linux串口编程read为什么不阻塞

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/select.h>
#include <errno.h>

#define SERIAL_PORT "/dev/ttyS0"
#define BUFFER_SIZE 1024

int main() {
    int fd;
    char buffer[BUFFER_SIZE];
    fd_set readfds;
    struct timeval timeout;

    // 打开串口设备
    fd = open(SERIAL_PORT, O_RDWR | O_NOCTTY | O_NDELAY);
    if (fd == -1) {
        perror("Failed to open serial port");
        return 1;
    }

    // 设置非阻塞模式
    fcntl(fd, F_SETFL, FNDELAY);

    while (1) {
        FD_ZERO(&readfds);
        FD_SET(fd, &readfds);

        // 设置超时时间为1秒
        timeout.tv_sec = 1;
        timeout.tv_usec = 0;

        int ret = select(fd + 1, &readfds, NULL, NULL, &timeout);
        if (ret == -1) {
            perror("select error");
            break;
        } else if (ret == 0) {
            printf("Timeout occurred, no data after 1 second.\n");
        } else {
            if (FD_ISSET(fd, &readfds)) {
                int bytesRead = read(fd, buffer, BUFFER_SIZE);
                if (bytesRead > 0) {
                    buffer[bytesRead] = '\0';
                    printf("Received data: %s\n", buffer);
                } else if (bytesRead == 0) {
                    printf("End of file or device disconnected.\n");
                    break;
                } else {
                    perror("read error");
                    break;
                }
            }
        }
    }

    close(fd);
    return 0;
}