Linux System v Message Queuing implements the MSGSND client/server and the MSGRCV function

First, msgsnd and MSGRCV functions

#include <sys/types.h>

#include <sys/ipc.h>

#include <sys/msg.h>

Function: Add a message to the message queue

Prototype int msgsnd (int msqid, const void *MSGP, size_t msgsz, int msgflg);

Parameters

MSGID: Message Queue identification code returned by the Msgget function

MSGP: is a pointer to the message structure that is ready to be sent

MSGSZ: is the message length that the MSGP points to, this length does not contain the long int that holds the message type

MSGFLG: Controls what happens when the current message queue is full or arrives on the system

Return value: successful return 0; failure return-1

Msgflg=ipc_nowait indicates that the queue is full without waiting and returns a Eagain error. For 0 indicates blocking wait

The message structure is constrained in two ways. First, its specific data must be smaller than the system-specified upper-bound value msgmax; second, it must begin with a long int, which will use this long integer to determine the type of message.

The message structure reference form is as follows:

struct MSGBUF {

Long Mtype;

char mtext [1];

};

The Mtext field is a array (or other structure) whose the size was specified by Msgsz, a nonnegative integer value. Messages of zero length (i.e., no mtext field) are permitted.

That is, the Mtex area can be a number or structure, and the size is indicated by the parameter msgsz.

Function: To receive messages from a message queue

Prototype ssize_t MSGRCV (int msqid, void *msgp, size_t msgsz, long Msgtyp, int msgflg);

Parameters

MSGID: Message Queue identification code returned by the Msgget function

MSGP: is a pointer to a message structure that is ready to receive

MSGSZ: is the message length that the MSGP points to, this length does not contain the long int that holds the message type

Msgtype: It can achieve a simple form of receive priority

MSGFLG: Control what happens when there is no corresponding type of message in the queue to receive

Return value: Successfully returns the number of characters actually placed in the receive buffer, failure returns-1

Msgtype=0 returns the first message of the queue

MSGTYPE>0 returns the message that the first type of the queue equals Msgtype

MSGTYPE<0 returns a message that the first type of the queue is less than or equal to the Msgtype absolute value and is the smallest message type that satisfies the condition

Msgflg=ipc_nowait, the queue does not wait for a readable message and returns a enomsg error.

Msgflg=msg_noerror, truncated when message size exceeds MSGSZ

Msgtype>0 and Msgflg=msg_except, the receiving type is not equal to the first message of Msgtype.

Second, the message queue implementation of the ECHO client/server

In the previous series, we used socket sockets to implement a MSGSND,MSGRCV client/server program, and now try to use Message Queuing to implement it, mainly using the two functions described above.