How to share memory between Linux processes

Shared memory IPC principle

The shared memory inter-process communication mechanism is mainly used to transfer large amounts of data between processes. The following figure shows a schematic diagram of using shared memory to transfer large amounts of data between processes:

Shared memory is a memory space that is opened separately in the memory. This memory space has its own unique data structure, including access permissions, size, and recent access time. The data structure is defined as follows:

from /usr/include/linux/shm.h

struct shmid_ds {
struct ipc_perm shm_perm; /* operation perms 操作权限 */
int shm_segsz; /* size of segment (bytes) 段长度大小 */
__kernel_time_t shm_atime; /* last attach time 最近attach时间 */
__kernel_time_t shm_dtime; /* last detach time 最近detach时间 */
__kernel_time_t shm_ctime; /* last change time 最近change时间 */
__kernel_ipc_pid_t shm_cpid; /* pid of creator 创建者pid */
__kernel_ipc_pid_t shm_lpid; /* pid of last operator 最近操作pid */
unsigned short shm_nattch; /* no. of current attaches */
unsigned short shm_unused; /* compatibility */
void *shm_unused2; /* ditto - used by DIPC */
void *shm_unused3; /* unused */
};

Before two processes use this shared memory space, they need to establish a connection between the process address space and the shared memory space, that is, mount the shared memory space into the process.

The system imposes the following restrictions on shared memory:

#define SHMMAX 0x2000000 /* max shared seg size (bytes) 最大共享段大小 */
#define SHMMIN 1 /* min shared seg size (bytes) 最小共享段大小 */
#define SHMMNI 4096 /* max num of segs system wide */
#define SHMALL (SHMMAX/getpagesize()*(SHMMNI/16))
#define SHMSEG SHMMNI /* max shared segs per process */

Linux shared memory management

1. Create shared memory

#include <sys> #include <sys></sys></sys>

/*
* The first parameter is the key value, generally generated by the ftok() function
* The second parameter is the size of the shared memory segment to be created (in bytes)
* The third parameter is used to identify the creation ID of the shared memory segment
*/

int shmget(key_t key, size_t size, int shmflg);

2. Shared memory control

#include <sys> #include <sys></sys></sys>

/*
* The first parameter is the shared memory identifier to be operated
* The second parameter is the operation to be performed
* The third parameter is the temporary shared memory variable information of the shmid_ds structure
*/

int shmctl(int shmid, int cmd, struct shmid_ds *buf);

3. Mapping shared memory objects

The system call shmat() function maps a shared memory segment to the data segment of the calling process and returns the first address of the memory space. Its function is declared as follows:

#include <sys>
#include <sys>
</sys></sys>

/*
* The first parameter is the shared memory identifier to be operated
* The second parameter is used to specify the mapped address of the shared memory. If it is not 0, this parameter will be allocated by the system.
* The third parameter is used to specify the access permissions and mapping conditions of the shared memory segment
*/

void *shmat(int shmid, const void *shmaddr, int shmflg);

4. Separate shared memory objects

After using the shared memory space, you need to use the shmdt() function call to separate it from the current process. The function declaration is as follows:

#include <sys>
#include <sys>
</sys></sys>

/*
* The parameter is the first address of the allocated shared memory
*/

int shmdt(const void *shmaddr);

The convention that shared memory follows between parent and child processes

1. After using the fork() function to create a child process, the process inherits the shared memory mounted by the parent process.

2. If exec() is called to execute a new program, all mounted shared memory will be automatically unmounted.

3. If the exit() function is called in a process, all mounted shared memory will be disconnected from the current process.

Program Example

Apply for a shared memory, the parent process stores an integer at the first address, and the child process reads it out.

#include
#include <sys>
#include <sys>
#include <sys>
#include
#include

#define SHM_SIZE 1024

int main()
{
int shm_id, pid;
int *ptr = NULL;
</sys></sys></sys>

/* Apply for shared memory */

shm_id = shmget((key_t)1004, SHM_SIZE, IPC_CREAT | 0600);

/* Map shared memory to process address space */

ptr = (int*)shmat(shm_id, 0, 0);

printf("Attach addr is %p \n", ptr);

*ptr = 1004;

printf("The Value of Parent is : %d \n", *ptr);

if((pid=fork()) == -1){
perror("fork Err");
exit(0);
}
else if(!pid){
printf("The Value of Child is : %d \n", *ptr);
exit(0);
}else{
sleep(1);

/* Unmap */

shmdt(ptr);

/* Delete shared memory */

shmctl(shm_id, IPC_RMID, 0);
}

return 0;
}

Output result:

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