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NAME

       msgget - get a 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 message queue identifier associated with the value of
       the key argument.  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.

       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 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 maximum number of message queues:  policy  dependent  (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 3.35 of the Linux man-pages project.  A description of the
       project, and information about reporting bugs, can be found at  http://man7.org/linux/man-
       pages/.