在 Linux 上用 C 语言读写串口 [英] Reading and writing to serial port in C on Linux

问题描述

我正在尝试使用 FTDI 通过 USB 端口发送/接收数据，因此我需要使用 C/C++ 处理串行通信.我正在使用 Linux (Ubuntu).

I'm trying to send/receive data over an USB Port using FTDI, so I need to handle serial communication using C/C++. I'm working on Linux (Ubuntu).

基本上，我连接到一个正在侦听传入命令的设备.我需要发送这些命令并读取设备的响应.命令和响应都是 ASCII 字符.

Basically, I am connected to a device which is listening for incoming commands. I need to send those commands and read device's response. Both commands and response are ASCII characters.

使用 GtkTerm 一切正常，但是当我切换到 C 编程时，我遇到了问题.

Everything works fine using GtkTerm but, when I switch to C programming, I encounter problems.

这是我的代码:

#include <stdio.h>      // standard input / output functions
#include <stdlib.h>
#include <string.h>     // string function definitions
#include <unistd.h>     // UNIX standard function definitions
#include <fcntl.h>      // File control definitions
#include <errno.h>      // Error number definitions
#include <termios.h>    // POSIX terminal control definitions

/* Open File Descriptor */
int USB = open( "/dev/ttyUSB0", O_RDWR| O_NONBLOCK | O_NDELAY );

/* Error Handling */
if ( USB < 0 )
{
cout << "Error " << errno << " opening " << "/dev/ttyUSB0" << ": " << strerror (errno) << endl;
}

/* *** Configure Port *** */
struct termios tty;
memset (&tty, 0, sizeof tty);

/* Error Handling */
if ( tcgetattr ( USB, &tty ) != 0 )
{
cout << "Error " << errno << " from tcgetattr: " << strerror(errno) << endl;
}

/* Set Baud Rate */
cfsetospeed (&tty, B9600);
cfsetispeed (&tty, B9600);

/* Setting other Port Stuff */
tty.c_cflag     &=  ~PARENB;        // Make 8n1
tty.c_cflag     &=  ~CSTOPB;
tty.c_cflag     &=  ~CSIZE;
tty.c_cflag     |=  CS8;
tty.c_cflag     &=  ~CRTSCTS;       // no flow control
tty.c_lflag     =   0;          // no signaling chars, no echo, no canonical processing
tty.c_oflag     =   0;                  // no remapping, no delays
tty.c_cc[VMIN]      =   0;                  // read doesn't block
tty.c_cc[VTIME]     =   5;                  // 0.5 seconds read timeout

tty.c_cflag     |=  CREAD | CLOCAL;     // turn on READ & ignore ctrl lines
tty.c_iflag     &=  ~(IXON | IXOFF | IXANY);// turn off s/w flow ctrl
tty.c_lflag     &=  ~(ICANON | ECHO | ECHOE | ISIG); // make raw
tty.c_oflag     &=  ~OPOST;              // make raw

/* Flush Port, then applies attributes */
tcflush( USB, TCIFLUSH );

if ( tcsetattr ( USB, TCSANOW, &tty ) != 0)
{
cout << "Error " << errno << " from tcsetattr" << endl;
}

/* *** WRITE *** */

unsigned char cmd[] = {'I', 'N', 'I', 'T', ' ', '
', ''};
int n_written = write( USB, cmd, sizeof(cmd) -1 );

/* Allocate memory for read buffer */
char buf [256];
memset (&buf, '', sizeof buf);

/* *** READ *** */
int n = read( USB, &buf , sizeof buf );

/* Error Handling */
if (n < 0)
{
     cout << "Error reading: " << strerror(errno) << endl;
}

/* Print what I read... */
cout << "Read: " << buf << endl;

close(USB);

read() 返回 0(根本没有读取字节)或阻塞直到超时(VTIME).我假设发生这种情况是因为 write() 没有发送任何内容.在这种情况下，设备不会收到命令，我也无法收到响应.事实上，在我的程序被阻止读取时关闭设备实际上成功地获得了响应(设备在关闭时发送了一些东西).

What happens is that read() returns 0 (no bytes read at all) or block until timeout (VTIME). I'm assuming this happens because write() does not send anything. In that case, device wouldn't receive command and I cannot receive response. In fact, turning off the device while my program is blocked on reading actually succeded in getting a response (device sends something while shutting down).

奇怪的是添加了这个

cout << "I've written: " << n_written << "bytes" << endl; 

在 write() 调用之后，我收到:

right after write() call, I receive:

I've written 6 bytes

这正是我所期望的.只有我的程序无法正常工作，例如 我的设备无法接收我实际在端口上写入的内容.

which is exactly what I expect. Only my program doesn't work as it should, like my device cannot receive what I'm actually writing on port.

我尝试了不同的方法和解决方案，也涉及数据类型(我尝试使用 std::string，例如 cmd = "INIT " 或 const char) 但没有任何效果.

I've tried different things and solution, also regarding data types (I've tried using std::string, such as cmd = "INIT " or const char) but nothing really worked.

谁能告诉我哪里错了?

提前谢谢你.

此代码的先前版本使用了

Previously version of this code used

unsigned char cmd[] = "INIT "

unsigned char cmd[] = "INIT "

还有 cmd[] = "INIT ".我更改了它，因为我的设备的命令 sintax 报告为

and also cmd[] = "INIT ". I changed it because command sintax for my device is reported as

<command><SPACE><CR>.

我也尝试在读取时避免使用 O_NONBLOCK 标志，但我只会一直阻塞.我试过使用 select() 但没有任何反应.只是为了尝试，我创建了一个等待循环，直到数据可用，但我的代码永远不会退出循环.顺便说一句，等待或 usleep() 是我需要避免的.报告的只是我的代码的摘录.完整的代码需要在实时环境中运行(特别是 OROCOS)，所以我真的不想要类似睡眠的功能.

I've also tried avoiding the O_NONBLOCK flag on reading, but then I only block until forever. I've tried using select() but nothing happens. Just for a try, I've created a waiting loop until data is avaliable, but my code never exit the loop. Btw, waiting or usleep() is something I need to avoid. Reported one is only an excerpt of my code. Complete code needs to work in a real-time environment (specifically OROCOS) so I don't really want sleep-like function.

推荐答案

我已经解决了我的问题，所以我在这里发布正确的代码以防有人需要类似的东西.

I've solved my problems, so I post here the correct code in case someone needs similar stuff.

开放端口

int USB = open( "/dev/ttyUSB0", O_RDWR| O_NOCTTY );

设置参数

struct termios tty;
struct termios tty_old;
memset (&tty, 0, sizeof tty);

/* Error Handling */
if ( tcgetattr ( USB, &tty ) != 0 ) {
   std::cout << "Error " << errno << " from tcgetattr: " << strerror(errno) << std::endl;
}

/* Save old tty parameters */
tty_old = tty;

/* Set Baud Rate */
cfsetospeed (&tty, (speed_t)B9600);
cfsetispeed (&tty, (speed_t)B9600);

/* Setting other Port Stuff */
tty.c_cflag     &=  ~PARENB;            // Make 8n1
tty.c_cflag     &=  ~CSTOPB;
tty.c_cflag     &=  ~CSIZE;
tty.c_cflag     |=  CS8;

tty.c_cflag     &=  ~CRTSCTS;           // no flow control
tty.c_cc[VMIN]   =  1;                  // read doesn't block
tty.c_cc[VTIME]  =  5;                  // 0.5 seconds read timeout
tty.c_cflag     |=  CREAD | CLOCAL;     // turn on READ & ignore ctrl lines

/* Make raw */
cfmakeraw(&tty);

/* Flush Port, then applies attributes */
tcflush( USB, TCIFLUSH );
if ( tcsetattr ( USB, TCSANOW, &tty ) != 0) {
   std::cout << "Error " << errno << " from tcsetattr" << std::endl;
}

写

unsigned char cmd[] = "INIT 
";
int n_written = 0,
    spot = 0;

do {
    n_written = write( USB, &cmd[spot], 1 );
    spot += n_written;
} while (cmd[spot-1] != '
' && n_written > 0);

绝对没有必要逐字节写入，int n_written = write(USB, cmd, sizeof(cmd) -1) 工作正常.

It was definitely not necessary to write byte per byte, also int n_written = write( USB, cmd, sizeof(cmd) -1) worked fine.

最后，阅读:

int n = 0,
    spot = 0;
char buf = '';

/* Whole response*/
char response[1024];
memset(response, '', sizeof response);

do {
    n = read( USB, &buf, 1 );
    sprintf( &response[spot], "%c", buf );
    spot += n;
} while( buf != '
' && n > 0);

if (n < 0) {
    std::cout << "Error reading: " << strerror(errno) << std::endl;
}
else if (n == 0) {
    std::cout << "Read nothing!" << std::endl;
}
else {
    std::cout << "Response: " << response << std::endl;
}

这个对我有用.谢谢大家！

This one worked for me. Thank you all!

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