lt;sys/ipc.h>&gt;
       #include <&lt;sys/msg.h>&gt;

       int msgget(key_t key, int msgflg);

DESCRIPTION
       The  msgget()  system call returns the message queue identifier associ-
       ated with the value of the key argument.  A new message queue  is  cre-
       ated if key has the value IPC_PRIVATE or key isn't IPC_PRIVATE, no mes-
       sage 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  sig-
       nificant 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  depen-
                  dent  (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  mes-
       sage 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_over-
       view(7), svipc(7)

COLOPHON
       This  page  is  part of release 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.



Linux                             2004-05-27                         MSGGET(2)