msgget - get a System V message queue identifier
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int msgget(key_t key, int msgflg);
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.
If successful, the return value will be the message queue identifier (a
nonnegative integer), otherwise -1 with
errno indicating the error.
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.
POSIX.1-2001, POSIX.1-2008, SVr4.
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.
Until version 2.3.20, Linux would return
EIDRM for a
msgget() on a
message queue scheduled for deletion.
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),
sysvipc(7)