deadlock again

[Alexander Bubnov]

Many thanks!

I see, nevertheless why is my program deadlocked? You wanted to said 
this problem from race condition? But why does it work correctly under 
some UNIX, UNIX-like OS?


         
David Xu wrote:

> Your program has obvious race condition:

> main: create thread A:
>
> LOOP:
>
> A: wait on Start
> main: signal Start
> main: wait on Restart
> A: resumed from Start
> A: Signal Restart
> main: resumed from Restart
> main: Create thread B
> goto loop
>
> This demo shows FreeBSD has better throughput then others.
>
> David Xu
>
> Alexander Bubnov wrote:
>
>> /*
>> Hello!
>> Could you tell me why this exemple is deadlocked under FreeBSD, not 
>> Linux?
>> (FreeBSD 4.10 and 5.2 , I have not tried under other versions)
>>
>> a string that includes this problem marked, see below the main function
>>
>> */
>> #include<sys/types.h>
>> #include<sys/ipc.h>
>> #include<sys/sem.h>
>> #include<errno.h>
>> #include<pthread.h>
>> #include<stdlib.h>
>> #include<stdio.h>
>>
>> int Exit=0;/* uses in hand(), changes in main() */
>> int ReStart,Start;/* semaphores ID which returns semget() */
>> /*
>>    Start semphore controls starting threads,
>>    EXETHREADS - how many threads can start in one time
>>       ReStart semaphore for wait when the starting threads display a 
>> string
>> */
>> struct sembuf dec={0},inc={0},op={0};/* they initilize in the main 
>> function*/
>> void*hand(void*p){
>>    do{
>>        semop(Start,&dec,1);
>>        if(Exit)break;
>>        puts(p);
>>        semop(ReStart,&inc,1);
>>    }while(1);
>>    fprintf(stderr,"bye, %s!\n",p);
>>    return NULL;
>> }
>>
>> #define MAXTHREADS 5
>> #define EXETHREADS 2 /* number of threads which is executed in one 
>> time*/
>> #define ITERAT 2 /* number of iterations, see the main function */
>>
>> int main(void){
>>    char*s[MAXTHREADS]={"Mike","Leo","Don","Raph","Splinter"};
>>    int i;
>>    pthread_t thread[MAXTHREADS];
>>    /*  to initlize semaphores operations */
>>    dec.sem_op=-1,inc.sem_op=1,op.sem_op=EXETHREADS;
>>       if( -1==(Start=semget('A',1,0666|IPC_CREAT)) ||
>>            -1==(ReStart=semget('B',1,0666|IPC_CREAT))
>>        ){perror("semget");return EXIT_FAILURE;}
>>       /* to initilize semaphores value */
>>    semctl(Start,0,SETVAL,0);
>>    semctl(ReStart,0,SETVAL,0);
>>    #ifndef linux    /* may be this string is not needed, I do not know*/
>>    if(pthread_setconcurrency(MAXTHREADS+1))perror("concurrency");
>> #endif
>>
>>    /* to create the threads */
>>    for(i=0;i<MAXTHREADS;++i)
>>        pthread_create(thread+i,NULL,hand,s[i]);
>>          for(i=0;i<ITERAT;++i){
>>        puts("____START____");
>>        semop(Start,&op,1);
>>        /* to wait EXETHREADS threads */
>>        op.sem_op*=-1;
>>        /*
>>            !!! this problem is here !!!
>>                       if I insert 'pause()' or 'while(1)' instend of 
>> 'semop(ReStart,&op,1)'
>>            that only one thread is executed two time, why?
>>                       this code does not help too (but I can see 
>> names that is printed by hand())
>>            op.sem_flg=IPC_NOWAIT
>>            while(-1==semop(ReStart,&op,1) && EAGAIN);
>>                   */
>>        semop(ReStart,&op,1);
>>        op.sem_op*=-1;
>>               puts("____FINISH____");
>>    }
>>          /* to wait the threads to exit*/
>>    semctl(Start,0,SETVAL,MAXTHREADS);
>>    Exit=1;
>>    for(i=0;i<MAXTHREADS;++i)
>>        pthread_join(thread[i],NULL);
>>      if(    -1==semctl(ReStart,0,IPC_RMID) ||
>>            -1==semctl(Start,0,IPC_RMID)
>>        ) perror("semdel");
>>           return EXIT_SUCCESS;
>> }
>>
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>>
>
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