Linux 从 UART 读取数据

Question

我想从 UART 读取数据，我关注了this tutorial，写入功能按预期工作，但是我遇到了读取功能的问题：

这是 uart_init 函数：

void uart_init()
{
 printf("\n +----------------------------------+");
 printf("\n |        Serial Port Write         |");
 printf("\n +----------------------------------+");

/*------------------------------- Opening the Serial Port -------------------------------*/

  fd = open("/dev/ttyUSB0",O_RDWR | O_NOCTTY| O_SYNC);      /* !!blocks the read  */
                                                            /* O_RDWR Read/Write access to serial port           */
                                                            /* O_NOCTTY - No terminal will control the process   */
                                                            /* O_NDELAY -Non Blocking Mode,Does not care about-  */
                                                            /* -the status of DCD line,Open() returns immediatly */                                        
                                
 if(fd == -1)                                               /* Error Checking */
  printf("\n  Error! in Opening ttyUSB0  ");
 else
  printf("\n  ttyUSB0 Opened Successfully ");


 /*---------- Setting the Attributes of the serial port using termios structure --------- */
    
struct termios SerialPortSettings;          /* Create the structure                          */

tcgetattr(fd, &SerialPortSettings);         /* Get the current attributes of the Serial port */

cfsetispeed(&SerialPortSettings,B19200);        /* Set Read  Speed as 19200                       */
cfsetospeed(&SerialPortSettings,B19200);        /* Set Write Speed as 19200                       */

SerialPortSettings.c_cflag &= ~PARENB;          /* Disables the Parity   Enable bit(PARENB),So No Parity   */
SerialPortSettings.c_cflag &= ~CSTOPB;          /* CSTOPB = 2 Stop bits,here it is cleared so 1 Stop bit */
SerialPortSettings.c_cflag &= ~CSIZE;           /* Clears the mask for setting the data size             */
SerialPortSettings.c_cflag |=  CS8;             /* Set the data bits = 8                                 */

SerialPortSettings.c_cflag &= ~CRTSCTS;         /* No Hardware flow Control                         */
SerialPortSettings.c_cflag |= CREAD | CLOCAL;   /* Enable receiver,Ignore Modem Control lines       */ 
    
    
SerialPortSettings.c_iflag &= ~(IXON | IXOFF | IXANY);          /* Disable XON/XOFF flow control both i/p and o/p */
SerialPortSettings.c_iflag &= ~(ICANON | ECHO | ECHOE | ISIG);  /* Non Cannonical mode                            */

SerialPortSettings.c_oflag &= ~OPOST;/*No Output Processing*/

/* Setting Time outs */
SerialPortSettings.c_cc[VMIN] = 10; /* Read at least 10 characters */
SerialPortSettings.c_cc[VTIME] = 0; /* Wait indefinetly   */

if((tcsetattr(fd,TCSANOW,&SerialPortSettings)) != 0) /* Set the attributes to the termios structure*/
 printf("\n  ERROR ! in Setting attributes");
else
 printf("\n  BaudRate = 19200 \n  StopBits = 1 \n  Parity   = none");
        
}

接收函数：

void uart_receive()
{
 char read_buffer[32];   /* Buffer to store the data received              */
 int  bytes_read = 0;    /* Number of bytes read by the read() system call */
 int i = 0;

 bytes_read = read(fd,&read_buffer,10); /* Read the data                   */
        
 printf("\n\n  Bytes Rxed %d", bytes_read); /* Print the number of bytes read */
 printf("\n\n  ");

 for(i=0;i<bytes_read;i++)   /*printing only the received characters*/
 printf("%c",read_buffer[i]);

 printf("\n +----------------------------------+\n\n\n");
}

主要功能：

void main(void)
{ 
  uart_init();
  /*------------------------------- Write data to serial port -----------------------------*/
  //uart_write_commande(write_buffer); //Write function works well
  uart_receive();

  close(fd);/* Close the Serial port */
}

我执行程序并等待 UART 接收到数据字节，我使用 UART 发送数据但读取功能一直被阻塞。

我正在使用带有 Ubunutu 14.04 的虚拟机，我不确定使用模拟 UART 是否会在接收期间引起问题。

Answer 1

您的程序在 read() 系统调用中挂起，因为它在等待行终止符时被阻塞。
您尝试使用以下语句将端口配置为非规范模式

SerialPortSettings.c_iflag &= ~(ICANON | ECHO | ECHOE | ISIG);  /* Non Cannonical mode  

但该操作是在错误的 termios 元素上进行的。
ICANON 属性是 lflag 元素的一部分（而不是 iflag）。 （此错误源于您引用的教程！）
因此，您的程序正在执行阻塞规范读取。

有一个方便的 termios 函数用于配置非规范模式：

   cfmakeraw()  sets the terminal to something like the "raw" mode of the old 
   Version 7 terminal driver: input is available character by
   character, echoing is disabled, and all special processing of  
   terminal  input  and  output  characters  is  disabled.   The  terminal
   attributes are set as follows:

       termios_p->c_iflag &= ~(IGNBRK | BRKINT | PARMRK | ISTRIP
                       | INLCR | IGNCR | ICRNL | IXON);
       termios_p->c_oflag &= ~OPOST;
       termios_p->c_lflag &= ~(ECHO | ECHONL | ICANON | ISIG | IEXTEN);
       termios_p->c_cflag &= ~(CSIZE | PARENB);
       termios_p->c_cflag |= CS8;

Answer 2

读取函数出错

bytes_read = read(fd,&read_buffer,10); /* Read the data 

应该是

bytes_read = read(fd,read_buffer,10); /* Read the data 

Answer 3

无论出于何种原因，您的 read() 函数都可能被阻止。 Here is a discussion 从串行端口读取，包括用于阻止/未阻止设置的代码。

也有可能没有数据正在传输，导致没有任何数据被读取。如果无法访问硬件设置，如果没有关于您所看到的非常具体的信息，就很难走得更远。

此外，除了正确传递 read_buffer（另一个答案）之外，至少还有两件事可以改进：

1）使用前检查read的返回：

bytes_read = read(fd,&read_buffer,10); /* Read the data*/
if(bytes_read > 0)
{
    ...
}

2) 改变：

for(i=0;i<bytes_read;i++)   /*printing only the received characters*/
 printf("%c",read_buffer[i]);

收件人：

//after successful read:
read_buffer[bytes_read]=0;//place null termination after last character read.
printf("%s",read_buffer);//note format specifier

这将打印读取的字符数，没有循环。