Beaglebone Black 无法从 C++ 中的 UART RX 引脚读取

Question

我目前正在尝试将串行数据从 ESP8266 NodeMCU 模块读取到 Beaglebone Black。本质上，我触发 NodeMCU 通过 UART 从 BBB 发送字符串并将字符串打印到 BBB 终端。以下是BBB上的代码：

#include<iostream>
#include<fstream>
#include<string>
#include<unistd.h>
#include<termios.h>
#include<stdio.h>
#include<fcntl.h>
using namespace std;

int main(){
   std::fstream fs;

   // Setup UART
   int file, count;
   if ((file = open("/dev/ttyO4", O_RDWR | O_NOCTTY | O_NDELAY))<0){
      perror("UART: Failed to open the file.\n");
      return -1;
   }
   struct termios options; // the termios structure is vital
   tcgetattr(file, &options); // sets the parameters associated with file
   // Set up the communications options:
   // 115200 baud, 8-bit, enable receiver, no modem control lines
   options.c_cflag = B115200 | CS8 | CREAD | CLOCAL;
   options.c_iflag = IGNPAR | ICRNL; // ignore partity errors, CR -> newline
   tcflush(file, TCIFLUSH); // discard file information not transmitted
   tcsetattr(file, TCSANOW, &options); // changes occur immmediately

   fs.open("/sys/class/gpio/export");
   fs << "48";
   fs.close();
   fs.open("/sys/class/gpio/gpio48/direction");
   fs << "out";
   fs.close();
   fs.open("/sys/class/gpio/gpio48/value");
   fs << "1"; // "0" for off
   fs.close();
   usleep(1000000);
   fs.open("/sys/class/gpio/gpio48/value");
   fs << "0";
   fs.close();

   unsigned char receive[1000]; // declare a buffer for receiving data
   int characters = read(file, (void*)receive, 1000);
   if (characters == -1){ // receive the data
      perror("Failed to read from the input\n");
      return -1;
   }
   if (characters==0) printf("There was no data available to read!\n");
   else {
      receive[characters] = 0;
      printf("%s\n",receive);
   }
   close(file);

   return 0;
}

NodeMCU上的代码如下：

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200, SERIAL_8N1);
  Serial.println();

  
  // Set D2 as input
  pinMode(4, INPUT);
}

void loop() {
  // put your main code here, to run repeatedly:
  if(digitalRead(4)==1){
    String msg = "Hola!";
    while(digitalRead(4)==1){
      
    }
    Serial.print(msg);
  }
}

当我运行 C 脚本时，我收到以下错误：

Failed to read from the input
: Resource temporarily unavailable

但是，我可以在 ttyO4 上的 BBB 上打开 Minicom 终端和“Hola！”字符串按预期打印一次。

我该如何克服这个错误？

Answer 1

已修复：

我在打开 ttyO4 端口的 if 语句下插入了这一行：

fcntl(file, F_SETFL, 0);

我还将Serial.print(msg); 行放在了while 循环上方的NodeMCU 代码中。