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Full Discussion: sem_wait, sem_post confusion?
Top Forums Programming sem_wait, sem_post confusion? Post 302582761 by gabam on Sunday 18th of December 2011 04:18:28 AM
Old 12-18-2011
sem_wait, sem_post confusion?
Hi friends,
I have written this small program using the concept of semaphores. I am a bit confused, hope u can help me with it.
The idea is that

1. Two threads are created
2. First will be displaying 0(zero) on the screen
3. Second will be displaing 1(one) on the screen
4. This process will go on upto three times for each thread, it would look like

Code:
 
0
 
1
 
0
 
1
 
0
 
1

But the program that i have written prints like this
0

0

0

1

1

1

I don't really know what I am missing here, could you pin point it to me, I think my concept of semaphores is corrct.
You can have a look at my code

Code:
 
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <pthread.h>
#include <sys/sem.h>
#include <semaphore.h>
sem_t mysem;
void *thread(void *);
int main()
{
        sem_init(&mysem,0,1);
        pthread_t tid[2];
 
        pthread_attr_t attr[2];
        pthread_attr_init(&attr[0]);
        pthread_attr_init(&attr[1]);
        {
        pthread_create(&tid[0],&attr[0],thread,(void *)0);
        pthread_create(&tid[1],&attr[1],thread,(void *)1);
        }
        {
        pthread_join(tid[0],NULL);
        pthread_join(tid[1],NULL);
        }
        printf("\nThreads finished!\n");
        return 0;
}
void *thread(void *n)
{
        int i;
        i = (int)n;
        int x = 0;
        while(x < 3)
        {
        sem_wait(&mysem);
        printf("%d\n\n",i);
        sem_post(&mysem);
        x++;
        }
        pthread_exit(0);
}

But the same program works when I put a sleep(1) in the middle like this

Code:
void *thread(void *n)
{
        int i;
        i = (int)n;
        int x = 0;
        while(x < 3)
        {
        sem_wait(&mysem);
        printf("%d\n\n",i);
        sem_post(&mysem);
        x++;
        sleep(1);
        }
        pthread_exit(0);
}

But I want to achieve what I want to achieve by using only semaphores, no sleep function,

Thanks
Looking forward to your lovely replies.
 

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LEARN ABOUT CENTOS

pthread_attr_setdetachstate

PTHREAD_ATTR_SETDETACHSTATE(3)				     Linux Programmer's Manual				    PTHREAD_ATTR_SETDETACHSTATE(3)

NAME

       pthread_attr_setdetachstate, pthread_attr_getdetachstate - set/get detach state attribute in thread attributes object

SYNOPSIS

       #include <pthread.h>

       int pthread_attr_setdetachstate(pthread_attr_t *attr, int detachstate);
       int pthread_attr_getdetachstate(pthread_attr_t *attr, int *detachstate);

       Compile and link with -pthread.

DESCRIPTION

       The pthread_attr_setdetachstate() function sets the detach state attribute of the thread attributes object referred to by attr to the value
       specified in detachstate.  The detach state attribute determines whether a thread created using the thread attributes object attr  will	be
       created in a joinable or a detached state.

       The following values may be specified in detachstate:

       PTHREAD_CREATE_DETACHED
	      Threads that are created using attr will be created in a detached state.

       PTHREAD_CREATE_JOINABLE
	      Threads that are created using attr will be created in a joinable state.

       The default setting of the detach state attribute in a newly initialized thread attributes object is PTHREAD_CREATE_JOINABLE.

       The  pthread_attr_getdetachstate()  returns  the  detach  state	attribute of the thread attributes object attr in the buffer pointed to by
       detachstate.

RETURN VALUE

       On success, these functions return 0; on error, they return a nonzero error number.

ERRORS

       pthread_attr_setdetachstate() can fail with the following error:

       EINVAL An invalid value was specified in detachstate.

CONFORMING TO

       POSIX.1-2001.

NOTES

       See pthread_create(3) for more details on detached and joinable threads.

       A thread that is created in a joinable state should eventually either be joined using pthread_join(3) or detached using	pthread_detach(3);
       see pthread_create(3).

       It  is  an  error  to  specify  the  thread  ID	of  a  thread that was created in a detached state in a later call to pthread_detach(3) or
       pthread_join(3).

EXAMPLE

       See pthread_attr_init(3).

SEE ALSO

       pthread_attr_init(3), pthread_create(3), pthread_detach(3), pthread_join(3), pthreads(7)

COLOPHON

       This page is part of release 3.53 of the Linux man-pages project.  A description of the project, and information about reporting bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2010-02-03					    PTHREAD_ATTR_SETDETACHSTATE(3)
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