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msgget() - Unix, Linux System Call
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NAME
msgget - get a message queue identifier
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
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int msgget(key_t key, int msgflg); DESCRIPTION
The
msgget() system call returns the message queue identifier associated
with the value of the
key argument.
A new message queue is created if
key has the value
IPC_PRIVATE or
key isnt
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 initialised as follows:
Tag | Description |
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msg_perm.cuid and
msg_perm.uid are set to the effective user ID of the calling process.
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msg_perm.cgid and
msg_perm.gid are set to the effective group ID of the calling process.
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The least significant 9 bits of
msg_perm.mode are set to the least significant 9 bits of
msgflg. |
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msg_qnum, msg_lspid, msg_lrpid, msg_stime and
msg_rtime are set to 0.
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msg_ctime is set to the current time.
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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
If successful, the return value will be the message queue identifier (a
nonnegative integer), otherwise -1
with
errno indicating the error.
ERRORS
On failure,
errno is set to one of the following values:
Tag | Description |
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.
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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.
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ENOSPC |
A message queue has to be created but the system limit for the maximum
number of message queues
(MSGMNI) would be exceeded.
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NOTES
IPC_PRIVATE isnt 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:
Tag | Description |
MSGMNI |
System wide maximum number of message queues: policy
dependent
(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.
CONFORMING TO
SVr4, POSIX.1-2001.
LINUX NOTES
Until version 2.3.20 Linux would return EIDRM for a
msgget() on a message queue scheduled for deletion.
SEE ALSO
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