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.1-2008.

HISTORY

       POSIX.1-2001, SVr4.

   Linux
       Until Linux 2.3.20, Linux would return EIDRM for a msgget() on a message queue scheduled for deletion.

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.

BUGS

       The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more clearly show its function.