msgget(2)

NAME

   msgget - get a System V 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 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.

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 in the user namespace that governs  its
          IPC namespace.

   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.

CONFORMING TO

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

NOTES

   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.

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 4.09 of the Linux  man-pages  project.   A
   description  of  the project, information about reporting bugs, and the
   latest    version    of    this    page,    can     be     found     at
   https://www.kernel.org/doc/man-pages/.

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