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。