msgctl(3)

NAME

msgctl - message control operations

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int
msgctl(int msqid, int cmd, struct msqid_ds *buf);

DESCRIPTION

The msgctl() system call performs some control operations on

the message
queue specified by msqid.

Each message queue has a data structure associated with it,

parts of
which may be altered by msgctl() and parts of which deter

mine the actions
of msgctl(). The data structure is defined in #include

<sys/msg.h>
and contains (amongst others) the following members:

struct msqid_ds {

struct ipc_perm msg_perm; /* msg queue permis

sion bits */
struct msg *msg_first; /* first message in the

queue */
struct msg *msg_last; /* last message in the queue

*/
u_long msg_cbytes; /* number of bytes in use on

the queue */
u_long msg_qnum; /* number of msgs in the

queue */
u_long msg_qbytes; /* max # of bytes on the

queue */
pid_t msg_lspid; /* pid of last msgsnd() */
pid_t msg_lrpid; /* pid of last msgrcv() */
time_t msg_stime; /* time of last msgsnd() */
long msg_pad1;
time_t msg_rtime; /* time of last msgrcv() */
long msg_pad2;
time_t msg_ctime; /* time of last msgctl() */
long msg_pad3;
long msg_pad4[4];

};

The ipc_perm structure used inside the shmid_ds structure is

defined in
#include <sys/ipc.h>
and looks like this:

struct ipc_perm {

ushort cuid; /* creator user id */
ushort cgid; /* creator group id */
ushort uid; /* user id */
ushort gid; /* group id */
ushort mode; /* r/w permission */
ushort seq; /* sequence # (to generate unique

msg/sem/shm id) */
key_t key; /* user specified msg/sem/shm key */

};

The operation to be performed by msgctl() is specified in

cmd and is one
of:

IPC_STAT Gather information about the message queue and

place it in the

structure pointed to by buf.

IPC_SET Set the value of the msg_perm.uid, msg_perm.gid,

msg_perm.mode

and msg_qbytes fields in the structure associated

with msqid.
The values are taken from the corresponding

fields in the
structure pointed to by buf. This operation can

only be executed by the super-user, or a process that has an

effective
user id equal to either msg_perm.cuid or

msg_perm.uid in the
data structure associated with the message queue.

The value
of msg_qbytes can only be increased by the super

user. Values
for msg_qbytes that exceed the system limit (MS

GMNB from are
silently truncated to that limit.

IPC_RMID Remove the message queue specified by msqid and

destroy the

data associated with it. Only the super-user or

a process
with an effective uid equal to the msg_perm.cuid

or
msg_perm.uid values in the data structure associ

ated with the
queue can do this.

The permission to read from or write to a message queue (see

msgsnd(3)
and msgrcv(3)) is determined by the msg_perm.mode field in

the same way
as is done with files (see chmod(2)), but the effective uid

can match
either the msg_perm.cuid field or the msg_perm.uid field,

and the effective gid can match either msg_perm.cgid or msg_perm.gid.

RETURN VALUES

The msgctl() function returns the value 0 if successful;

otherwise the
value -1 is returned and the global variable errno is set to

indicate the
error.

ERRORS

The msgctl() function will fail if:

[EPERM] The cmd argument is equal to IPC_SET or

IPC_RMID and

the caller is not the super-user, nor

does the effective uid match either the msg_perm.uid or msg_perm.cuid fields of the data struc

ture associated
with the message queue.

An attempt is made to increase the value

of msg_qbytes
through IPC_SET but the caller is not the

super-user.

[EACCES] The command is IPC_STAT and the caller

has no read

permission for this message queue.

[EINVAL] The msqid argument is not a valid message

queue iden

tifier.

cmd is not a valid command.

[EFAULT] The buf argument specifies an invalid ad

dress.

SEE ALSO

msgget(3), msgrcv(3), msgsnd(3)

HISTORY

Message queues appeared in the first release of AT&T System

V UNIX.

BSD November 24, 1997

BSD