DragonFly On-Line Manual Pages
MSGCTL(2) DragonFly System Calls Manual MSGCTL(2)
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 determine the actions
of msgctl(). The data structure is defined in <sys/msg.h> and contains
(amongst others) the following members:
struct msqid_ds {
struct ipc_perm msg_perm; /* msg queue permission 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 msqid_ds structure is defined in
<sys/ipc.h> and looks like this:
struct ipc_perm {
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; /* 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 (MSGMNB
from <sys/msg.h>) 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 associated
with the queue can do this.
The permission to read from or write to a message queue (see msgsnd(2)
and msgrcv(2)) 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.
ENVIRONMENT
The XSI Interprocess Communication family of functions is also available
as an implementation in userspace. To use it, the sysvipcd(8) daemon has
to be running.
If the USR_SYSVIPC variable is set in a process' environment, the process
and its children will use the userspace implementation.
ERRORS
The msgctl() system call 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 structure 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
identifier.
cmd is not a valid command.
[EFAULT] The buf argument specifies an invalid address.
SEE ALSO
msgget(2), msgrcv(2), msgsnd(2)
HISTORY
Message queues appeared in the first release of AT&T System V UNIX.
AUTHORS
The DragonFly specific userspace implementation (see ENVIRONMENT) was
written by Larisa Grigore.
DragonFly 5.7-DEVELOPMENT September 13, 2019 DragonFly 5.7-DEVELOPMENT