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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 initialised 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.

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).

BUGS

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

CONFORMING TO

       SVr4, SVID.  Until version  2.3.20  Linux  would  return  EIDRM  for  a
       msgget() on a message queue scheduled for deletion.

SEE ALSO

       msgctl(2), msgrcv(2), msgsnd(2), ftok(3), ipc(5), capabilities(7)