Multiprocess programming (4): shared memory

Definition ：

Shared memory （Shared Memory） It allows two or more processes to access the same memory space , Is the most efficient way to communicate in multiple processes .
The operating system arranges the shared memory between different processes into the same physical memory , Processes can connect shared memory to their own address space , If a process modifies data in shared memory , The data read by other processes will also change . Because shared memory will become part of the process user space , All this communication does not require kernel intervention .
Shared memory does not provide a locking mechanism , in other words , When a process reads and writes shared memory , It will not prevent other processes from reading and writing to it , There may be data disorder .

function ：

1、 Required header file

#include <sys/ipc.h>
#include <sys/shm.h>

2、shmget() Get or create shared memory

//  Get or create shared memory 
int shmget(key_t key, size_t size, int shmflg);
- key The memory key value is shared , It's an integer , Is the number of shared memory in the system , Different shared memory has different numbers . Generally, hexadecimal is used .
- size Indicates the size of the shared memory to be created , In bytes 
- shmflg Indicates access to shared memory , The same permissions as files ,
	0666 | IPC_CREAT  Indicates that all users can read and write it 
-  Return value ： Returns the shared memory ID 

3、shmat() Connect the shared memory to the address space of the current process

void *shmat(int shm_id, const void *shm_addr, int shmflg);
- shm_id By  shmget Function return   Identification number of shared memory 
- shm_addr Specifies the location of the shared memory connection to the current process , Usually it is  NULL, Let the system choose the address of shared memory .
- shm_flg Is a set of flag bits , Usually it is 0
-  Return value ：
	 Successful call ： Returns a pointer to the first byte of shared memory 
	 Call failed ： return  -1

4、shmdt() Separate shared memory from the current process , amount to shmat() The reverse operation of a function

int shmdt(const void *shmaddr);
- shmaddr yes  shmat function   Return address 
-  Return value ：
	 Successful call ： return  0
	 Call failed ： return  -1

5、shmctl() Delete shared memory

int shmctl(int shm_id, int command, struct shmid_ds *buf);
- shm_id Is the identification number of shared memory 
- command  Fill in  IPC_RMID
- buf  Fill in  0
-  Return value ：
	 Successful call ： return  0
	 Call failed ： return  -1

Use steps

• call shmget() Function to create a new shared memory segment or get the identification number of an existing shared memory segment . Return the required shared memory identifier that can be called later .
• Use in process shmat() Function with shared memory segment , That is, make the shared memory segment a part of the virtual memory of the calling process （ Process binding shared memory segment ）
• Use shmat() Returned memory address pointer , You can operate on the shared memory segment in the program （ Write operations / Read operations ）.
• call shmdt() function , Separate the process from the shared memory segment , The process can no longer operate on shared memory .（ Just separation , Shared memory still exists ）
• call shmctl() function , Delete shared memory segment , Only when the processes bound to the shared memory segment are separated , Will destroy , Only one process needs to perform this step .

give an example

// write_shm.c
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/ipc.h>
#include <sys/shm.h> 

int main()
{
    
	int shmid; //  Shared memory identifier 
 
  	//  Create shared memory , The key value is 0x5005, common 1024 byte .
  	if ( (shmid = shmget((key_t)0x5005, 1024, 0666 | IPC_CREAT)) == -1)
  	{
     
		printf("shmat(0x5005) failed\n"); 
		return -1; 
	}

  	char *ptext = NULL;   //  Pointer to shared memory 
 
  	//  Connect shared memory to the address space of the current process , from ptext The pointer points to it 
  	ptext = (char *)shmat(shmid, NULL, 0);
    
  	//  To operate this procedure ptext The pointer , Is to operate shared memory 
    char *str = "Hello world!";
  	memcpy(ptext, str, strlen(str));
    
  	//  Detach shared memory from the current process 
  	shmdt(ptext);

    return 0;
}

// read_shm.c

int main()
{
    
	int shmid; //  Shared memory identifier 
 
  	//  Create shared memory , The key value is 0x5005, common 1024 byte .
  	if ( (shmid = shmget((key_t)0x5005, 1024, 0666 | IPC_CREAT)) == -1)
  	{
     
		printf("shmat(0x5005) failed\n"); 
		return -1; 
	}

  	char *ptext = NULL;   //  Pointer to shared memory 
 
  	//  Connect shared memory to the address space of the current process , from ptext The pointer points to it 
  	ptext = (char *)shmat(shmid, NULL, 0);
 
  	//  To operate this procedure ptext The pointer , Is to operate shared memory 
  	printf("%s\n", ptext);
 
  	//  Detach shared memory from the current process 
  	shmdt(ptext);
   
  	//  Delete shared memory 
  	if (shmctl(shmid, IPC_RMID, 0) == -1)
  	{
     
		printf("shmctl(0x5005) failed\n"); 
		return -1; 
	}
}

read_shm.c Output result of ：

Hello world!

It can be seen that ,write_shm.c Request shared memory , And write data ,read_shm Get the shared memory identification number , And bind the process , Read the data in the shared memory .

View shared memory

ipcs -m #  View all existing shared memory segments 

ipcrm -m shmid #  According to the identification number of the shared memory segment , Delete shared memory