Linux Message Queuing instances

Objective:

A message queue is a linked list of messages. You can think of a message as a record, with a specific format and a specific priority. A process that has write permission to a message queue can add new messages to it by a certain rule, and a process that has read access to a message queue can read messages from the message queue

Function:
1. Create a new message queue or get an existing message queue

Prototype: int msgget (key_t key, int msgflg);

Parameters:

Key: Can be thought of as a port number, or it can be generated by the function Ftok.

The Msgflg:ipc_creat value, which, if there is no queue, creates one and returns a new identifier, or returns the original identifier if it already exists.

A IPC_EXCL value that returns 1 if there is no queue, or 0 if it already exists.

2. Read/write messages to the queue

Prototype:

MSGRCV messages from the queue: ssize_t msgrcv (int msqid, void *msgp, size_t msgsz, long Msgtyp, int msgflg);

MSGSND put data into Message Queuing: int msgsnd (int msqid, const void *MSGP, size_t msgsz, int msgflg);

Parameters:

MSQID: Message Queue identification code

MSGP: A pointer to the message buffer, which is used to temporarily store messages sent and received, is a generic, user-definable structure, in the following form:

structmsgstru{    long mtype; //大于0    charmtext[512];};

MSGSZ: The size of the message.

Msgtyp: The message pattern read from within the message queue. A value of zero indicates that all messages in the message queue will be read.

MSGFLG: Used to indicate the action that the core program should take if the queue has no data. If the MSGFLG and constant ipc_nowait are pooled, then if the message queue is full at msgsnd () execution, then MSGSND () will not block, and will immediately return-1, if the MSGRCV () is performed, then when the message queue is empty, do not wait for the immediate return-1, and set the error code to enomsg. When MSGFLG is 0 o'clock, msgsnd () and MSGRCV () take a blocking wait processing mode when the queue is full or empty.

3. Set Message Queuing properties

Prototype: int msgctl (int msgqid, int cmd, struct msqid_ds *buf);

Parameters: The MSGCTL system invokes a CMD operation on the message queue identified by the msgqid, and the system defines 3 cmd operations: Ipc_stat, Ipc_set, Ipc_rmid
Ipc_stat: This command is used to obtain the MSQID_DS data structure for the message queue and save it to the BUF specified address space.
Ipc_set: The command is used to set the properties of the message queue, and the properties to be set are stored in BUF.

Ipc_rmid: Removes the MSQID identity message queue from the kernel.

Instance:

Message Send side: SEND.C

 1/*send.c*/2 #include <stdio.h> 3 #include <sys/types.h> 4 #include <sys/ipc.h> 5 #include <sys/msg.h> 6 #include <errno.h> 7 8 #define Msgkey 9-struct Msgstru Each {a long MS  Gtype   -Char msgtext[2048];  14};  Main () Msgstru msgs;  Msg_type int;  (+ char str[256];  int ret_value;  msqid int;  Msqid=msgget (MSGKEY,IPC_EXCL);  /* Check if Message Queuing exists * * if (Msqid < 0) {msqid = Msgget (msgkey,ipc_creat|0666);/* Create Message Queue * * if (msqid <0) { printf ("Failed to create MSQ |  errno=%d [%s]\n], Errno,strerror (errno));  Exit (-1);  (1) {The "Input message type (end:0):");  scanf ("%d", &msg_type);  if (Msg_type = = 0) PNs break;  * printf ("input message to be sent:");  scanf ("%s", str);  Msgs.msgtype = Msg_type;  strcpy (Msgs.msgtext, str); 42/* Send Message Queue */Ret_value = msgsnd (msqid,&msgs,sizeof (struct msgstru), ipc_nowait);  if (Ret_value < 0) {"msgsnd () write msg failed,errno=%d[%s]\n", Errno,strerror (errno));  (-1); Msgctl (msqid,ipc_rmid,0);   Delete Message Queuing 50}

Message Receiving end RECEIVE.C

 1/*receive.c */2 #include <stdio.h> 3 #include <sys/types.h> 4 #include <sys/ipc.h> 5 #inc Lude <sys/msg.h> 6 #include <errno.h> 7 8 #define Msgkey 9 Msgstru struct  Ong Msgtype;  -Char msgtext[2048];  14};  15 16/* Child process, listener message Queue */void Childproc () {Msgstru msgs;  Msgid,ret_value int;  (+ char str[512]; while (1) {MsgId = Msgget (MSGKEY,IPC_EXCL);/* check if Message Queuing exists */if (MsgId < 0) {pri NTF ("Msq not existed!  errno=%d [%s]\n], Errno,strerror (errno));  Sleep (2);  Continue;  28} 29/* Receive Message Queue */Ret_value = MSGRCV (msgid,&msgs,sizeof (struct msgstru), 0,0);  To printf ("text=[%s] pid=[%d]\n", msgs.msgtext,getpid ());  (+)-return;  () + void Main () PNS {i,cpid int;  Max/* Create 5 child process */i=0;i<5;i++ {cpid = fork (); if (Cpid < 0) * printf ("fork failed\n");  Cpid ==0/*child process*/childproc ();   47}

Linux Message Queuing instance