信号量应用

#include <linux/sem.h> 
 
#include <stdio.h> 
 
#include <errno.h> 
 
#define SEM_PATH "/home/my_sem" 
 
#define max_tries 3  
 
 
 
int semid; 
 
main() 
 
{ 
 
int flag1,flag2,key,i,init_ok,tmperrno; 
 
struct semid_ds sem_info; 
 
struct seminfo sem_info2; 
 
union semun arg;    //union semun£º Çë²Î¿ŒžœÂŒ2 
 
struct sembuf askfor_res, free_res; 
 
flag1=IPC_CREAT|IPC_EXCL|00666; 
 
flag2=IPC_CREAT|00666; 
 
key=ftok(SEM_PATH,''a''); 
 
//error handling for ftok here; 
 
init_ok=0; 
 
semid=semget(key,1,flag1);//create a semaphore set that only includes one semphore. 
 
if(semid<0) 
 
{ 
 
 tmperrno=errno; 
 
 perror("semget"); 
 
if(tmperrno==EEXIST) 
 
//errno is undefined after a successful library call( including perror call) so it is saved //in tmperrno. 
 
  { 
 
  semid=semget(key,1,flag2); 
 
//flag2 Ö»°üº¬ÁËIPC_CREAT±êÖŸ, ²ÎÊýnsems(ÕâÀïΪ1)±ØÐëÓëÔ­ÀŽµÄÐźŵÆÊýÄ¿Ò»Ö 
 
  arg.buf=&sem_info; 
 
  for(i=0; i<max_tries; i  ) 
 
  { 
 
   if(semctl(semid, 0, IPC_STAT, arg)==-1) 
 
   { perror("semctl error"); i=max_tries;} 
 
   else 
 
   {  
 
    if(arg.buf->sem_otime!=0){ i=max_tries;  init_ok=1;} 
 
    else  sleep(1);  
 
   } 
 
  } 
 
  if(!init_ok) 
 
  // do some initializing, here we assume that the first process that creates the sem will  
 
  // finish initialize the sem and run semop in max_tries*1 seconds. else it will not run  
 
  // semop any more. 
 
  { 
 
   arg.val=1; 
 
   if(semctl(semid,0,SETVAL,arg)==-1) perror("semctl setval error"); 
 
  }  
 
 } 
 
 else 
 
 {perror("semget error, process exit");  
 
 exit(1);  
 
 } 
 
} 
 
else //semid>=0; do some initializing   
 
{ 
 
 arg.val=1; 
 
 if(semctl(semid,0,SETVAL,arg)==-1) 
 
  perror("semctl setval error"); 
 
} 
 
//get some information about the semaphore and the limit of semaphore in redhat8.0 
 
 arg.buf=&sem_info; 
 
 if(semctl(semid, 0, IPC_STAT, arg)==-1) 
 
  perror("semctl IPC STAT");   
 
 printf("owner''s uid is %d\n",  arg.buf->sem_perm.uid); 
 
 printf("owner''s gid is %d\n",  arg.buf->sem_perm.gid); 
 
 printf("creater''s uid is %d\n",  arg.buf->sem_perm.cuid); 
 
 printf("creater''s gid is %d\n",  arg.buf->sem_perm.cgid); 
 
 
 
 arg.__buf=&sem_info2; 
 
 if(semctl(semid,0,IPC_INFO,arg)==-1) 
 
  perror("semctl IPC_INFO"); 
 
 printf("the number of entries in semaphore map is %d \n",    arg.__buf->semmap); 
 
 printf("max number of semaphore identifiers is %d \n",     arg.__buf->semmni); 
 
 printf("mas number of semaphores in system is %d \n",     arg.__buf->semmns); 
 
 printf("the number of undo structures system wide is %d \n",   arg.__buf->semmnu); 
 
 printf("max number of semaphores per semid is %d \n",     arg.__buf->semmsl); 
 
 printf("max number of ops per semop call is %d \n",     arg.__buf->semopm); 
 
 printf("max number of undo entries per process is %d \n",     arg.__buf->semume); 
 
 printf("the sizeof of struct sem_undo is %d \n",         arg.__buf->semusz); 
 
 printf("the maximum semaphore value is %d \n",      arg.__buf->semvmx); 
 
  
 
//now ask for available resource:  
 
 askfor_res.sem_num=0; 
 
 askfor_res.sem_op=-1; 
 
 askfor_res.sem_flg=SEM_UNDO;   
 
   
 
  if(semop(semid,&askfor_res,1)==-1)//ask for resource 
 
   perror("semop error"); 
 
  
 
 sleep(3); //do some handling on the sharing resource here, just sleep on it 3 seconds 
 
 printf("now free the resource\n");  
 
  
 
//now free resource  
 
 free_res.sem_num=0; 
 
 free_res.sem_op=1; 
 
 free_res.sem_flg=SEM_UNDO; 
 
 
 
 if(semop(semid,&free_res,1)==-1)//free the resource. 
 
  if(errno==EIDRM) 
 
   printf("the semaphore set was removed\n"); 
 
//you can comment out the codes below to compile a different version:    
 
 if(semctl(semid, 0, IPC_RMID)==-1) 
 
  perror("semctl IPC_RMID"); 
 
 else printf("remove sem ok\n"); 
 
} 

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