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man:msgget

MSGGET(2) Linux Programmer's Manual 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.  It may be  used  either
     to  obtain  the  identifier of a previously created message queue (when
     msgflg is zero and key does not have the value IPC_PRIVATE), or to cre-
     ate a new set.
     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  sig-
     nificant 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 mes-
     sage 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_over-
     view(7), svipc(7)

COLOPHON

     This page is part of release 4.16 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/.

Linux 2018-04-30 MSGGET(2)

/data/webs/external/dokuwiki/data/pages/man/msgget.txt · Last modified: 2019/05/17 09:47 by 127.0.0.1

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