deadlock

[Alexander Bubnov]

Many thanks!

Mikko Tyo"la"ja"rvi wrote:

>
> Hi Alexander,
>
> On Fri, 26 Nov 2004, 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
>
>
> Not needed. Also, it does not set errno, it returns the error value.
> Use strerror().
>
>>
>> /* 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;
>> }
>
>
> Silently wondering why on earth anybody would want to use SYSV
> semaphores to synchronize threads in the same program, I compiled and
> ran your code on some different boxes:
>
> FreeBSD 5.3, i386 - runs ok (?)
> FreeBSD 5.3, sparc64 - hangs after first line of output
> Linux 2.6/Debian 3.1, i386 - hangs after last line of output
> Solaris 9, sparc64 - dumps core in semctl
>
> I believe your program perhaps is not 100% correct... :-)
>
> $.02,
> /Mikko
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