msgget(2) — Linux manual page


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

NAME         top

       msgget - get a System V message queue identifier

SYNOPSIS         top

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

       int msgget(key_t key, int msgflg);

DESCRIPTION         top

       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

       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

       · 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         top

       If successful, the return value will be the message queue identifier
       (a nonnegative integer), otherwise -1 with errno indicating the

ERRORS         top

       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

       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         top

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

NOTES         top

       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         top

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

SEE ALSO         top

       msgctl(2), msgrcv(2), msgsnd(2), ftok(3), capabilities(7),
       mq_overview(7), sysvipc(7)

COLOPHON         top

       This page is part of release 5.07 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

Linux                            2019-08-02                        MSGGET(2)

Pages that refer to this page: ipcrm(1)ipc(2)msgctl(2)msgop(2)msgrcv(2)msgsnd(2)syscalls(2)umask(2)ftok(3)mq_overview(7)svipc(7)sysvipc(7)