msgget(2) System Calls Manual msgget(2)
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 al-
ready 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 ar-
gument 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 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.
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_overview(7), sysvipc(7)
Linux man-pages 6.9.1 2024-05-02 msgget(2)
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