msgget(2) — Linux manual page


msgget(2)                  System Calls Manual                 msgget(2)

NAME         top

       msgget - get a System V message queue identifier

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #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 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         top

       On success, msgget() returns the message queue identifier (a
       nonnegative integer).  On failure, -1 is returned, and errno is
       set to indicate the error.

ERRORS         top

       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

STANDARDS         top


HISTORY         top

       POSIX.1-2001, SVr4.

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

NOTES         top

       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

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)

Linux man-pages (unreleased)     (date)                        msgget(2)

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