如何通知select()立即返回？

#include<stdio.h>
#include <unistd.h>
#include <sys/select.h>

int main() {
    int pipefd[2];
    char buf;

    // 创建管道
    pipe(pipefd);

    fd_set readfds;
    struct timeval timeout;

    // 初始化文件描述符集合
    FD_ZERO(&readfds);
    FD_SET(pipefd[0], &readfds);

    // 设置超时时间
    timeout.tv_sec = 5;
    timeout.tv_usec = 0;

    // 调用select()函数
    int ret = select(pipefd[0] + 1, &readfds, NULL, NULL, &timeout);

    if (ret == -1) {
        perror("select");
        return 1;
    } else if (ret == 0) {
        printf("select() timed out\n");
    } else {
        // 如果select()函数返回，说明管道中有数据可读
        read(pipefd[0], &buf, 1);
        printf("received signal: %c\n", buf);
    }

    return 0;
}

char signal = 'a';
write(pipefd[1], &signal, 1);