为什么父进程卡在sem_wait（＆s2）中，而s2被子进程使用sem_post（＆s2）解锁？

Question

我要测试两个进程之间的信号量，为什么父进程没有在sem_wait(&s2之后输入代码，请注意我已经在子进程中使用sem_post(&s2)发出了s2信号。

include <linux/unistd.h>
#include <pthread.h>
#include <stdlib.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <string.h>
#include <time.h>
#include <sys/time.h>
#include <sys/wait.h>
#include <fcntl.h>
#include <math.h>
#include <semaphore.h>
sem_t s1,s2;

int value;

int main(){

    sem_init(&s1,1,1);

    sem_init(&s2,1,0);

    pid_t   childpid;

    int generate[100];

    if((childpid = fork()) == -1)
    {
            perror("fork");
            exit(1);
    }

    if(childpid == 0)
    {
        while(1)
        {
            printf("im child befor enter critical section\n");
            sem_wait(&s1);

            printf("im child inside crtical section\n");
            generate[0]=1;
            generate[1]=2;

            sem_getvalue(&s2,&value);
            printf("value of s2 befor post is %d\n",value);
            sem_post(&s2);
            sem_getvalue(&s2,&value);
            printf("im child exiting after post parent,the value after post s2 is %d \n",value);

        }

    }
    else
    { 
        while(1)
        {    

            printf("im parent befor enter critical section\n");

            sem_wait(&s2);
            printf("im parent inside critical section\n"); 

            printf("Received integer: %d\n", generate[0]);
            printf("Received integer: %d\n", generate[1]);
            sem_post(&s1);
            printf("im parent exit after post child \n");

        }
    }
    return(0);
}

这是输出：

im parent befor enter critical section

im child befor enter critical section

im child inside crtical section

value of s2 befor post is 0

im child exiting after post parent,the value after post s2 is 1 

im child befor enter critical section