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。
