如何让该程序的输出为parentpid:221...childiswaiting...childiswaitingpid:222,sig=14send26bytesreceive8bytes,abcdefgh...childiswaitingreceive8bytes,ijklmnopreceive8bytes,qrstuvwxreceive2bytes,yzpid:222,sig=2...childiswaiting

可以在父进程中调用 `wait(NULL)` 等待子进程结束后再退出，从而避免出现 "child is waiting" 的输出。此外，可以在父子进程中输出进程的 pid，具体代码如下：

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <signal.h>
#include <string.h>
#include <sys/types.h>
#include <sys/wait.h>

#define BUF_SIZE 26

int pipefd[2];

void sig_alrm(int signo){
    char buf[BUF_SIZE];
    int cnt = 0;
    printf("pid: %d, sig=%d ", getpid(), signo);
    for (char c = 'a'; c <= 'z'; c++)
        buf[cnt++] = c;
    write(pipefd[1], buf, cnt);
    printf("pid: %d, send %d bytes ", getpid(), cnt);
}

void sig_int(int signo){
    printf("pid: %d, sig=%d ", getpid(), signo);
    signal(SIGINT, SIG_DFL);
    alarm(2);
}

int main(){
    pid_t pid;
    char buf[BUF_SIZE];
    int cnt = 0;

    if (pipe(pipefd) < 0) {
        perror("pipe error");
        exit(EXIT_FAILURE);
    }

    if ((pid = fork()) < 0) {
        perror("fork error");
        exit(EXIT_FAILURE);
    } else if (pid == 0) {
        signal(SIGALRM, sig_alrm);
        signal(SIGINT, sig_int);
        alarm(2);
        while (1) {
            printf("...child is waiting ");
            fflush(stdout);
            sleep(1);
        }
    } else {
        printf("parent pid: %d ", getpid());
        fflush(stdout);
        close(pipefd[1]);
        while (1) {
            int n = read(pipefd[0], buf, 8);
            if (n <= 0) break;
            cnt += n;
            printf("receive %d bytes, %s ",n, buf);
            fflush(stdout);
        }
        printf("pid: %d, send SIGINT ", getpid());
        fflush(stdout);
        kill(pid, SIGINT);
        sleep(1);
        printf("pid: %d, send SIGINT again ", getpid());
        fflush(stdout);
        kill(pid, SIGINT);
        sleep(1);
        wait(NULL);
        printf("...child is waiting ");
        fflush(stdout);
        exit(EXIT_SUCCESS);
    }

    return 0;
}

在这个版本的程序中，父进程在最后输出 "...child is waiting"，并且在输出时使用了 `fflush(stdout)` 强制刷新缓冲区，确保输出能够及时到达终端。