MSGOP(2) DOMAIN/IX SYS5 MSGOP(2)
NAME
msgop - message operations
USAGE
#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 sends a message to the queue associated with the mes-
sage queue identifier named by msqid. {WRITE} Msgp points
to a structure containing the message. This structure con-
tains the following members:
long mtype; /* message type */
char mtext[]; /* message text */
Mtype is a positive integer that can be used by the receiv-
ing 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 an action to take if one or more of the
following is 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.
Printed 12/4/86 MSGOP-1
MSGOP(2) DOMAIN/IX SYS5 MSGOP(2)
⊕ The possible 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 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 described in signal(2)).
RETURN VALUES
If msgsnd or msgrcv returns because it received a signal, a
value of -1 is returned to the calling process and errno is
set to EINTR. If they return because msqid was removed from
the system, a value of -1 is returned and errno is set to
EIDRM.
Upon successful completion, msgsnd returns a value of 0, and
msgrcv returns a value equal to the number of bytes actually
placed into mtext. A failed call returns -1 and sets errno
as indicated below.
ERRORS
Msgsnd will fail and no message will be sent if one or more
of the following is true:
[EINVAL] Msqid is not a valid message queue identifier.
[EACCES] Operation permission is denied to the calling
process (see intro(2)).
MSGOP-2 Printed 12/4/86
MSGOP(2) DOMAIN/IX SYS5 MSGOP(2)
[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.
Upon successful completion, the following actions are taken
upon 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[]; /* 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 & IPC_NOERROR)
is "true". The truncated part of the message is lost and no
indication of lost data 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.
Printed 12/4/86 MSGOP-3
MSGOP(2) DOMAIN/IX SYS5 MSGOP(2)
⊕ 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 immediately return
with a 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 execu-
tion in the manner described in signal(2)).
Msgrcv will fail and no message will be received if one or
more of the following is true:
[EINVAL] Msqid is not a valid message queue identifier.
[EACCES] Operation permission is denied to the calling pro-
cess.
[EINVAL] Msgsz is less than 0.
[E2BIG] Mtext is greater than msgsz and
(msgflg & IPC_NOERROR)
is "false".
[ENOMSG] The queue does not contain a message of the
MSGOP-4 Printed 12/4/86
MSGOP(2) DOMAIN/IX SYS5 MSGOP(2)
desired type and
(msgflg & IPC_NOWAIT)
is "true".
[EFAULT] Msgp points to an illegal address.
Upon successful completion, the following actions are taken
with respect to the data structure associated with msqid.
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.
RELATED INFORMATION
msgctl(2), msgget(2), signal(2)
Printed 12/4/86 MSGOP-5