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NAME
msgget - get a 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 message queue identifier associ‐
ated with the value of the key argument. A new message queue is cre‐
ated if key has the value IPC_PRIVATE or key isn’t IPC_PRIVATE, no mes‐
sage 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.
If successful, the return value will be the message queue identifier (a
non-negative 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.
EEXIST A message queue exists for key and msgflg specified both
IPC_CREAT and IPC_EXCL.
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.
SVr4, POSIX.1-2001.
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 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 maximum number of message queues: policy depen‐
dent (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.
msgctl(2), msgrcv(2), msgsnd(2), ftok(3), capabilities(7),
mq_overview(7), svipc(7)