#include <sys/types.h>

#include <sys/ipc.h>

#include <sys/sem.h>

int semget(key_t key, int nsems, int semflg);

返回值：成功信号量ID，出错-

key：函数ftok返回值或IPC_PRIVATE（适合用在有亲缘关系的进程中）

nsems：新的信号量集合中要创建的信号量个数，如果不是新创建的为0

semflg：　　0取信号量集标识符，若不存在则函数会报错

　　　　　　IPC_CREAT：当semflg&IPC_CREAT为真时，如果不存在与key值相等的信号量集，则创建。否则返回此信号量集的标识符。

　　　　　　IPC_CREAT|IPC_EXCL：如果不存在与key值相等的信号量集，则创建。否则存在这样的信号集报错

#include <sys/types.h>

#include <sys/ipc.h>

#include <sys/sem.h>

int semctl(int semid, int semnum, int cmd, ...);
除了GETALL以外的所有命令，成功0失败-1
semid：信号量标识符
semnum：信号量集数组上的下标，表示某个信号量
cmd：下面有说明
arg：semnum联合体

union semun {

               int              val;    /* Value for SETVAL */

               struct semid_ds *buf;    /* Buffer for IPC_STAT, IPC_SET */

               unsigned short  *array;  /* Array for GETALL, SETALL */

               struct seminfo  *__buf;  /* Buffer for IPC_INFO

                                           (Linux-specific) */

           };

union semun

struct semid_ds {

               struct ipc_perm sem_perm;  /* Ownership and permissions */

               time_t          sem_otime; /* Last semop time */

               time_t          sem_ctime; /* Last change time */

               unsigned long   sem_nsems; /* No. of semaphores in set */

           };

struct semid_ds

下列10种命令

IPC_STAT        对此集合取semid_ds结构，并存储在arg.buf指向的结构中

IPC_SET          设置一个信号量集合的semid_ds结构中ipc_perm域的值，并从semun的buf中取出值

IPC_RMID       从内核中删除信号量集合

GETALL           从信号量集合中获取所有信号量的值，并存到semun中的指针数组

GETNCNT        返回当前等待100%自远离用的进程个数

GETPID          返回最后执行semop的进程PID

GETVAL          返回信号量集合内单个信号量的值

GETZCNT         返回当前等待100%资源利用的进程个数

SETALL            与GETALL正好相反

SETVAL            用联合体中val成员设置信号量集合中单个信号的值

cmd指令

#include <sys/types.h>

#include <sys/ipc.h>

#include <sys/sem.h>

int semop(int semid, struct sembuf *sops, size_t nsops);
返回值：成功0出错-1
semid：信号量标识符
sops：是指向结构体数组的指针
nsops：操作结构体的数量，恒大于或等于1

struct sembuf {

           unsigned short sem_num;  /* semaphore number */

           short          sem_op;   /* semaphore operation */

           short          sem_flg;  /* operation flags */

}

struct sembuf

 #include <stdio.h>

 #include <sys/types.h>

 #include <sys/ipc.h>

 #include <sys/sem.h>

 int semid = ;

 union semun {

     int val;

     struct semid_ds *buf;

     unsigned short *arry;

 };

 int set_semvalue()

 {

     union semun sem_union;

     sem_union.val = ;

     if(semctl(semid, , SETVAL, sem_union) == -)

         return -;

     return ;

 }

 void del_semvalue()

 {

     union semun sem_union;

     if(semctl(semid, , IPC_RMID, sem_union) == -)

         fprintf(stderr, "Failed to delete semaphore\n");

 }

 int P()

 {

     struct sembuf sem_b;

     sem_b.sem_num = ;

     sem_b.sem_op = -;

     sem_b.sem_flg = SEM_UNDO;

     if(semop(semid, &sem_b, ) == -)

     {

         fprintf(stderr, "P() failed\n");

         return -;

     }

     return ;

 }

 int V()

 {

     struct sembuf sem_b;

     sem_b.sem_num = ;

     sem_b.sem_op = ;

     sem_b.sem_flg = SEM_UNDO;

     if(semop(semid, &sem_b, ) == -)

     {

         fprintf(stderr, "V() failed\n");

         return -;

     }

     returi ;

 }

 int main(int argc, char *argv[])

 {

     char msg = 'X';

     int i=;

     if(argc >  && !strcmp(argv[], ""))

     {

         /* first user sem */

         if(set_semvalue() == -)

         {

             fprintf(stderr, "Failed to initialize semaphore\n");

             return -;

         }

         msg = argv[][];

         sleep();

     }

     semid = semget((key_t), , |IPC_CREAT);

     if(semid == -)

     {

         printf("semget error\n");

         return -;

     }

     for(i=;i<;i++)

     {

         if(P() == -)

             return -;

         printf("%c", msg);

         fflush(stdout);

         sleep(rand()%);

         printf("%c", msg);

         fflush(stdout);

         if(V() == -)

             return -;

         sleep(rand()%);

     }

     sleep();

     printf("\n%d - finished\n", getpid());

     if(argc > )

         del_semvalue();

     return ;

 }

 #include <stdio.h>

 #include <stdlib.h>

 #include <sys/types.h>

 #include <unistd.h>

 int main(int argc, char *argv[])

 {

     char msg = 'X';

     int i =;

     if(argc >)

         msg = argv[][];

     for(i=;i<;i++)

     {

         printf("%c", msg);

         fflush(stdout);

         sleep(rand()%);

         printf("%c", msg);

         fflush(stdout);

         sleep(rand()%);

     }

     sleep();

     printf("\n%d - finished\n", getpid());

     return ;

 }

./a.out  &./a.out