C语言进程通过信号量进入临界区

#include<sys/sem.h>#include<unistd.h>#include<stdio.h>#include<stdlib.h>int res;    // 返回结果int sem_id;     //信号标识符union semun   //定义semun，不同的系统可能不同，需要自己定义{   int val;   struct semid_ds *buf;   unsigned short *array;   struct seminfo *__buf;};void sem_setval()  //初始化信号量{  union semun sem_union;  sem_union.val=1;  //信号量的初始值  res=semctl(sem_id,0,SETVAL,sem_union);  //初始化信号量       if(-1==res)  {     perror("semctl failed \n");     exit(0);  }}void sem_remove() //销毁信号量{  union semun sem_union;  res=semctl(sem_id,0,IPC_RMID,sem_union);  //销毁信号量  if(-1==res)  {    perror("sem remove control fail\n");    exit(0);  }}void sem_p() //减少信号量{   struct sembuf sema_buff;   sema_buff.sem_num=0;     //信号编号，信号量编号从0开始   sema_buff.sem_op=-1;      //对信号量的操作值，使信号量减1   sema_buff.sem_flg=SEM_UNDO; //让系统跟进这个信号量的修改  res=semop(sem_id,&sema_buff,1);//对信号量操作，如果信号量为0则等待。   if(-1==res)   {     perror("semop failed \n");     exit(0);   }}void sem_v()  //增加信号量{  struct sembuf sema_buff;  sema_buff.sem_num=0;  sema_buff.sem_op=1;  //使信号量加1  sema_buff.sem_flg=SEM_UNDO;  res=semop(sem_id,&sema_buff,1);  if(res==-1)  {    perror("semov failed\n");    exit(0);  }}int main(){   char CHR;  //这是本程序的临界变量，即一次只能一个进程或线程访问   sem_id=semget((key_t)1024,1,0666|IPC_CREAT);   if(sem_id<=0)   {     perror("semget failed\n");     exit(0);   }   sem_setval();   res=fork();   //创建新进程   if(res==-1)   {     perror("fork error\n");     exit(0);   }   if(res==0) //子进程   {      int i=0;      CHR='O';      for(i;i<=10;i++)      {       sem_p();       printf("%c",CHR);       fflush(stdout);       printf("%c",CHR);       fflush(stdout);       sleep(1);       sem_v();      }     exit(0);   }else{    //父进程      int i=0;      CHR='X';      for(i;i<=10;i++)      {       sem_p();       printf("%c",CHR);       printf("%c",CHR);       fflush(stdout);       sleep(1);       sem_v();      }     wait(NULL);     sem_remove();     exit(0);   }}