Manual page for MSGOP(2)
msgop, msgsnd, msgrcv - message operations
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int msgsnd(msqid, msgp, msgsz, msgflg)
int msqid;
struct msgbuf *msgp;
int msgsz, msgflg;
int msgrcv(msqid, msgp, msgsz, msgtyp, msgflg)
int msqid;
struct msgbuf *msgp;
int msgsz;
long msgtyp;
int msgflg;
DESCRIPTION
msgsnd()
is used to send a message to the queue associated with the message
queue identifier specified by
msqid.
[WRITE]
(see
msgctl.2
msgp
points to a structure containing the message.
This structure is composed of the following members:
-
long mtype; /* message type */
char mtext[1]; /* message text */
mtype
is a positive integer that can be used by the receiving process for
message selection (see
msgrcv()
below).
mtext
is any text of length
msgsz
bytes.
msgsz
can range from 0 to a system-imposed maximum.
msgflg
specifies the action to be taken if one or more of the following are
true:
- The number of bytes already on the queue is equal to
msg_qbytes
(see
intro.2
- The total number of messages on all queues system-wide is equal to the
system-imposed limit.
These actions are as follows:
- If
(msgflg &
IPC_NOWAIT)
is ``true'', the message will not be sent and the calling process will
return immediately.
- If
(msgflg &
IPC_NOWAIT)
is ``false'',
the calling process will suspend execution until one of the following occurs:
-
- The condition responsible for the suspension no longer exists, in which case
the message is sent.
- msqid
is removed from the system (see
msgctl.2
When this occurs,
errno
is set equal to
EIDRM,
and a value of -1 is returned.
- The calling process receives a signal that is to be caught.
In this case the message is not sent and the calling process resumes
execution in the manner prescribed in
signal.3v
Upon successful completion, the following actions are taken with respect to
the data structure associated with
msqid
(see
intro.2
- msg_qnum
is incremented by 1.
- msg_lspid
is set equal to the process
ID
of the calling process.
- msg_stime
is set equal to the current time.
msgrcv()
reads a message from the queue associated with the message queue identifier
specified by
msqid
and places it in the structure pointed to by
msgp.
[READ]
This structure is composed of the following members:
-
long mtype; /* message type */
char mtext[1]; /* message text */
mtype
is the received message's type as specified by the sending process.
mtext
is the text of the message.
msgsz
specifies the size in bytes of
mtext.
The received message is truncated to
msgsz bytes
if it is larger than
msgsz
and
(msgflg &
MSG_NOERROR)
is ``true''.
The truncated part of the message is lost and no indication of the truncation is
given to the calling process.
msgtyp
specifies the type of message requested as follows:
- If
msgtyp
is equal to 0, the first message on the queue is received.
- If
msgtyp
is greater than 0, the first message of type
msgtyp
is received.
- If
msgtyp
is less than 0,
the first message of the lowest type that is less than or equal
to the absolute value of
msgtyp
is received.
msgflg
specifies the action to be taken if a message of the desired type
is not on the queue.
These are as follows:
- If
(msgflg &
IPC_NOWAIT)
is ``true'', the calling process will return immediately with a return value
of -1 and
errno
set to
ENOMSG.
- If
(msgflg &
IPC_NOWAIT)
is ``false'', the calling process will suspend execution until one of the
following occurs:
-
- A message of the desired type is placed on the queue.
- msqid
is removed from the system.
When this occurs,
errno
is set equal to
EIDRM,
and a value of -1 is returned.
- The calling process receives a signal that is to be caught.
In this case a message is not received and the calling process resumes
execution in the manner prescribed in
signal.3v
Upon successful completion, the following actions are taken with respect to
the data structure associated with
msqid
(see
intro.2
- msg_qnum
is decremented by 1.
- msg_lrpid
is set equal to the process
ID
of the calling process.
- msg_rtime
is set equal to the current time.
RETURN VALUES
msgsnd()
returns:
- 0
-
on success.
- -1
-
on failure and sets
errno
to indicate the error.
msgrcv()
returns
the number of bytes actually placed into
mtext
on success.
On failure,
it returns
-1
and sets
errno
to indicate the error.
ERRORS
msgsnd()
will fail and no message will be sent if one or more of the following are true:
- EACCES
-
Operation permission is denied to the calling process (see
intro.2
- EAGAIN
-
The message cannot be sent for one of the reasons cited above and
(msgflg &
IPC_NOWAIT)
is ``true''.
- EFAULT
-
msgp
points to an illegal address.
- EIDRM
-
The message queue referred to by
msqid
was removed from the system.
- EINTR
-
The call was interrupted by the delivery of a signal.
- EINVAL
-
msqid
is not a valid message queue identifier.
mtype
is less than 1.
msgsz
is less than zero or greater than the system-imposed limit.
msgrcv()
will fail and no message will be received if one or more of the following are
true:
- E2BIG
-
mtext
is greater than
msgsz
and
(msgflg &
MSG_NOERROR)
is ``false''.
- EACCES
-
Operation permission is denied to the calling process.
- EFAULT
-
msgp
points to an illegal address.
- EIDRM
-
The message queue referred to by
msqid
was removed from the system.
- EINTR
-
The call was interrupted by the delivery of a signal.
- EINVAL
-
msqid
is not a valid message queue identifier.
msgsz
is less than 0.
- ENOMSG
-
The queue does not contain a message of the desired type and
(msgtyp &
IPC_NOWAIT)
is ``true''.
SEE ALSO
intro.2
msgctl.2
msgget.2
signal.3v
Created by unroff & hp-tools.
© somebody (See intro for details). All Rights Reserved.
Last modified 11/5/97