Child Process zombie OR pipe block error. 如果父进程没有主动回收子进程的return status且父进程没有退出, 子进程在exit后进行zombie状态;进程间建立了pipe,一方关闭将导致另一方无法通信且默认会block

linux_fork_pipe_waitpid_error_demo.c

#include <unistd.h>

#include <sys/types.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>

int main()
{
    int pipe_fd[2];
    pid_t pid;
    char r_buf[10];
    char test[10];
    int res;

    strcpy(test,"just test");

    memset(r_buf,0,sizeof(r_buf));

    if(pipe(pipe_fd)<0)
    {
        printf("pipe create error ");
        return -1;
    }
    int readpipe = pipe_fd[0];
    int writepipe = pipe_fd[1];

    if((pid=fork())>0)//父进程：解析从管道中获取的命令，并作相应的处理
    {
        close(writepipe);
        //read(readpipe,r_buf,10);//default options will block this invoke
        //close(readpipe);//If closed, the child will block @ write
        while(1){
            printf("Self=%d,Child=%d\n",getpid(),pid);
            sleep(20);
            waitpid(pid, &res, WNOHANG);//Without this line, the child return status will be lost, and the child will convert to zombie process
        }
    } else if(pid==0) {
        //child: send commands to parent
        printf("create child process successfully\n");
        close(readpipe);
        printf("Child Sleep 10s\n");
        sleep(1);
        printf("Self=%d,Parent=%d\n",getpid(),getppid());
        res=write(writepipe,test,sizeof(test));//Will block if its parent's readpipe is closed before this write
        printf("res=%d,%s\n",res,strerror(errno));
        close(writepipe);//Even if the child has exit, but the parent does NOT get return value, this child will be zombied
    }
    return 0;
}