使用system V实现读者写者问题

 #include <stdio.h>
 #include <sys/sem.h>
 #include <sys/ipc.h>
 #include <string.h>
 #include <errno.h>
 #include <unistd.h>
 #include <pthread.h>
 #include <sys/types.h>
 #include <sys/wait.h>

 typedef int semophore;
 semophore mutex = ;
 semophore cnt = ;
 semophore db = ;
 //char mutex[] = "mutex";
 //char db[] = "db";
 int semId;
 void read(int cnt)
 {
     fprintf(stdout, "NO.%d reading...\n",cnt);
     //for (int i = 0; i < 1000000; i++);
     sleep();
     fprintf(stdout, "NO.%d read finished.\n",cnt);

 }
 void write()
 {
     fprintf(stdout, "writing...\n");
     //for (int i = 0; i < 1000000; i++);
     sleep();
     fprintf(stdout, "write finished.\n");
 }

 union semun {
     int val;
     struct semid_ds *buf;
     unsigned short int *array;
     struct seminfo *__buf;
 };
 /*
 struct  sembuf{                     //结构定义在 linux/sem.h，无需重复定义
     unsigned short   sem_num;     //信号在信号集中的索引，0代表第一个信号，1代表第二个信号
     short          sem_op;     //操作类型
     short          sem_flg;   //操作标志
 };
 */

 /*
 void p(const char *s)
 {
     /*
     semun arg;
     int tmp;
     int r;
     if (strcmp(s, mutex) == 0)
     {
         //fprintf(stdout,"mutex:%d\n",semctl(semId, 0, GETVAL, arg));
         while(1){
             if(semctl(semId, 0, GETVAL, arg)<=0)
                 wait(&r);
             else break;
         }
         tmp = semctl(semId, 0, GETVAL, arg);
         //if(tmp<=0) waitpid();
         arg.val = tmp - 1;
         semctl(semId, 0, SETVAL, arg);
     }
     else if (strcmp(s, db) == 0)
     {
         //fprintf(stdout,"db:%d\n",semctl(semId, 1, GETVAL, arg));
         while(1){
             if(semctl(semId, 1, GETVAL, arg)<=0)
                 wait(&r);
             else break;
         }
         tmp = semctl(semId, 1, GETVAL, arg);
         arg.val = tmp - 1;
         semctl(semId, 1, SETVAL, arg);
     }

     sembuf arg;
     arg.sem_op = 1;
     arg.sem_flg = IPC_UNDO;
     if(strcmp(s, mutex)==0)
     {
         arg.sem_num = 0;
         semop(semId,&arg,1);
     }
     else if(strcmp(s, db)==0)
     {
         arg.sem_num = 1;
         semop(semId,&arg,1);
     }

 }
 */
 /*
 void v(const char *s)
 {
     /*
     semun arg;
     int tmp;
     if (strcmp(s, mutex) == 0)
     {
         tmp = semctl(semId, 0, GETVAL, arg);
         arg.val = tmp + 1;
         semctl(semId, 0, SETVAL, arg);
     }
     else if (strcmp(s, db) == 0)
     {
         tmp = semctl(semId, 1, GETVAL, arg);
         arg.val = tmp + 1;
         semctl(semId, 1, SETVAL, arg);
     }

     sembuf arg;
     arg.sem_op = -1;
     arg.sem_flg = IPC_UNDO;
     if(strcmp(s, mutex)==0)
     {
         arg.sem_num = 0;
         semop(semId,&arg,1);
     }
     else if(strcmp(s, db)==0)
     {
         arg.sem_num = 1;
         semop(semId,&arg,1);
     }
 }
 */

 //P操作函数
 int p(int index)
 {
     struct sembuf buf = {, -};

     if (index < )
     {
         perror("index of array cannot equals a minus value!\n");
         return -;
     }
     buf.sem_num = index;
     if (semop(semId, &buf, ) == -)
     {
         perror(" a wrong operation to semaphore occurred!\n");
         return -;
     }
     return ;
 }

 //V操作函数
 int v(int index)
 {
     struct sembuf buf = {, };

     if (index < )
     {
         perror("index of array cannot equals a minus value!\n");
         return -;
     }
     buf.sem_num = index;
     if (semop(semId, &buf, ) == -)
     {
         perror(" a wrong operation to semaphore occurred!\n");
         return -;
     }
     return ;
 }
 void* reader(void* args)
 {
     p(mutex);
     cnt++;
     if (cnt == )
     {
         p(db);
     }
     v(mutex);

     read(cnt);

     p(mutex);
     cnt--;
     if (cnt == )
     {
         v(db);
     }
     v(mutex);
 }
 void* writer(void* args)
 {
     p(db);
     write();
     v(db);
 }

 int main()
 {
     semun arg;

     key_t key = ;
     if ((key = ftok(".", )) == -)
     {
         perror("ftok error:");
         _exit();
     }
     semctl(semId, , IPC_RMID, arg);

     //semid = semget(key, 1, IPC_CREAT|0660);

     if ((semId = semget(key, , IPC_CREAT | IPC_EXCL | )) >= )
     {
         arg.val = ;
         if (semctl(semId, , SETVAL, arg) < )
         {
             fprintf(stdout, "semctl error %s\n", strerror(errno));
             return -;
         }
         if (semctl(semId, , SETVAL, arg) < )
         {
             fprintf(stdout, "semctl error %s\n", strerror(errno));
             return -;
         }

     }
     else if (errno == EEXIST)
     {
         semId = semget(key, , );
         //fprintf(stdout,"Taskr :: errno==EEXIST\n");
         arg.val = ;
         if (semctl(semId, , SETVAL, arg) < )
         {
             fprintf(stdout, "semctl error %s\n", strerror(errno));
             return -;
         }
         if (semctl(semId, , SETVAL, arg) < )
         {
             fprintf(stdout, "semctl error %s\n", strerror(errno));
             return -;
         }
     }
     else
     {
         fprintf(stdout, "semget error %s\n", strerror(errno));
         return -;
     }
     pthread_t tids[];
     for(int i = ; i < ; ++i)
     {

         //参数依次是：创建的线程id，线程参数，调用的函数，传入的函数参数
         if(i== || i== || i==)
         {
             fprintf(stdout,"turn NO.%d want to read.\n",i);
             int ret = pthread_create(&tids[i], NULL, reader, NULL);
             if (ret != )
             {
                 fprintf(stdout, "reading error\n");
             }
         }
         else{
             fprintf(stdout,"turn NO.%d want to write.\n",i);
             int ret = pthread_create(&tids[i], NULL, writer, NULL);
             if (ret != )
             {
                 fprintf(stdout, "writing error\n");
             }
         }
     }

     pthread_exit(NULL);
     return ;
 }

结果如图（读者优先）：

非常感谢 @神一城 老师的指点，之前自己没有学清楚 system V 中关于信号量的内容，使用了 semctl 这种直接赋值的操作，其实这样和直接使用 int 变量再加一些条件判断实现一样，而这样就等同于没有原子性，完全没有体现出信号量的作用，只是把信号量当作了一个普通变量。

这里向之前受到错误博客内容误导的朋友道歉。