Multi process programming (III): message queue

Message queue

** Message queue , Is a linked list of messages , Is a list of messages stored in the kernel .** User processes can add messages... To message queues , You can also read messages to the message queue .
Message queuing overcomes the shortcomings that the pipeline can only carry unformatted byte streams and the buffer size is limited .
The advantage is to specify a specific message type for each message , When receiving, you can receive specific types of messages according to custom conditions , It is not necessary to receive data in a first in first out manner like the pipeline .

Message data consists of structures

struct msgbuf {
    
	long msg_type; //  Information types 
	char msg[size_t]; //  An array that holds information 
};

function ：

1、 Required header file

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

2、 Create a message queue

int msgget(key_t key, int msgflg);
- key It's a key value , It is used to identify a globally unique message queue , Generally, hexadecimal numbers are used to represent 
- msgflg Indicates the required operations and permissions , It can be used to control and create a message queue 
	IPC_CREAT
	IPC_EXCL
	IPC_CREAT | IPC_EXCL
-  Return value ：
	 Successful call ： Return the identification number of the message queue 
	 Call failed ： return  -1

3、 Write a message to the message queue

int msgsnd(int msgid, const void *ptr, size_t nbytes, int flag);
- msgid Indicates the identification number of the message queue 
- ptr Pointer to the message structure （ Pointer to the structure storing data ）
- nbytes Represents the size of the character array in the message structure 
- flag Indicates the mode selected by the message queue 
	0 --  When the message queue is full ,msgsnd It's blocking the process , Until the message can be written into the message queue or the message queue is deleted 
	IPC_NOWAIT --  Does not block the process , Will return immediately  EAGAIN
-  Return value ：
	 Successful call ： return  0
	 Call failed ： return  -1

4、 Read information from the message queue

size_t msgrcv(int msgid, void *ptr, size_t nbytes, long type, int flag);
- msgid Indicates the identification number of the message queue 
- ptr Pointer to the message structure （ Pointer to the structure storing data ）
- nbytes Represents the size of the character array in the message structure 
- type It means reading the data in the message queue according to the type 
	0 --  Return the first message in the queue 
	>0 --  The message type in the return queue is  type  The first message 
	<0 --  The message type value in the return queue is less than or equal to  type Absolute news , If there are many such messages , Then take the message with the smallest type 
- flag:
	0 --  Blocking process , Until the information is successfully read 
	IPC_NOWAIT --  If you can't read the information, return immediately , The error code is  ENOMSG
-  Return value ：
	 Successful call ： Returns the length of the message data 
	 Call failed ： return  -1

5、 Delete message queue

int msgctl(int msgid, int cmd, struct msqid_ds *buf);
- msgid Indicates the identification number of the message queue 
- cmd:
	IPC_RMID
- buf Set to  NULL

give an example

// write_msg.c
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <string.h>

struct msgbuf {
    
        long int mymsg_type;
        char msg[100];
} msg;

int main() {
    
    //  Create a message queue 
    int msgid = msgget((key_t)0x5005, IPC_CREAT | 0666);
    if (msgid == -1) {
    
        perror("msgget!");
    }
		
		//  Prepare data in advance , And copy it to the message queue structure 
    msg.mymsg_type = 1;
    char buf[100] = "Hello world!\n This is my show!";
    memcpy(msg.msg, buf, strlen(buf));
		
		//  Send a message to a message queue 
    int res = msgsnd(msgid, &msg, 100, 0);
    if (res == -1) {
    
        perror("msgsnd!");
    }

    return 0;
}

// read_msg.c
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <string.h>

struct msgbuf {
    
        long int mymsg_type;
        char msg[100];
} msg;

int main() {
    
    //  Create a message queue 
    int msgid = msgget((key_t)0x5005, IPC_CREAT | 0666);
    if (msgid == -1) {
    
        perror("msgget!");
    }
    
    //  Read the data in the message queue 
    int res = msgrcv(msgid, &msg, 100, 1, IPC_NOWAIT); //  Non blocking read 
    if (res == -1) {
    
        perror("msgsnd!");
    }
    
    //  Print data 
    printf("%s\n", msg.msg);

    return 0;
}

View message queues

ipcs -q #  View existing message queues 

ipcrm -q msgid #  According to the message queue identification number , Delete message queue