MSGGET(2)                 (2019-08-02)                  MSGGET(2)

     NAME
          msgget - get a System V 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 System V message queue
          identifier associated with the value of the key argument.
          It may be used either to obtain the identifier of a previ-
          ously created message queue (when msgflg is zero and key
          does not have the value IPC_PRIVATE), or to create a new
          set.

          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 mes-
          sage 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 fol-
          lows:

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

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

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

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

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          +o msg_ctime is set to the current time.

          +o msg_qbytes is set to the system limit MSGMNB.

          If the message queue already exists the permissions are ver-
          ified, 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 nonnegative 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 in the user
               namespace that governs its IPC namespace.

          EEXIST
               IPC_CREAT and IPC_EXCL were specified in msgflg, but a
               message queue already exists for key.

          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
          POSIX.1-2001, POSIX.1-2008, SVr4.

     NOTES
          The inclusion of <sys/types.h> and <sys/ipc.h> isn't
          required on Linux or by any version of POSIX.  However, some
          old implementations required the inclusion of these header
          files, and the SVID also documented their inclusion.  Appli-
          cations intended to be portable to such old systems may need
          to include these header files.

          IPC_PRIVATE isn't a flag field but a key_t type.  If this
          special value is used for key, the system call ignores

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          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 limit on the number of message queues.
               Before Linux 3.19, the default value for this limit was
               calculated using a formula based on available system
               memory.  Since Linux 3.19, the default value is 32,000.
               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), sysvipc(7)

     COLOPHON
          This page is part of release 5.10 of the Linux man-pages
          project.  A description of the project, information about
          reporting bugs, and the latest version of this page, can be
          found at https://www.kernel.org/doc/man-pages/.

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