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       msgget - get a System V message queue identifier


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

       int msgget(key_t key, int msgflg);


       The  msgget() system call returns the System V 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.


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


       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 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.


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


       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

       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.


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


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


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