libc/sys/msgctl.2

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.Dd August 25, 1999
.Dt MSGCTL 2
.Os
.Sh NAME
.Nm msgctl
.Nd message control operations
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/msg.h
.Ft int
.Fn msgctl "int msqid" "int cmd" "struct msqid_ds *buf"
.Sh DESCRIPTION
The
.Fn msgctl
system call performs control operations on the message queue specified
by
.Fa msqid .
.Pp
Each message queue has a
.Sy msqid_ds
structure associated with it which contains the following members:
.Bd -literal
    struct ipc_perm msg_perm; /* msg queue permission bits */
    msgqnum_t msg_qnum;       /* # of msgs in the queue */
    msglen_t  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() */
    time_t    msg_rtime;      /* time of last msgrcv() */
    time_t    msg_ctime;      /* time of last msgctl() */
.Ed
.Pp
The
.Sy ipc_perm
structure used inside the
.Sy msgid_ds
structure is defined in
.In sys/ipc.h
and contains the following members:
.Bd -literal
    uid_t cuid;  /* creator user id */
    gid_t cgid;  /* creator group id */
    uid_t uid;   /* user id */
    gid_t gid;   /* group id */
    mode_t mode; /* permission (lower 9 bits) */
.Ed
.Pp
The operation to be performed by
.Fn msgctl
is specified in
.Fa cmd
and is one of:
.Bl -tag -width IPC_RMIDX
.It Dv IPC_STAT
Gather information about the message queue and place it in the
structure pointed to by
.Fa buf .
.It Dv IPC_SET
Set the value of the
.Va msg_perm.uid ,
.Va msg_perm.gid ,
.Va msg_perm.mode
and
.Va msg_qbytes
fields in the structure associated with
.Fa msqid .
The values are taken from the corresponding fields in the structure
pointed to by
.Fa buf .
This operation can only be executed by the super-user, or a process that
has an effective user id equal to either
.Va msg_perm.cuid
or
.Va msg_perm.uid
in the data structure associated with the message queue.
The value of
.Va msg_qbytes
can only be increased by the super-user.
Values for
.Va msg_qbytes
that exceed the system limit
.Pf ( Dv MSGMNB
from
.In sys/msg.h )
are silently truncated to that limit.
.It Dv IPC_RMID
Remove the message queue specified by
.Fa msqid
and destroy the data associated with it.
Only the super-user or a process with an effective uid equal to the
.Va msg_perm.cuid
or
.Va msg_perm.uid
values in the data structure associated with the queue can do this.
.El
.Pp
The permission to read from or write to a message queue (see
.Xr msgsnd 2
and
.Xr msgrcv 2 )
is determined by the
.Va msg_perm.mode
field in the same way as is done with files (see
.Xr chmod 2 ) ,
but the effective uid can match either the
.Va msg_perm.cuid
field or the
.Va msg_perm.uid
field, and the
effective gid can match either
.Va msg_perm.cgid
or
.Va msg_perm.gid .
.Sh RETURN VALUES
Upon successful completion, a value of 0 is returned.
Otherwise, \-1 is returned and the global variable
.Va errno
is set to indicate the error.
.Sh ERRORS
.Fn msgctl
will fail if:
.Bl -tag -width Er
.It Bq Er EPERM
.Fa cmd
is equal to
.Dv IPC_SET
or
.Dv IPC_RMID
and the caller is not the super-user, nor does
the effective uid match either the
.Va msg_perm.uid
or
.Va msg_perm.cuid
fields of the data structure associated with the message queue.
.Pp
An attempt was made to increase the value of
.Va msg_qbytes
through
.Dv IPC_SET ,
but the caller is not the super-user.
.It Bq Er EACCES
.Fa cmd
is
.Dv IPC_STAT
and the caller has no read permission for this message queue.
.It Bq Er EINVAL
.Fa msqid
is not a valid message queue identifier.
.Pp
.Fa cmd
is not a valid command.
.It Bq Er EFAULT
.Fa buf
specifies an invalid address.
.El
.Sh SEE ALSO
.Xr msgget 2 ,
.Xr msgrcv 2 ,
.Xr msgsnd 2
.Sh STANDARDS
The
.Nm
system call conforms to
.St -xsh5 .
.Sh HISTORY
Message queues appeared in the first release of
.At V .