使用sem_post信号量进行线程同步

写了一小段程序，测试一下线程同步的问题，如下：

#include <stdio.h>

#include <string.h>

#include <semaphore.h>

#include <pthread.h>

#include <sys/types.h>

#include <unistd.h>



sem_t sp; 



int val = -1;

int semPost(sem_t * pSem, int nMaxCount)

{

    int nRet, nSemCount;



    sem_getvalue(pSem, &nSemCount);

    if (nSemCount>=nMaxCount)

    {

        return 0;

    }

    else

    {

        nRet=sem_post(pSem);

        return nRet;

    }

}



int semWait(sem_t * pSem)

{

    int nRet;

    nRet=sem_wait(pSem);



    while (sem_trywait(pSem)==0) {}

    return nRet;

}



void xxx()

{

    while(1)

    {

        semWait(&sp);

        printf("xxx  val  %d\n",val);

    }



}



void yyy()

{

    int i = 0;

    while(1)

    {

    //    semPost(&sp,3);

        printf("yyy :  %d\n",i);

        sleep(1);

        if(i++ >= 3)

        {

            i = 0;

            semPost(&sp,3);

            val = 9;

            printf("now xxx can run !!!\n");

        }

    }



}



int main(int argc,char **argv)

{

    pthread_t x;

    pthread_t y;



    sem_init(&sp,0,0);



    pthread_create(&x,NULL,(void *)xxx,NULL);

    pthread_create(&y,NULL,(void *)yyy,NULL);



    while(1)

    {

        sleep(1);

    }

    

    return 0;

}