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»ÖÂ

  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
",  arg.buf->sem_perm.uid);

 printf("owner's gid is %d
",  arg.buf->sem_perm.gid);

 printf("creater's uid is %d
",  arg.buf->sem_perm.cuid);

 printf("creater's gid is %d
",  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 
",    arg.__buf->semmap);

 printf("max number of semaphore identifiers is %d 
",     arg.__buf->semmni);

 printf("mas number of semaphores in system is %d 
",     arg.__buf->semmns);

 printf("the number of undo structures system wide is %d 
",   arg.__buf->semmnu);

 printf("max number of semaphores per semid is %d 
",     arg.__buf->semmsl);

 printf("max number of ops per semop call is %d 
",     arg.__buf->semopm);

 printf("max number of undo entries per process is %d 
",     arg.__buf->semume);

 printf("the sizeof of struct sem_undo is %d 
",         arg.__buf->semusz);

 printf("the maximum semaphore value is %d 
",      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
");



//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
");

//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
");

}

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