LINUX study notes 15 -- inter-process communication 4 Message Queue

A) essence: message linked list B) classification: 1. POSIX message queue: Port Operating System Interface 2. system V: as the kernel continues, it will be deleted only after kernel restart and manual deletion. Each message queue corresponds to a unique key value c) get the key value: Key_tftok (char * pathname,... a) essence: message linked list
 
B) Category:
 
1. POSIX message queue: Port operating system interfaces
 
2. System V: The System V will be deleted only after kernel restart and manual deletion. Each message queue corresponds to a unique key value
 
C) obtain the key value: Key_t ftok (char * pathname, char proj)
 
1. function: return the key value corresponding to the file name, essentially a file
 
2. # include , # Include
 
3. Pathname: file name
 
4. Proj: project name, not 0
 
D) get the description: int msgget (key_t key, int msgflg)
 
1. # include , # Include , # Include
 
2. Key: Key value, obtained by ftok
 
3. Msgflg: Flag
 
A) IPC_CREAT: if there is no message queue corresponding to the key value, create a new message queue.
 
B) IPC_EXCL: used together with IPC_CREAT. If an error is returned
 
C) IPC_NOWWAIT: if it cannot be met, it is not blocked.
 
4. return value: file descriptor msgid
 
E) send message: int msgsnd (int msqid, struct msgbuf * msgp, int msgsz, int msgflg)
 
1. # include , # Include , # Include
 
2. Msqid: message queue descriptor
 
3. Msgp: message storage structure:
 
A) struct msgbuf
 
B) {long mtype; message type, defined by yourself. one type can store multiple messages.
 
C) Char mtext [1];} first address of the message data
 
4. Msgsz: length
 
5. Msgflg: flag, IPC_NOWAIT: indicates whether to wait when the message queue does not have enough space to accommodate the message to be sent.
 
F) receive message: int msgrcv (int msqid, struct msgbuf * msgp, int msgsz, long msgtype, int msgflg)
 
1. function: read msgtype messages from the message queue, and the messages are no longer read.
 
2. Others are the same as above:
 
3. message length: length = sizeof (struct mymsgbuf)-sizeof (long)