Day 23rd: Shared memory. Message Queuing and MySQL database usage

Shared memory and Message Queuing are also ways of communicating between processes.

Shared memory (similar to semaphore operation)

#include <sys/shm.h>

1, int shmget (key_t key,size_t size,int SHMFLG);

Function: Create a new piece of memory or return the built-in memory

Parameter: Key, which is used to indicate the opening of a memory, each process accesses the same piece of memory through this flag

Size, amount of memory

SHMFLG, which is exactly the same as the file operation, indicates that a bitwise or ipc_create creates a piece of memory, and if the memory that the key represents is already established, even if the addition of Ipc_create does not create a new chunk of memory, the memory associated with the key is returned.

Return value: Returns an identifier, other operations on shared memory, with the return value

2, void *shmat (int shm_id,const void * Shm_addr,int SHMFLG);

Role: Talk about a shared memory connection to the current process

Parameter: return value of Shm_id,shmget

SHM_ADDR is connected to the address location of the current process, which is generally null, indicating that the system chooses

Shmfig:shm_rnd, combined with shm_addr, to control the connection address

Shm_rdonly, read-only

Generally set to 0

Return value: Pointer to the first byte of shared memory

3, int shmdt (const void * shm_addr);

Role: Separating shared memory from the current process

Parameter: return value of Shmat

4, int shmctl (int shm_id,int command,struct shmid_ds * buf);

Structshmid_ds {

uid_t Shm_perm.uid;

uid_t Shm_perm.gid;

mode_t Shm_prem.mode;

}

Role: Control of shared memory

Parameter: return value of Shm_id,shm_get

Commond,ipc_stat: Sets the value in Shmid_ds to the current shared memory state value

Ipc_set: Set the shared memory state to a value in Shmid_ds

IPC_RMID: Deleting shared memory segments

Message Queuing

#include <sys/msg.h>

1, int msgget (key_t key,int MSGFLG);

Role: Create and access a message queue

Parameter: Key, which represents a message queue

MSGFLG, permission flag bit, similar to Shmget flag bit

Return value: Returns a descriptor for use in other Message Queuing functions

2, int msgsnd (int msqid,const void * msg_ptr,size_t msg_sz,int MSGFLG);

The message is generally expressed in the structure below.

struct my_message{

A long int message_type;//represents the data type

/*the data wish to transfer*/

}

Role: Adding messages to a message queue

Parameter: return value of Msqid,msgget

Msg_ptr, pointing to a pointer ready to send a message

MSG_SZ, message length does not include long shaping message type variables

MSGFLG controls what happens when the current queue is full or reaches the maximum system, typically set to 0

3, int MSGRCV (int msqid,void * msg_ptr,size_t msg_sz,long intmsgtype,int MSGFLG);

Role: Receive messages from Message Queuing

Parameter: The return value of the Msqid,msgget function

Msg_ptr, pointing to the pointer ready to receive the message

MSG_SZ, accepts the size of the message, does not include the first variable that represents a type

Msgtype, Acceptance type

MSGFLG, which controls what happens when no message is received, is typically set to 0

5, int msgctl (int msqid,int commond,struct msqid_ds * buf);

Structmsqid_ds {

uid_t Msg_perm.uid;

uid_t Msg_perm.gid;

mode_t Msg_perm.mode;

}

http://blog.csdn.net/coderlrx/article/details/8762017

Day 23rd: Shared memory. Message Queuing and MySQL database usage