Provided by: manpages-dev_3.54-1ubuntu1_all bug


       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 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.


       SVr4, POSIX.1-2001.


       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  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.


       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)


       This page is part of release 3.54 of the Linux man-pages project.  A  description  of  the
       project,     and    information    about    reporting    bugs,    can    be    found    at