msgctl — System V message control operations
#include <sys/ipc.h> #include <sys/msg.h>
int
msgctl( |
int msqid, |
| int cmd, | |
struct msqid_ds *buf); |
msgctl() performs the
control operation specified by cmd on the System V 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;time_t msg_stime;time_t msg_rtime;time_t msg_ctime;
modification by msgctl() */unsigned long msg_cbytes;msgqnum_t msg_qnum;msglen_t msg_qbytes;pid_t msg_lspid;pid_t msg_lrpid;};
The fields of the msgid_ds structure are as
follows:
msg_permThis is an ipc_perm structure (see
below) that specifies the access permissions on the
message queue.
msg_stimeTime of the last msgsnd(2) system call.
msg_rtimeTime of the last msgrcv(2) system call.
msg_ctimeTime of creation of queue or time of last
msgctl() IPC_SET operation.
msg_cbytesNumber of bytes in all messages currently on the message queue. This is a nonstandard Linux extension that is not specified in POSIX.
msg_qnumNumber of messages currently on the message queue.
msg_qbytesMaximum number of bytes of message text allowed on the message queue.
msg_lspidID of the process that performed the last msgsnd(2) system call.
msg_lrpidID of the process that performed the last msgrcv(2) system call.
The ipc_perm
structure is defined as follows (the highlighted fields are
settable using IPC_SET):
struct ipc_perm { key_t __key;uid_t uid;gid_t gid;uid_t cuid;gid_t cgid;unsigned short mode;unsigned short __seq;};
The least significant 9 bits of the mode field of the ipc_perm structure define the
access permissions for the message queue. The permission bits
are as follows:
| 0400 | Read by user |
| 0200 | Write by user |
| 0040 | Read by group |
| 0020 | Write by group |
| 0004 | Read by others |
| 0002 | Write by others |
Bits 0100, 0010, and 0001 (the execute bits) are unused by the system.
Valid values for cmd are:
IPC_STATCopy 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_SETWrite 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_SYS_RESOURCE capability) is
required to raise the msg_qbytes value beyond
the system parameter MSGMNB.
IPC_RMIDImmediately 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. The third argument to
msgctl() is ignored in
this case.
IPC_INFO (Linux-specific)Return 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;
used to hold message data;
unused within kernel */int msgmap;
map; unused within kernel */int msgmax;
written in a single message */int msgmnb;
written to queue; used to initialize
msg_qbytes during queue creation
(msgget(2)) */int msgmni;int msgssz;
unused within kernel */int msgtql;
in system; unused within kernel */unsigned short msgseg;
unused within kernel */};
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)Return 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)Return 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.
MSG_STAT_ANY (Linux-specific, since
Linux 4.17)Return a msqid_ds
structure as for MSG_STAT. However, msg_perm.mode is not
checked for read access for msqid meaning that any
user can employ this operation (just as any user may
read /proc/sysvipc/msg to
obtain the same information).
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
or MSG_STAT_ANY operations to
obtain information about all queues on the system.) A
successful MSG_STAT or
MSG_STAT_ANY operation returns
the identifier of the queue whose index was given in
msqid.
On failure, −1 is returned and errno is set to indicate the error.
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 in the user
namespace that governs its IPC namespace.
The argument cmd has the value
IPC_SET or IPC_STAT, but the address pointed to
by buf isn't
accessible.
The message queue was removed.
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.
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 caller is not privileged (Linux:
does not have the CAP_SYS_ADMIN capability).
An attempt (IPC_SET)
was made to increase msg_qbytes beyond the
system parameter MSGMNB,
but the caller is not privileged (Linux: does not have
the CAP_SYS_RESOURCE
capability).
The inclusion of <sys/ipc.h>
isn't required on Linux or by any version of POSIX. However,
some old implementations required the inclusion of this
header file, and the SVID also documented its inclusion.
Applications intended to be portable to such old systems may
need to include this header file.
The IPC_INFO, MSG_STAT, and MSG_INFO operations are used by the
ipcs(1) program to provide
information on allocated resources. In the future these may
modified or moved to a /proc
filesystem interface.
Various fields in the struct
msqid_ds were typed as short under Linux 2.2 and have become
long 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.)
This page is part of release 5.11 of the Linux man-pages project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
|
Copyright 1993 Giorgio Ciucci (giorgiocrcc.it) and Copyright 2004, 2005 Michael Kerrisk <mtk.manpagesgmail.com> %%%LICENSE_START(VERBATIM) 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. %%%LICENSE_END Modified Tue Oct 22 08:11:14 EDT 1996 by Eric S. Raymond <esrthyrsus.com> Modified Sun Feb 18 01:59:29 2001 by Andries E. Brouwer <aebcwi.nl> Modified, 27 May 2004, Michael Kerrisk <mtk.manpagesgmail.com> Added notes on CAP_IPC_OWNER requirement Modified, 17 Jun 2004, Michael Kerrisk <mtk.manpagesgmail.com> Added notes on CAP_SYS_ADMIN requirement for IPC_SET and IPC_RMID Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpagesgmail.com> 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 2018-03-20, dbueso: Added MSG_STAT_ANY description. |