msgget

MSGGET(2)                   Linux Programmer's Manual                  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 previously 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 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
       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.  Applications 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 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), svipc(7)

COLOPHON
       This page is part of release 4.16 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/.



Linux                              2018-04-30                          MSGGET(2)