使用eventfd进行同步时无法获取锁定

Question

I am trying to use eventfd for synchronization b/w two threads. Please look below code. In this if main thread acquires a lock first it wont release unless I uncomment sleep after write function. It is true if thread gets the lock first. Please let me know how to handle without using sleep after write.

#include <pthread.h>
#include <stdint.h> 
#include <stdio.h>
int event_fd;
uint64_t counter;
void * Thread1(void *p)
{
    printf("\n%s eventfd = %d\n",(char*)p, event_fd);

    while(1)
    {
        read(event_fd, &counter,sizeof(counter));
        printf("\n %s function counter = %llu\n",(char*)p,counter);
        sleep(1);
        write(event_fd,&counter,sizeof(counter));
        //sleep(1);     
     }

    return NULL;
}

void main()
{
    char str[]="In Thread1";
    int ret;

    pthread_t p_th;
    printf("Events demonstration pid = %d sizeof counter %lu\n ",getpid(),sizeof(counter));
    event_fd=eventfd(1,0);
    printf("event_fd %d\n",event_fd);
    pthread_create(&p_th,NULL,Thread1, str);    
    while(1)
    {
        read(event_fd, &counter,sizeof(counter));
        printf("\n In main function counter = %llu\n",counter);
        sleep(1);
        write(event_fd,&counter,sizeof(counter));   
        //sleep(1); 
    }

    pthread_exit (NULL);
}

Answer 1

Firstly, they aren't locks. They are eventfd semaphores.

Secondly, they contain no provision for fairness amongst threads. So if you omit the sleep() clause, the semaphore will be grabbed again the moment you release it by the next read . eventfd is more often used in a producer consumer environment where one end is doing the write and the other the read .