CSAPP中8.5.5那个信号处理的例程

Question

#include "csapp.h"
#define BUF_SIZE 16

void handler_sigchld(int sig);

int main()
{
    int i;
    char *buf = (char *)malloc(BUF_SIZE * sizeof(char));

    signal(SIGCHLD, handler_sigchld);

    for(i = 0; i < 3; i++){
        if(Fork() == 0){
            printf("Child process %d\n", (int)getpid());
            sleep((i + 1) * 5);
            exit(0);
        }
    }

    while(read(STDIN_FILENO, buf, BUF_SIZE)){
        printf("Parent process running, got the input: %s\n", buf);
        memset(buf, '\0', BUF_SIZE);
    }

    free(buf);
    return 0;
}

void handler_sigchld(int sig)
{
    int pid, olderrno = errno;

    while((pid = waitpid(-1, NULL, 0)) > 0){
        printf("Got process %d's SIGCHLD\n", pid);
    }

    if(errno == ECHILD){
        printf("done.\n");
    }

    errno = olderrno;
}

以上是我小改的例程代码,下面是运行结果:

我的疑问是:那个处理子进程结束信号的函数handler_sigchld被调用了3次,为什么只有2次正常的结束(也就是只输出了2次done.)?