Ubuntu Manpage: msgget - get a System V message queue identifier

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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 create 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  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.

COLOPHON

       This  page  is  part  of  release  5.10  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/.