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msgop(S)


msgop -- message operations

Syntax

cc . . . -lc

#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

msgrcv- receives a message

The msgsnd system call is used to send a message to the queue associated with the message queue identifier specified by msqid. {WRITE} msgp points to a structure containing the message. This structure is composed of the following members:

   long    mtype;         /* message type */
   char    mtext[];       /* message text */
The mtype integer is positive and can be used by the receiving process for message selection (see msgrcv below). The array mtext is any text of length msgsz bytes. The msgsz argument can range from 0 to a system-imposed maximum.

msgflg specifies the action to be taken if one or more of the following is true:


These actions are:

msgsnd fails and no message is sent if one or more of the following is true:


[EACCES]
Operation permission is denied to the calling process (see Intro(S)).

[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.

[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.

Upon successful completion, the following actions are taken with respect to the data structure associated with msqid (see Intro(S)):

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 & 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:


msgflg specifies the action to be taken if a message of the desired type is not on the queue. These are as follows:

msgrcv fails and no message is received if one or more of the following is 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.

[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''.

Upon successful completion, the following actions are taken with respect to the data structure associated with msqid. msg_qnum is decremented by 1.


Diagnostics

If msgsnd or msgrcv return due to the receipt of a signal, a value of -1 is returned to the calling process and errno is set to EINTR. If they return due to removal of msqid from the system, a value of -1 is returned and errno is set to EIDRM.

Upon successful completion, the return value is as follows:

Otherwise, a value of -1 is returned, and errno is set to indicate the error.

See also

msgctl(S), msgget(S), sigaction(S), signal(S)

Standards conformance

msgrcv and msgsnd are conformant with:

X/Open Portability Guide, Issue 3, 1989 .


© 2003 Caldera International, Inc. All rights reserved.
SCO OpenServer Release 5.0.7 -- 11 February 2003