|
|
|
|
| |
|
|
Name
msgctl — message control operations
Synopsis
int
msgctl( |
int |
msqid, |
| |
int |
cmd, |
| |
struct msqid_ds
* |
buf); |
DESCRIPTION
msgctl() performs the
control operation specified by cmd on the message queue with
identifier msqid.
The msqid_ds
data structure is defined in <sys/msg.h> as
follows:
| struct |
msqid_ds { |
| |
struct ipc_perm |
|
msg_perm; |
/* Ownership and permissions */ |
| |
time_t |
|
msg_stime; |
/* Time of last msgsnd(2) */ |
| |
time_t |
|
msg_rtime; |
/* Time of last msgrcv(2) */ |
| |
time_t |
|
msg_ctime; |
/* Time of last change */ |
| |
unsigned long |
|
__msg_cbytes; |
/* Current number of bytes in queue (non-standard) */ |
| |
msgqnum_t |
|
msg_qnum; |
/* Current number of messages in queue */ |
| |
msglen_t |
|
msg_qbytes; |
/* Maximum number of bytes allowed in queue */ |
| |
pid_t |
|
msg_lspid; |
/* PID of last msgsnd(2) */ |
| |
pid_t |
|
msg_lrpid; |
/* PID of last msgrcv(2) */ |
| }; |
The ipc_perm
structure is defined in <sys/ipc.h> as follows (the
highlighted fields are settable using IPC_SET):
| struct |
ipc_perm { |
| |
key_t |
|
key; |
/* Key supplied to msgget(2) */ |
| |
uid_t |
|
uid; |
/* Effective UID of owner */ |
| |
gid_t |
|
gid; |
/* Effective GID of owner */ |
| |
uid_t |
|
cuid; |
/* Effective UID of creator */ |
| |
gid_t |
|
cgid; |
/* Effective GID of creator */ |
| |
unsigned short |
|
mode; |
/* Permissions */ |
| |
unsigned short |
|
seq; |
/* Sequence number */ |
| }; |
Valid values for cmd are:
IPC_STAT-
Copy information from the kernel data structure
associated with msqid into the msqid_ds structure
pointed to by buf. The caller must have
read permission on the message queue.
IPC_SET-
Write the values of some members of the msqid_ds structure
pointed to by buf to the kernel data
structure associated with this message queue, updating
also its msg_ctime member. The
following members of the structure are updated:
msg_qbytes,
msg_perm.uid,
msg_perm.gid,
and (the least significant 9 bits of) msg_perm.mode. The
effective UID of the calling process must match the
owner (msg_perm.uid) or
creator (msg_perm.cuid) of the
message queue, or the caller must be privileged.
Appropriate privilege (Linux: the CAP_IPC_RESOURCE capability) is
required to raise the msg_qbytes value beyond
the system parameter MSGMNB.
IPC_RMID-
Immediately remove the message queue, awakening all
waiting reader and writer processes (with an error
return and errno set to
EIDRM). The calling
process must have appropriate privileges or its
effective user ID must be either that of the creator or
owner of the message queue.
IPC_INFO (Linux specific)-
Returns information about system-wide message queue
limits and parameters in the structure pointed to by
buf. This
structure is of type msginfo (thus, a cast
is required), defined in <sys/msg.h> if
the _GNU_SOURCE feature test macro is defined:
| struct |
msginfo { |
|
|
int |
|
msgpool; |
/* Size in bytes of buffer pool used to hold message data; unused */ |
|
|
int |
|
msgmap; |
/* Max. # of entries in message map; unused */ |
|
|
int |
|
msgmax; |
/* Max. # of bytes that can be written in a single message */ |
|
|
int |
|
msgmnb; |
/* Max. # of bytes that can be written to queue; used to initialize msg_qbytes during queue creation (msgget(2)) */ |
|
|
int |
|
msgmni; |
/* Max. # of message queues */ |
|
|
int |
|
msgssz; |
/* Message segment size; unused */ |
|
|
int |
|
msgtql; |
/* Max. # of messages on all queues in system; unused */ |
|
|
unsigned short int |
|
msgseg; |
/* Max. # of segments; unused */ |
| }; |
The msgmni, msgmax, and msgmnb settings can be
changed via /proc files
of the same name; see proc(5) for
details.
MSG_INFO (Linux specific)-
Returns a msginfo structure
containing the same information as for IPC_INFO, except that the following
fields are returned with information about system
resources consumed by message queues: the msgpool field returns
the number of message queues that currently exist on
the system; the msgmap field returns
the total number of messages in all queues on the
system; and the msgtql field returns
the total number of bytes in all messages in all queues
on the system.
MSG_STAT (Linux specific)-
Returns a msqid_ds structure as
for IPC_STAT. However,
the msqid
argument is not a queue identifier, but instead an
index into the kernel's internal array that maintains
information about all message queues on the system.
RETURN VALUE
On success, IPC_STAT,
IPC_SET, and IPC_RMID return 0. A successful
IPC_INFO or MSG_INFO operation returns the index of the
highest used entry in the kernel's internal array recording
information about all message queues. (This information can
be used with repeated MSG_STAT
operations to obtain information about all queues on the
system.) A successful MSG_STAT
operation returns the identifier of the queue whose index was
given in msqid.
On error, −1 is returned with errno indicating the error.
ERRORS
On failure, errno is set to
one of the following:
- EACCES
-
The argument cmd is equal to
IPC_STAT or MSG_STAT, but the calling process
does not have read permission on the message queue
msqid, and does
not have the CAP_IPC_OWNER capability.
- EFAULT
-
The argument cmd has the value
IPC_SET or IPC_STAT, but the address pointed to
by buf isn't
accessible.
- EIDRM
-
The message queue was removed.
- EINVAL
-
Invalid value for cmd or msqid. Or: for a
MSG_STAT operation, the
index value specified in msqid referred to an
array slot that is currently unused.
- EPERM
-
The argument cmd has the value
IPC_SET or IPC_RMID, but the effective user ID
of the calling process is not the creator (as found in
msg_perm.cuid) or the
owner (as found in msg_perm.uid) of the
message queue, and the process is not privileged
(Linux: it does not have the CAP_SYS_ADMIN capability).
CONFORMING TO
SVr4, POSIX.1-2001.
NOTES
The IPC_INFO, MSG_STAT and MSG_INFO operations are used by the
ipcs(8) program to provide
information on allocated resources. In the future these may
modified or moved to a /proc file system interface.
Various fields in the struct
msqid_ds were shorts under Linux 2.2 and have
become longs under Linux 2.4. To take advantage of this, a
recompilation under glibc-2.1.91 or later should suffice.
(The kernel distinguishes old and new calls by an IPC_64 flag
in cmd.)
SEE ALSO
msgget(2), msgrcv(2), msgsnd(2), capabilities(7), mq_overview(7), svipc(7)
Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
and Copyright 2004, 2005 Michael Kerrisk <mtk-manpages@gmx.net>
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.
Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date. The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein. The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working
professionally.
Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.
Modified Tue Oct 22 08:11:14 EDT 1996 by Eric S. Raymond <esr@thyrsus.com>
Modified Sun Feb 18 01:59:29 2001 by Andries E. Brouwer <aeb@cwi.nl>
Modified, 27 May 2004, Michael Kerrisk <mtk-manpages@gmx.net>
Added notes on CAP_IPC_OWNER requirement
Modified, 17 Jun 2004, Michael Kerrisk <mtk-manpages@gmx.net>
Added notes on CAP_SYS_ADMIN requirement for IPC_SET and IPC_RMID
Modified, 11 Nov 2004, Michael Kerrisk <mtk-manpages@gmx.net>
Language and formatting clean-ups
Added msqid_ds and ipc_perm structure definitions
2005-08-02, mtk: Added IPC_INFO, MSG_INFO, MSG_STAT descriptions
|
|
|
|
|
|
|
|