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 kibibytes of buffer pool
used to hold message data;
unused within kernel */
int msgmap; /* Maximum number of entries in message
map; unused within kernel */
int msgmax; /* Maximum number of bytes that can be
written in a single message */
int msgmnb; /* Maximum number of bytes that can be
written to queue; used to initialize
msg_qbytes during queue creation
(msgget(2)) */
int msgmni; /* Maximum number of message queues */
int msgssz; /* Message segment size;
unused within kernel */
int msgtql; /* Maximum number of messages on all queues
in system; unused within kernel */
unsigned short int msgseg;
/* Maximum number of segments;
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)
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
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 3.14 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
