bionic (2) msgget.2.gz

Provided by: manpages-dev_4.15-1_all bug

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 4.15 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/.