To send or receive a message, the calling process allocates a structure
that looks like the following
struct msgbuf {
long mtype;
/* message type, must be > 0 */
char mtext[1];
/* message data */
};
but with an array
mtext
of size
msgsz ,
a non-negative integer value.
The structure member
mtype
must have a strictly positive integer value that can be
used by the receiving process for message selection
(see the section about
msgrcv ).
The calling process must have write access permissions to send
and read access permissions to receive a message on the queue.
The
msgsnd
system call enqueue a copy of the message pointed to by the
msgp
argument
on the message queue whose identifier is specified by the value
of the
msqid
argument.
The argument
msgflg
specifies the system call behaviour if enqueuing the new message
will require more than
msg_qbytes
in the queue.
Asserting
IPC_NOWAIT
the message will not be sent and the system call fails returning with
errno
set to
EAGAIN .
Otherwise the process is suspended until the condition for the
suspension no longer exists (in which case the message is sent and the
system call succeeds),
or the queue is removed (in which case the system call fails
with
errno
set to
EIDRM ),
or the process receives a signal that has to be
caught (in which case the system call fails
with
errno
set to
EINTR ).
Upon successful completion the message queue data structure is updated
as follows:
msg_lspid
is set to the process-\ID of the calling process.
msg_qnum
is incremented by 1.
msg_stime
is set to the current time.
The system call
msgrcv
reads a message from the message queue specified by
msqid
into the
msgbuf
pointed to by the
msgp
argument removing from the queue, on success, the read message.
The argument
msgsz
specifies the maximum size in bytes for the member
mtext
of the structure pointed to by the
msgp
argument.
If the message text has length greater than
msgsz ,
then if the
msgflg
argument asserts
MSG_NOERROR ,
the message text will be truncated (and the truncated part will be
lost), otherwise the message isn't removed from the queue and
the system call fails returning with
errno
set to
E2BIG .
The argument
msgtyp
specifies the type of message requested as follows:
If
msgtyp
is
0 ,
then the message on the queue's front is read.
If
msgtyp
is greater than
0 ,
then the first message on the queue of type
msgtyp
is read if
MSG_EXCEPT
isn't asserted by the
msgflg
argument, otherwise
the first message on the queue of type not equal to
msgtyp
will be read.
If
msgtyp
is less than
0 ,
then the first message on the queue with the lowest type less than or
equal to the absolute value of
msgtyp
will be read.
The
msgflg
argument asserts none, one or more (or-ing them) among the following
flags:
IPC_NOWAIT
For immediate return if no message of the requested type is on the queue.
The system call fails with errno set to
ENOMSG .
MSG_EXCEPT
Used with
msgtyp
greater than
0
to read the first message on the queue with message type that differs
from
msgtyp .
MSG_NOERROR
To truncate the message text if longer than
msgsz
bytes.
If no message of the requested type is available and
IPC_NOWAIT
isn't asserted in
msgflg ,
the calling process is blocked until one of the following conditions occurs:
A message of the desired type is placed on the queue.
The message queue is removed from the system.
In such a case the system call fails with
errno
set to
EIDRM .
The calling process receives a signal that has to be caught.
In such a case the system call fails with
errno
set to
EINTR .
Upon successful completion the message queue data structure is updated
as follows:
msg_lrpid
is set to the process-\ID of the calling process.
msg_qnum
is decremented by 1.
msg_rtime
is set to the current time.