Linux信号量的用法

基本函数：

//semaphore function





int create_sem(key_t key, int members)

{

    int cntr;

    union semun semopts;

    int semid;

    

    jprintf("Attempting to create new semaphore set with %d members/n",members);

    if((semid = semget(key, members, IPC_CREAT|IPC_EXCL|0666)) == -1)

    {

        jprintf("Semaphore set already exists!/n");

        return -1;

    }

    semopts.val = 1;    //最大的资源个数，这里就是互斥锁的功能，所以就是 0 | 1 , 二值信号量



    /* Initialize all members (could be done with SETALL) */

    for(cntr=0; cntr 0) {

        jprintf("it has been unlocked , I did nothing /n");

        return 0;

    }

/* Attempt to lock the semaphore set */

    if((semop(semid, &sem_unlock, 1)) == -1)

    {

        fprintf(stderr, "Unlock failed/n");

        return -1;

    }

    else

        printf("Semaphore resources incremented by one (unlocked)/n");

    semval = semctl(semid, member, GETVAL, 0);

    jprintf("semval for member %d is %d/n", member, semval);

    jprintf("----------------------------unlock sem --------------------------------/n");

    return 0;

}

---

例子：

/**

 * query those files which are downloading from share memory to get status info

 * generally , conf_name is /etc/shm.conf

    for CGI , should have a version for Socket query

    int QueryShmForClient(DownloadFileInfoList *DList ,const char *conf_name , const char *client_ip);

    

 **/

int QueryShm(DownloadFileInfoList *DList, const char *conf_name) //etc/shm.conf



{

    char ptr[128];

    FILE *fp = NULL;

    

    int fd = 0;

    char *seg = NULL;

    key_t key;

    int shmid;    //share memory ID



    int semid;    //semaphore ID



    void *segptr = NULL;

    int downloading_num = 0;

    int rc = 0;

    

    assert(DList);

    fp = fopen(conf_name,"rt");

    if(!fp) {

        fatal_err("can't open /etc/shm.conf/n");

        LogMesg("can't open /etc/shm.conf , maybe /etc/shm.conf doesn't exist any longer /n");

        

        return SERVER_CANNOT_OPEN_SHM_CONF;

    }

    assert(fp);

    fd = fileno(fp);

    //lockf(fd,F_LOCK,0);



    readw_lock(fd,0,SEEK_SET,0);

    while(fgets(ptr,sizeof(ptr),fp)) {    //get a line



        DownloadFileInfo *doingJob_entry = (DownloadFileInfo *)malloc(sizeof(DownloadFileInfo));

        

        //assert(doingJob_entry);



        if(!doingJob_entry) {

            fatal_err("can't malloc DownloadFileInfo struct entry /n");

            continue;

        }

                

        jprintf("ptr = %s/n",ptr);

        seg = strrchr(ptr,':');

        if(seg == NULL) {

            jprintf("can't find /':/' in %s , maybe this line in shm.conf has problem ,please check %s/n",ptr,SHM_CONF);

            continue;

        }

        *seg = '/0';

        //now ptr should be fullpathname ,ie, /harddisk/hd~2/data/public/bob/linux-2.4.21.tar.gz



        

        //---------------------------Start reading share memory----------------------------



        key = ftok(ptr,0);

        

    // ----------------------------------semaphore mutex lock -------------------------------------    



    #if 0

        semid = open_sem(key);    //get a ID of a existing semaphore set



        if(semid < 0) {

            fatal_err("get semaphore failure , job name (%s) /n",ptr);

            continue;

        }

        rc = lock_sem(semid ,0);

        if(rc < 0) {

            fatal_err("Sorry ,lock failed for(%s) /n",ptr);

            continue;

        }

        jprintf("haha , you have get the mutex lock , you can read the share memory segment !/n");

    #endif    

    // ----------------------------------------------------------------------------------------------



        

             if((shmid = shmget(key, SEGSIZE, 0444)) == -1) {

                    fatal_err("can't get share memory ,key(%s) /n",key);

                    LogMesg("the key (%s) of file (%s) 's share memory looks that has problem(shmget error ) ,please check /n",key,SHM_CONF);

                      //return SERVER_SHMGET_FAIL;



                      continue;

                }

          else {

            jprintf("shmid = %d/n",shmid);

                    jprintf("has attached memory segment/n");

          }

        /* Attach (map) the shared memory segment into the current process */

        if((segptr = shmat(shmid, 0, 0)) == NULL) {

            fatal_err("can't shmat share memory ,key (%s) /n",key);

            LogMesg("the key (%s) of file (%s) 's share memory looks that has problem(shmat error ) ,please check /n",key,SHM_CONF);

            //return SERVER_SHMAT_ERROR;



            continue;

        }

        //memcpy(doingJob_entry,(DownloadFileInfo *)segptr,sizeof(DownloadFileInfo));    ////fill the doingJob_entry , if problem ,single assign ,bob 2005-12-29 10:51



        *doingJob_entry = *((DownloadFileInfo *)segptr);

        shmdt(segptr);

        

    //unlock the mutex lock



    #if 0

        unlock_sem(semid,0);

    #endif

        AddDownloadFileInfoList(DList,doingJob_entry);    //add the node into the List



        downloading_num++;

    } //endof while(fgets(ptr,sizeof(ptr),fp))



    

    //lockf(fd,F_ULOCK,0);



    un_lock(fd,0,SEEK_SET,0);

    fclose(fp);

    if(downloading_num==0) {

        jprintf("there is no job downloading in share memory /n");

    }

    else {

        jprintf("there is %d files downloading /n",downloading_num);

    }

    

    return 0;

}