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

NAME

       msgget - get a System V message queue identifier

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #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

       On success, msgget() returns the message queue identifier  (a  nonnegative  integer).   On
       failure, -1 is returned, and errno is set to indicate the error.

ERRORS

       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.

STANDARDS

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

NOTES

       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  Linux  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.