如何在C中从串口打开，读取和写入? [英] How to open, read, and write from serial port in C?

问题描述

我对读取和写入串行端口有点困惑.我在 Linux 中有一个 USB 设备，它使用 FTDI USB 串行设备转换器驱动程序.当我插入它时，它会创建:/dev/ttyUSB1.

I am a little bit confused about reading and writing to a serial port. I have a USB device in Linux that uses the FTDI USB serial device converter driver. When I plug it in, it creates: /dev/ttyUSB1.

我认为用 C 语言打开和读/写它很简单.我知道波特率和奇偶校验信息，但似乎没有标准?

I thought itd be simple to open and read/write from it in C. I know the baud rate and parity information, but it seems like there is no standard for this?

我是否遗漏了什么，或者有人能指出我正确的方向吗?

Am I missing something, or can someone point me in the right direction?

推荐答案

我很久以前写过这个 (从 1985 年到 1992 年，从那时起只进行了一些调整)，只需将所需的位复制并粘贴到每个项目中即可.

I wrote this a long time ago (from years 1985-1992, with just a few tweaks since then), and just copy and paste the bits needed into each project.

您必须在从 tcgetattr 获得的 tty 上调用 cfmakeraw.您不能将 struct termios 置零，配置它，然后使用 tcsetattr 设置 tty.如果您使用归零方法，那么您将遇到无法解释的间歇性故障，尤其是在 BSD 和 OS X 上.无法解释的间歇性故障"包括挂在 read(3) 中.

You must call cfmakeraw on a tty obtained from tcgetattr. You cannot zero-out a struct termios, configure it, and then set the tty with tcsetattr. If you use the zero-out method, then you will experience unexplained intermittent failures, especially on the BSDs and OS X. "Unexplained intermittent failures" include hanging in read(3).

#include <errno.h>
#include <fcntl.h> 
#include <string.h>
#include <termios.h>
#include <unistd.h>

int
set_interface_attribs (int fd, int speed, int parity)
{
        struct termios tty;
        if (tcgetattr (fd, &tty) != 0)
        {
                error_message ("error %d from tcgetattr", errno);
                return -1;
        }

        cfsetospeed (&tty, speed);
        cfsetispeed (&tty, speed);

        tty.c_cflag = (tty.c_cflag & ~CSIZE) | CS8;     // 8-bit chars
        // disable IGNBRK for mismatched speed tests; otherwise receive break
        // as 00 chars
        tty.c_iflag &= ~IGNBRK;         // disable break processing
        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_iflag &= ~(IXON | IXOFF | IXANY); // shut off xon/xoff ctrl

        tty.c_cflag |= (CLOCAL | CREAD);// ignore modem controls,
                                        // enable reading
        tty.c_cflag &= ~(PARENB | PARODD);      // shut off parity
        tty.c_cflag |= parity;
        tty.c_cflag &= ~CSTOPB;
        tty.c_cflag &= ~CRTSCTS;

        if (tcsetattr (fd, TCSANOW, &tty) != 0)
        {
                error_message ("error %d from tcsetattr", errno);
                return -1;
        }
        return 0;
}

void
set_blocking (int fd, int should_block)
{
        struct termios tty;
        memset (&tty, 0, sizeof tty);
        if (tcgetattr (fd, &tty) != 0)
        {
                error_message ("error %d from tggetattr", errno);
                return;
        }

        tty.c_cc[VMIN]  = should_block ? 1 : 0;
        tty.c_cc[VTIME] = 5;            // 0.5 seconds read timeout

        if (tcsetattr (fd, TCSANOW, &tty) != 0)
                error_message ("error %d setting term attributes", errno);
}


...
char *portname = "/dev/ttyUSB1"
 ...
int fd = open (portname, O_RDWR | O_NOCTTY | O_SYNC);
if (fd < 0)
{
        error_message ("error %d opening %s: %s", errno, portname, strerror (errno));
        return;
}

set_interface_attribs (fd, B115200, 0);  // set speed to 115,200 bps, 8n1 (no parity)
set_blocking (fd, 0);                // set no blocking

write (fd, "hello!
", 7);           // send 7 character greeting

usleep ((7 + 25) * 100);             // sleep enough to transmit the 7 plus
                                     // receive 25:  approx 100 uS per char transmit
char buf [100];
int n = read (fd, buf, sizeof buf);  // read up to 100 characters if ready to read

速度值分别为B115200、B230400、B9600、B19200、B38400code>、B57600、B1200、B2400、B4800等，校验位为0(无奇偶校验)、PARENB|PARODD(开启奇偶校验使用奇数)、PARENB(开启奇偶校验使用偶数)、PARENB|PARODD|CMSPAR(标记奇偶校验)和 PARENB|CMSPAR(空间奇偶校验).

The values for speed are B115200, B230400, B9600, B19200, B38400, B57600, B1200, B2400, B4800, etc. The values for parity are 0 (meaning no parity), PARENB|PARODD (enable parity and use odd), PARENB (enable parity and use even), PARENB|PARODD|CMSPAR (mark parity), and PARENB|CMSPAR (space parity).

"Blocking" 设置端口上的 read() 是否等待指定数量的字符到达.设置无阻塞意味着 read() 返回多少可用字符而无需等待更多字符，直到缓冲区限制.

"Blocking" sets whether a read() on the port waits for the specified number of characters to arrive. Setting no blocking means that a read() returns however many characters are available without waiting for more, up to the buffer limit.

附录:

CMSPAR 仅用于选择标记和空间奇偶校验，这是不常见的.对于大多数应用程序，它可以省略.我的头文件 /usr/include/bits/termios.h 仅在定义了预处理器符号 __USE_MISC 时才启用 CMSPAR 的定义.该定义与

CMSPAR is needed only for choosing mark and space parity, which is uncommon. For most applications, it can be omitted. My header file /usr/include/bits/termios.h enables definition of CMSPAR only if the preprocessor symbol __USE_MISC is defined. That definition occurs (in features.h) with

#if defined _BSD_SOURCE || defined _SVID_SOURCE
 #define __USE_MISC     1
#endif

的介绍性注释说:

/* These are defined by the user (or the compiler)
   to specify the desired environment:

...
   _BSD_SOURCE          ISO C, POSIX, and 4.3BSD things.
   _SVID_SOURCE         ISO C, POSIX, and SVID things.
...
 */

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