共享内存同步机制

1. 使用posix有名信号量进行同步

有名信号量既可用于线程间的同步，又可用于进程间的同步。

两个进程，对同一个共享内存读写，可利用有名信号量来进行同步。一个进程写，另一个进程读，利用两个有名信号量semr, semw。semr信号量控制能否读，初始化为0。 semw信号量控制能否写，初始为1。

示例代码如下：

//读共享内存
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/types.h>
#include <semaphore.h>
#include <stdio.h>
#include <errno.h>
#include <fcntl.h>

typedef struct _Teacher
{
    char name[64];
    int age;
}Teacher;

int main()
{
    int shmid = -1;
    key_t key = 0x2234;
    Teacher *p = NULL;
    sem_t *semr = NULL, *semw = NULL;
    semr = sem_open("sem_r", O_CREAT | O_RDWR, 0666, 0);
    if (semr == SEM_FAILED )
    {
        printf("errno = %d\n", errno );
        return -1;
    }

    semw = sem_open("sem_w", O_CREAT | O_RDWR, 0666, 1 );
    if (semw == SEM_FAILED)
    {
        printf("errno = %d\n", errno );
        return -1;
    }
    
    shmid = shmget(key, 0, 0 );
    if ( shmid == -1 )
    {
        printf("shmget failed\n");
        perror("shmget err");
        return -1;
    }

    p = (Teacher*)shmat(shmid, NULL, 0);
    if (p == (Teacher*)(-1))
    {
        printf("shmat failed\n");
        perror("shmat");
        return -1;
    }

    while(1)
    {
        sem_wait(semr);
        printf("name:%s\n", p->name);
        printf("age:%d\n", p->age);
        sem_post(semw);
    }

    //shmdt(p);
    return 0;
}

//写共享内存
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/types.h>
#include <semaphore.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <fcntl.h> //declare O_CREAT O_RDWR

typedef struct _Teacher
{
    char name[64];
    int age;
}Teacher;

int main()
{
    int shmid = -1;
    key_t key = 0x2234;
    Teacher *p = NULL;
    int count = 0;
    sem_t *semr = NULL, *semw = NULL;
    semr = sem_open("sem_r", O_CREAT | O_RDWR, 0666, 0);
    if (semr == SEM_FAILED )
    {
        printf("errno = %d\n", errno );
        return -1;
    }

    semw = sem_open("sem_w", O_CREAT | O_RDWR, 0666, 1 );
    if (semw == SEM_FAILED)
    {
        printf("errno = %d\n", errno );
        return -1;
    }
    
    shmid = shmget(key, sizeof(Teacher), 0666 | IPC_CREAT );
    if ( shmid == -1 )
    {
        perror("shmget");
        return -1;
    }

    p = (Teacher*)shmat(shmid, NULL, 0);
    if (p == (Teacher*)(-1))
    {
        perror("shmat");
        return -1;
    }

    while(1)
    {
        sem_wait(semw);
        //printf(">name:");
        strcpy(p->name, "aaaa");
        p->age = count;
        ++count;
        sem_post(semr);
    }
    return 0;
}

注意：编译上面的代码需要链接动态库-lpthread

2. 使用posix无名信号量进行同步

POSIX无名信号量是基于内存的信号量，可以用于线程间同步也可以用于进程间同步。若实现进程间同步，需要在共享内存中来创建无名信号量。

因此，共享内存需要定义以下的结构体：

typedef struct
{
    sem_t semr;
    sem_t semw;
    char buf[MAXSIZE];
}SHM;

3. 使用system V的信号灯实现同步

System V的信号灯是一个或者多个信号灯的一个集合。其中的每一个都是单独的计数信号灯。而Posix信号灯指的是单个计数信号灯。

System V 信号灯由内核维护，主要函数semget，semop，semctl 。

一个进程写，另一个进程读，信号灯集中有两个信号灯，下标0代表能否读，初始化为0。 下标1代表能否写，初始为1。
示例代码如下：

//进程A
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/types.h>
#include <sys/sem.h>
#include <semaphore.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>
#include <signal.h>
#include <fcntl.h> //declare O_CREAT O_RDWR

int shm_id, sem_id;
char* addr;

void ser_exit(int signo)
{
    semctl(sem_id, 0, IPC_RMID);
    semctl(sem_id, 1, IPC_RMID);
    shmdt(addr);
    shmctl(shm_id, IPC_RMID, NULL);

    printf("server exit ...\n");
    exit(0);
}

union semun
{
    int val;
    struct semid_ds *buf;
    unsigned short *array;
    struct seminfo *__buf;
};

int main()
{
    struct sigaction act;
    act.sa_handler = ser_exit;

    key_t shm_key = ftok("./readshm", 1);
    if (shm_key == -1 )
    {
        perror("ftok error");
        return -1;
    }

    int shm_id = shmget(shm_key, 1024, IPC_CREAT | IPC_EXCL | 0755);
    if (shm_id == -1)
    {
        perror("shmget");
        return -1;
    }
    
    char* addr = (char*)shmat(shm_id, NULL, 0);
    if (addr == (char*)(-1))
    {
        perror("shmat");
        return -1;
    }

    int sem_id = semget(shm_key, 2, IPC_CREAT|IPC_EXCL|0755);
    if (sem_id == -1 )
    {
        perror("semget");
        return -1;
    }

    union semun init;
    init.val = 0;

    semctl(sem_id, 0, SETVAL, init);
    semctl(sem_id, 1, SETVAL, init);

    struct sembuf v = {0, 1, SEM_UNDO};
    struct sembuf p = {1, -1, SEM_UNDO};

    sigaction(SIGINT, &act, NULL);
    while(1)
    {
        printf("ser:>");
        scanf("%s", addr);
        semop(sem_id, &v, 1);
        semop(sem_id, &p, 1);
        printf("cli:>%s\n", addr);
    }

    return 0;
}

//进程B
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/types.h>
#include <sys/sem.h>
#include <semaphore.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>
#include <signal.h>
#include <fcntl.h> //declare O_CREAT O_RDWR

union semun
{
    int val;
    struct semid_ds *buf;
    unsigned short *array;
    struct seminfo *__buf;
};

void cli_exit(int signo)
{
    printf("client exit ...\n");
    exit(0);
}

int main()
{
    struct sigaction act;
    act.sa_handler = cli_exit;

    key_t shm_key = ftok("./readshm", 1);
    if (shm_key == -1 )
    {
        perror("ftok error");
        return -1;
    }

    int shm_id = shmget(shm_key, 0, 0);
    if (shm_id == -1)
    {
        perror("shmget");
        return -1;
    }
    
    char* addr = (char*)shmat(shm_id, NULL, 0);
    if (addr == (char*)(-1))
    {
        perror("shmat");
        return -1;
    }

    int sem_id = semget(shm_key, 0, 0 );
    if (sem_id == -1 )
    {
        perror("semget");
        return -1;
    }

    struct sembuf v = {1, 1, SEM_UNDO};
    struct sembuf p = {0, -1, SEM_UNDO};

    sigaction(SIGINT, &act, NULL);
    while(1)
    {
        semop(sem_id, &p, 1);
        printf("ser:>%s\n", addr );

        printf("cli:>");
        scanf("%s", addr);
        semop(sem_id, &v, 1);
    }

    return 0;
}

4. 使用信号实现共享内存的同步

其实就是使用kill和signal发送信号来实现，这里不再实现。