man-pages/man2/msgctl.2

.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
.\" and Copyright 2004, 2005 Michael Kerrisk <mtk-manpages@gmx.net>
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.\" 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
.\"
.TH MSGCTL 2 2004-11-10 "Linux" "Linux Programmer's Manual"
.SH NAME
msgctl \- message control operations
.SH SYNOPSIS
.nf
.B
#include <sys/types.h>
.B
#include <sys/ipc.h>
.B
#include <sys/msg.h>
.fi
.sp
.BI "int msgctl(int " msqid ,
.BI "int " cmd ,
.BI "struct msqid_ds *" buf );
.SH DESCRIPTION
.BR msgctl ()
performs the control operation specified by
.I cmd
on the message queue with identifier
.IR msqid .
.PP
The
.I msqid_ds
data structure is defined in <sys/msg.h> as follows:
.nf
.in +4n

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) */
};
.in -4n
.fi
.PP
The
.I ipc_perm
structure is defined in <sys/ipc.h> as follows
(the highlighted fields are settable using
.BR IPC_SET ):
.PP
.nf
.in +4n
struct ipc_perm {
    key_t key;            /* Key supplied to msgget(2) */
    uid_t \fBuid\fP;            /* Effective UID of owner */
    gid_t \fBgid\fP;            /* Effective GID of owner */
    uid_t cuid;           /* Effective UID of creator */
    gid_t cgid;           /* Effective GID of creator */
    unsigned short \fBmode\fP;  /* Permissions */
    unsigned short seq;   /* Sequence number */
};
.in -4n
.fi
.PP
Valid values for
.I cmd
are:
.TP
.B IPC_STAT
Copy information from the kernel data structure associated with
.I msqid
into the
.I msqid_ds
structure pointed to by
.IR buf .
The caller must have read permission on the message queue.
.TP
.B IPC_SET
Write the values of some members of the
.I msqid_ds
structure pointed to by
.I buf
to the kernel data structure associated with this message queue,
updating also its
.I msg_ctime
member.
The following members of the structure are updated:
.IR msg_qbytes ,
.IR msg_perm.uid ,
.IR msg_perm.gid ,
and (the least significant 9 bits of)
.IR msg_perm.mode .
The effective UID of the calling process must match the owner
.RI ( msg_perm.uid )
or creator
.RI ( msg_perm.cuid )
of the message queue, or the caller must be privileged.
Appropriate privilege (Linux: the
.B CAP_IPC_RESOURCE
capability) is required to raise the
.I msg_qbytes
value beyond the system parameter
.BR MSGMNB .
.TP
.B IPC_RMID
Immediately remove the message queue,
awakening all waiting reader and writer processes (with an error
return and
.I errno
set to
.BR 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.
.TP
.BR IPC_INFO " (Linux specific)"
Returns information about system-wide message queue limits and
parameters in the structure pointed to by
.IR buf .
This structure is of type
.IR msginfo
(thus, a cast is required),
defined in
.I <sys/msg.h>
if the _GNU_SOURCE feature test macro is defined:
.nf
.in +2n

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 */
};

.in -2n
.fi
The
.IR msgmni ,
.IR msgmax ,
and
.I msgmnb
settings can be changed via
.I /proc
files of the same name; see
.BR proc (5)
for details.
.TP
.BR MSG_INFO " (Linux specific)"
Returns a
.I msginfo
structure containing the same information as for
.BR IPC_INFO ,
except that the following fields are returned with information
about system resources consumed by message queues: the
.I msgpool
field returns the number of message queues that currently exist
on the system; the
.I msgmap
field returns the total number of messages in all queues
on the system; and the
.I msgtql
field returns the total number of bytes in all messages
in all queues on the system.
.TP
.BR MSG_STAT " (Linux specific)"
Returns a
.I msqid_ds
structure as for
.BR IPC_STAT .
However, the
.I 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.
.SH "RETURN VALUE"
On success,
.BR IPC_STAT ,
.BR IPC_SET ,
and
.BR IPC_RMID
return 0.
A successful
.B IPC_INFO
or
.B 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
.B MSG_STAT
operations to obtain information about all queues on the system.)
A successful
.B MSG_STAT
operation returns the identifier of the queue whose index was given in
.IR msqid .

On error, \-1 is returned with
.I errno
indicating the error.
.SH ERRORS
On failure,
.I errno
is set to one of the following:
.TP 11
.B EACCES
The argument
.I cmd
is equal to
.BR IPC_STAT
or
.BR MSG_STAT ,
but the calling process does not have read permission on the message queue
.IR msqid ,
and does not have the
.B CAP_IPC_OWNER
capability.
.TP
.B EFAULT
The argument
.I cmd
has the value
.B IPC_SET
or
.BR IPC_STAT ,
but the address pointed to by
.I buf
isn't accessible.
.TP
.B EIDRM
The message queue was removed.
.TP
.B EINVAL
Invalid value for
.I cmd
or
.IR msqid .
Or: for a
.B MSG_STAT
operation, the index value specified in
.I msqid
referred to an array slot that is currently unused.
.TP
.B EPERM
The argument
.I cmd
has the value
.B IPC_SET
or
.BR IPC_RMID ,
but the effective user ID of the calling process is not the creator
(as found in
.IR msg_perm.cuid )
or the owner
(as found in
.IR msg_perm.uid )
of the message queue,
and the process is not privileged (Linux: it does not have the
.B CAP_SYS_ADMIN
capability).
.SH "CONFORMING TO"
SVr4, POSIX.1-2001.
.\" SVID does not document the EIDRM error condition.
.SH NOTES
The
.BR IPC_INFO ,
.BR MSG_STAT
and
.B MSG_INFO
operations are used by the
.BR 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 \fIstruct msqid_ds\fP 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
.IR cmd .)
.SH "SEE ALSO"
.BR msgget (2),
.BR msgrcv (2),
.BR msgsnd (2),
.BR capabilities (7),
.BR mq_overview (7),
.BR svipc (7)