Linux Message Queuing practices (1)

Message Queuing Basic Concepts

Message Queuing provides a way to send a piece of data from one process to another (limited to native only)

Each block of data is considered to have a type, and the receiver process receives a data block that can have different type values

Message Queuing also has the same pipeline of shortcomings, that is, the maximum length of each message is capped (Msgmax), the total number of bytes per message queue is capped (MSGMNB), the total number of message queues on the system also has an upper limit (Msgmni)

Pipeline vs. Message Queuing:

Pipeline: Stream Pipeline message: bounded

Advanced first out can be entered later, first out

Message Queue Size Three limitations

Cat/proc/sys/kernel/msgmax #最大消息长度限制

CAT/PROC/SYS/KERNEL/MSGMNB #消息队列总的字节数

Cat/proc/sys/kernel/msgmni #消息条目数

IPC Object Data Structure

The kernel maintains a data structure for each IPC object

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2. struct Ipc_perm
4. {
6. key_t __key; /* Key supplied to Msgget (2) */   
8. uid_t uid; / * Effective UID of owner * /   
10. gid_t GID; / * Effective GID of owner * /   
12. uid_t cuid; / * Effective UID of Creator * /   
14. gid_t Cgid; / * Effective GID of Creator * /   
16. unsigned short mode; / * Permissions * /
18. unsigned short __seq; / * Sequence number * /
20. };

Message Queuing-specific structures

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2. struct Msqid_ds
4. {
6. struct     Ipc_perm msg_perm; / * Ownership and Permissions * /
8. time_t    Msg_stime; /* Time of Last msgsnd (2) */
10. time_t    Msg_rtime; /* Time of Last MSGRCV (2) */
12. time_t    Msg_ctime; /* Time of last change * /
14. Unsigned long __msg_cbytes; /* Current number of bytes in
16. Queue (nonstandard) */   
18. Msgqnum_t Msg_qnum; /* Current number of messages
20. In queue * /   
22. Msglen_t msg_qbytes; / * Maximum number of bytes
24. allowed in queue * /   
26. pid_t Msg_lspid; /* PID of Last MSGSND (2) */   
28. pid_t Msg_lrpid; /* PID of Last MSGRCV (2) */   
30. };

Message Queuing representation in the kernel

Message Queuing Function Example

msgget function

function: Used to create and access a message queue

Prototype:

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1. int  Msgget (key_t key, int MSGFLG);

Parameters:

Key: The name of a message queue

MSGFLG: Consisting of nine permission flags, such as 0644, whose usage is the same as the mode pattern flag used when creating the file (but Message Queuing does not have X (execute) permission)

return value:

Successfully returns the message queue number, which is the identification code for the message queue; 1 failure return

Programming practices

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2. //Practice 1:IPC_PRIVATE: Macro with a value of 0   
4. int Main ()
6. {
8. 　　//ipc_private each time you create a message queue, the descriptor is different!   
10. 　　//So, the timing MessageID (key) is transmitted to other processes (unless there is an associated process),   
12. 　　//Other processes also cannot use this message queue (except Blood fork)   
14. int  MsgId = Msgget (ipc_private,0666);
16. if (MsgId < 0)
18. {
20. //If Message Queuing does not exist, an error is turned on, Errno=enoent   
22. if (errno = = ENOENT)
24. {
26. cout << "ENOENT" << Endl;
28. }
30. Err_exit ("Mesget error");
32. }
34. Else   
36. {
38. cout << "MsgId =" << msgid << Endl;
40. }
44. return  0;
46. }
48. /**
50. The message queue created by Ipc_private can only be used in processes that are related to the current process!
52. */   

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2. //Practice 2:ipc_creat   
4. int Main ()
6. {
8. //Specify Ipc_creat, the message queue must be created   
10. int MsgId = Msgget (0x1235,0666|  Ipc_creat);
12. if (MsgId < 0)
14. {
16. //If Message Queuing does not exist, an error is turned on, Errno=enoent   
18. if (errno = = ENOENT)
20. {
22. cout << "ENOENT" << Endl;
24. }
26. Err_exit ("Mesget error");
28. }
30. Else   
32. {
34. cout << "MsgId =" << msgid << Endl;
36. }
40. return  0;
42. }

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2. //Practice 3:ipc_creat| IPC_EXCL
4. int Main ()
6. {
8. //Specify IPC_EXCL, if it already exists, the report file already exists (error)   
10. int MsgId = Msgget (0x1235,0666| Ipc_creat|  IPC_EXCL);
12. if (MsgId < 0)
14. {
16. Err_exit ("Mesget error");
18. }
20. Else   
22. {
24. cout << "MsgId =" << msgid << Endl;
26. }
30. return  0;
32. }

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2. //Practice 4: Low privilege creation, high privilege open   
4. int Main ()
6. {
8. //Low privilege creation   
10. int MsgId = msgget (0x255,0444 |  Ipc_creat);
12. if (MsgId < 0)
14. {
16. Err_exit ("Mesget error");
18. }
20. Else   
22. {
24. cout << "Create Mes OK, MsgId =" << msgid << Endl;
26. }
30. //High Privilege open   
32. MsgId = Msgget (0x255,0644 | Ipc_creat);
34. if (MsgId < 0)
36. {
38. Err_exit ("Mesget error");
40. }
42. Else   
44. {
46. cout << "Create Mes OK, MsgId =" << msgid << Endl;
48. }
52. return  0;
54. }

Msgget function parameter diagram

Linux Message Queuing practices (1)