Detailed description of Linux interprocess communication-Message Queuing

Preamble: The process of communication between signals and pipelines is discussed, and Message Queuing is discussed next.

First, the System V IPC

Three systems V IPC: There are many similarities between Message Queuing, semaphores, and shared memory (shared memory).

　　The I P C structure (message queue, semaphore, or shared bucket) in each core uses an identifier of a nonnegative integer
(i d e n t i f i e R) be referenced.

　　Whenever you create an I P-C structure (call m S G G e t, s E m g e t or s h M g e t) , you should specify a keyword (k e y ), off

The above is a brief introduction to the IPC, which is useful for understanding the message queues, semaphores, and shared memory that are described next.

second, Message Queuing1. Introduction

　　Message Queuing is a linked table of messages , stored in the kernel and identified by message queue identifiers. We will say that the news team is listed as
"Queue" with the identifier "queue I D". m S G G e T is used to create a new queue or open an existing queue. m S G S n d
Used to add new messages to the end of the queue. Each message contains a positive long integer type field, a non-negative length, and an actual
Data bytes (corresponding to length), all of which are passed to m s G S n d when the message is added to the queue. the M S G R C v is used to
The queue for fetching messages. We do not have to get messages in FIFO order or in the Type field of the message.

2. Function Introduction

• Ftok function

#include <sys/types.h>
#include <sys/ipc.h>

key_t Ftok (const char *pathname, int proj_id);//"/home/linux", ' a '
Function: Generate a key (value)

• Msgget function

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>

int Msgget (key_t key, int msgflg);

Function: Create or get a Message Queuing object
Returns: The ID of the message queue object the same key gets the same object
Format: Msgget (Key,flag|mode);
Flag: can be 0 or ipc_creat (created if not present)
Mode: Same as file permissions

• msgsnd function

int msgsnd (int msqid, const void *MSGP, size_t msgsz, int msgflg);
Function: Writes MSGP message to message queue identified as MsgId
MSGP:
struct MSGBUF {
Long Mtype; /* Message type, must be > 0 * * The type of message must be >0
Char mtext[1]; /* Message Data */length arbitrary
};

MSGSZ: The size of the message to be sent does not include the 4 bytes of the message type
MSGFLG: If it is 0 when the message team is listed as full MSGSND will block
If ipc_nowait is not blocked when the message queue is full, return immediately

Return value: Successful return ID failure return-1

• MSGRCV function

ssize_t MSGRCV (int msqid, void *msgp, size_t msgsz, Long Msgtyp,
int MSGFLG);

Function: Receives a specified type of message from the message queue with identifier MsgId and stores it in MSGP to delete the message from the message queue
Msgtyp: 0 means that no matter what type you can receive
MSGP: The structure that holds the message
MSGSZ: The size of the message to receive does not contain the 4 bytes that the message type occupies
MSGFLG: If the 0 ID is waiting for a message without a specified type
If it's ipc_nowait, it means no waiting.

• Msgctl function

int msgctl (int msqid, int cmd, struct msqid_ds *buf);
Msgctl (msgid,ipc_rmid,null);//Delete Message Queuing objects

Program 2-2 will simply demonstrate Message Queuing:

---snd.c---

#include"My.h"typedefstruct{    Longtype; Charname[ -]; intAge ;} MSG;intMain () {key_t key= Ftok ("/HOME/LIUDW",'6'); printf ("key:%x\n", key); intMsgId = Msgget (key,ipc_creat| o_wronly|0777); if(msgid<0) {perror ("Msgget error!"); Exit (-1);    } MSG m; Puts ("Please input your type name:"); scanf ("%ld%s%d",&m.type,m.name,&m.age); Msgsnd (MsgId,&m,sizeof(m)-sizeof(M.type),0); return 0;}

---rcv.c---

#include"My.h"typedefstruct{    Longtype; Charname[ -]; intAge ;} MSG;intMain () {key_t key= Ftok ("/HOME/LIUDW",'6'); printf ("key:%x\n", key); intMsgId =Msgget (key,o_rdonly); if(msgid<0) {perror ("Msgget error!"); Exit (-1);    } MSG Rcv; Longtype; Puts ("Please input type want!"); scanf ("%ld",&type); MSGRCV (MsgId,&AMP;RCV,sizeof(RCV)-sizeof(type), type,0); printf ("rcv--name:%s age:%d\n", Rcv.name,rcv.age);    Msgctl (Msgid,ipc_rmid,null); return 0;}

Run the Demo:

Third, detailed Ftok function

• Ftok extracts the file information according to the path name, and then synthesizes the key based on the file information and project ID, which can be arbitrarily set.
• The path must exist, and Ftok only takes a value based on the uniqueness of the file inode within the system, regardless of the permissions of the file.
• PROJ_ID can be arbitrarily set according to their own conventions. This number, some call it project ID; On UNIX systems, it has a value of 1 to 255;

To verify the above view, modify the program 2-2 slightly and modify the path and proj_id as follows:

Program 3-1 is as follows:

---snd.c---

#include"My.h"typedefstruct{    Longtype; Charname[ -]; intAge ;} MSG;intMain () {key_t key= Ftok (" /Home",'a'); printf ("key:%x\n", key); intMsgId = Msgget (key,ipc_creat| o_wronly|0777); if(msgid<0) {perror ("Msgget error!"); Exit (-1);    } MSG m; Puts ("Please input your type name:"); scanf ("%ld%s%d",&m.type,m.name,&m.age); Msgsnd (MsgId,&m,sizeof(m)-sizeof(M.type),0); return 0;}

---rcv.c---

#include"My.h"typedefstruct{    Longtype; Charname[ -]; intAge ;} MSG;intMain () {key_t key= Ftok (" /Home",'a'); printf ("key:%x\n", key); intMsgId =Msgget (key,o_rdonly); if(msgid<0) {perror ("Msgget error!"); Exit (-1);    } MSG Rcv; Longtype; Puts ("Please input type want!"); scanf ("%ld",&type); MSGRCV (MsgId,&AMP;RCV,sizeof(RCV)-sizeof(type), type,0); printf ("rcv--name:%s age:%d\n", Rcv.name,rcv.age);    Msgctl (Msgid,ipc_rmid,null); return 0;}

Run a demo such as:

　

　　Summary: mainly describes the process of communication between the message queue, there are questions can leave a message, must be answered immediately.

Detailed description of Linux interprocess communication-Message Queuing