msgop(S)
msgop: msgsnd, msgrcv --
message operations
Synopsis
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int msgsnd(int msqid, const void msgp,
size_t msgsz, int msgflg);
int msgrcv(int msqid, void msgp,
size_t msgsz, long msgtyp, int msgflg);
Description
msgsnd sends a message to the queue associated with the message
queue identifier specified by
msqid.
msgp points to a user defined buffer
that must contain first a field of type long integer that
will specify the type of the message, and then a data portion
that will hold the text of the message.
The following is an example of members that might be in a user defined buffer.
long mtype; /* message type */
char mtext[]; /* message text */
mtype is a positive integer that can be used
by the receiving process for message selection.
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(S)].
-
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 is not sent and the caller
returns immediately.
-
If (msgflg&IPC_NOWAIT)
is false,
the caller suspends 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(S)].
When this occurs, errno is set to EIDRM,
and a value of -1 is returned.
-
The caller receives a signal that is to be caught.
In this case the message is not sent and the caller resumes
execution in the manner prescribed in
signal(M).
On success, the following actions are taken with respect to
the data structure associated with
msqid
[see
intro(S)].
-
msg_qnum
is incremented by 1.
-
msg_lspid
ID of the caller.
-
msg_stime
is set 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 user defined structure pointed to by
msgp.
The structure must contain a message type field followed
by the area for the message text (see the structure mymsg above).
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 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 caller returns immediately with a return value
of -1 and sets errno to ENOMSG.
-
If
(msgflg&IPC_NOWAIT)
is false, the caller suspends 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 to EIDRM, and a value of -1 is returned.
-
The caller receives a signal that is to be caught.
In this case a message is not received and the caller resumes
execution in the manner prescribed in
signal(S).
On success, 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 to the process ID of the caller.
-
msg_rtime
is set to the current time.
Return values
On success:
-
msgsnd
returns 0.
-
msgrcv
returns the number of bytes actually placed into
mtext.
On failure, msgsnd and msgrcv return -1 and set errno to identify the error.
Errors
In the following conditions, msgsnd and msgrcv fail and set errno to:
EINTR-
msgsnd or msgrcv
returned due to the receipt of a signal.
EIDRM-
msgsnd or msgrcv
returned due to removal of
msqid from the system.
In the following conditions, msgsnd fails and sets errno to:
EINVAL-
msqid is not a valid message queue identifier.
EACCES-
Write
permission is denied to the caller.
[see
intro(S)].
EINVAL-
mtype is less than 1.
EAGAIN-
The message cannot be sent for one of the reasons cited above and
(msgflg&IPC_NOWAIT) is true.
EINVAL-
msgsz is less than zero or greater than the system-imposed limit.
EFAULT-
msgp points to an illegal address.
In the following conditions, msgrcv fails and sets errno to:
EINVAL-
msqid is not a valid message queue identifier.
EACCES-
Read
permission is denied to the caller.
EINVAL-
msgsz is less than 0.
E2BIG-
The length of mtext is greater than msgsz and
(msgflg&MSG_NOERROR)
is false.
ENOMSG-
The queue does not contain a message of the desired type and
(msgtyp&IPC_NOWAIT)
is true.
EFAULT-
msgp points to an illegal address.
References
intro(S),
msgctl(S),
msgget(S),
signal(S)
Notices
Considerations for threads programming
While one thread is blocked, siblings might still be executing.
© 2005 The SCO Group, Inc. All rights reserved.
SCO OpenServer Release 6.0.0 - 01 June 2005