IPC---Message Queuing

Introduction to Message Queuing

The implementation and function of each communication mode is different, and the scene is not the same, the message queue is a linked list, it provides a

The struct Msqid_ds *msgque[msgmni] Vector maintains a list of message queues because the maximum number of messages supported by the Linux system is determined by the Msgque array, and each msgid_ds represents a message queue;

1. Structure of a message queue:

The data members in struct Msqid_ds are described below:

{

struct Ipc_perm msg_perm;

struct MSG *msg_first; /* Message Queue header pointer */

struct MSG *msg_last; /* Message Queue tail pointer */

__kernel_time_t Msg_stime; /* The last time the message queue message was inserted */

__kernel_time_t Msg_rtime; /* The last time a message is received deletes the message in the queue */

__kernel_time_t Msg_ctime;

struct Wait_queue *wwait; /* Send message waiting for process queue */

struct Wait_queue *rwait;

unsigned short msg_cbytes;

unsigned short msg_qnum; /* Number of messages in Message Queuing */

unsigned short msg_qbytes;

__kernel_ipc_pid_t Msg_lspid; /* pid*/of the last message sending process

__kernel_ipc_pid_t Msg_lrpid; /* pid*/of the last message sending process

};

2. Use of Message Queuing

(1) Acquisition of message queue key

To use Message Queuing in a program, you must be able to get to Message Queuing key, because the process cannot directly access the kernel message queue structure, so you need a Message Queuing flag to let the process know which message queue is currently operating, and also to guarantee the key value uniqueness of the message queue.

key_t Kkey=ftok (".", "a");

The function maps a message queue key with a pathname

(2) Gets or opens a message queue

Qid=msgget (key_t key,int msgflag);

Key: Message Queue key

-msgflag:

Ipc_private: Creates a message queue that is exclusive to the process, and other processes cannot access the message queue

Ipc_creat: Creates a new message queue if the message queue does not exist, and returns the message queue if the message queue exists

Ipc_creat| IPC_EXCL:: Ensure only new queues are created and return errors if Message Queuing is present

Ipc_noeait: Small queues are not blocked for blocking

function Reason:

1) If key==ipc_private request a piece of memory, create a new message queue

2) in the Msgque vector table to find the key value of the message queue, if there are no results found in two cases:

Msgflag indicates that a new message queue is not created and returns an error

Msgflag means to create a new message queue (ipc_creat) to create a new message queue

3. Sending a message queue

int msgsnd (int msqid,struct msgbuf* msgp,size_t msgsz,int msgflg)

--msqid: QID for Message Queuing

--MSGP: pointer to corresponding message content struct body

--MSGSZ: The size of the message is the size of the message structure that the pointer points to

--MSFLG: Message Flags

0: Ignore the flag bit to send messages to Message Queuing in a blocked manner

4, receive a message from message queue to msgbuf*

int MSGRCV (int msqid,struct msgbuf* msgp,size) T Msgsz,long msgtyp,int MSGFLG)

--msgqid: qid of Message Queuing

--MSGP: is the buffer to which the message queue will be received

--MSGSZ: The size of the message

--msgtyp: The type of message expected to be received

--MSGFLG: Logo

5. Control of Message Queuing

int msgctl (int msqid,int cmd,struct msqid_ds* buf)

--msqid: QID for Message Queuing

--cmd: The action to be performed on the message queue for the function

Ipc_atat: Takes out the msqid_ds result body of the message queue and stores the parameters in the Msqid_ds structure object pointed to by the BUF

Ipc_set: Sets the Msg_perm member in the MSQID_DS data of the message queue, given by the MSQID_DS structure that the buf points to.

Settings for Message Queuing and operations are determined by CMD

IPC---Message Queuing