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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

       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

              msg_perm.cgid  and  msg_perm.gid  are  set to the effective group ID of the calling

              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 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.


       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 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.


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


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


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