如何打開，讀取和寫入串行端口C

Question

我對讀取和寫入串行端口有點困惑。我在Linux中有一個使用FTDI USB串行設備轉換器驅動程序的USB設備。當我插入時，它會創建：/ dev/ttyUSB1。

我認爲itd很容易在C中打開並讀寫它。我知道波特率和奇偶校驗信息，但似乎沒有這個標準嗎？

我是否錯過了某些東西，或者有人能指引我朝着正確的方向嗎？

Answer 1

我很久以前寫了這個，只是將需要的位複製並粘貼到每個項目中。

#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; 
     memset (&tty, 0, sizeof 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 \000 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!\n", 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 

速度的值B115200，B230400，B9600，B19200，B38400，B57600，B1200，B2400，B4800等奇偶值是0（意味着沒有奇偶校驗），PARENB|PARODD（啓用奇偶校驗和使用奇數），PARENB（啓用奇偶校驗和使用偶數），PARENB|PARODD|CMSPAR（標記奇偶校驗）和PARENB|CMSPAR（空間奇偶校驗）。

「Blocking」設置端口上的read()是否等待指定數量的字符到達。設置沒有阻止意味着read()返回，但許多字符都可用，無需等待更多，直到達到緩衝區限制。

---

附錄：

CMSPAR只需要爲選擇標記和空間奇偶校驗，這是罕見的。對於大多數應用程序，可以省略。我的頭文件/usr/include/bits/termios.h只有在定義了預處理器符號__USE_MISC時才能定義CMSPAR。這一定義時（在features.h）與

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

的<features.h>的開場白說：

/* 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. 
... 
*/ 

Answer 2

對於爲Setting Terminal Modes Properly 和Serial Programming Guide for POSIX Operating Systems描述，下面是提供符合POSIX標準的演示代碼。
它基本上是從其他答案得出的，但不正確和誤導性的評論已被糾正。

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

int set_interface_attribs(int fd, int speed) 
{ 
    struct termios tty; 

    if (tcgetattr(fd, &tty) < 0) { 
     printf("Error from tcgetattr: %s\n", strerror(errno)); 
     return -1; 
    } 

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

    tty.c_cflag |= (CLOCAL | CREAD); /* ignore modem controls */ 
    tty.c_cflag &= ~CSIZE; 
    tty.c_cflag |= CS8;   /* 8-bit characters */ 
    tty.c_cflag &= ~PARENB;  /* no parity bit */ 
    tty.c_cflag &= ~CSTOPB;  /* only need 1 stop bit */ 
    tty.c_cflag &= ~CRTSCTS; /* no hardware flowcontrol */ 

    /* setup for non-canonical mode */ 
    tty.c_iflag &= ~(IGNBRK | BRKINT | PARMRK | ISTRIP | INLCR | IGNCR | ICRNL | IXON); 
    tty.c_lflag &= ~(ECHO | ECHONL | ICANON | ISIG | IEXTEN); 
    tty.c_oflag &= ~OPOST; 

    /* fetch bytes as they become available */ 
    tty.c_cc[VMIN] = 1; 
    tty.c_cc[VTIME] = 1; 

    if (tcsetattr(fd, TCSANOW, &tty) != 0) { 
     printf("Error from tcsetattr: %s\n", strerror(errno)); 
     return -1; 
    } 
    return 0; 
} 

void set_mincount(int fd, int mcount) 
{ 
    struct termios tty; 

    if (tcgetattr(fd, &tty) < 0) { 
     printf("Error tcgetattr: %s\n", strerror(errno)); 
     return; 
    } 

    tty.c_cc[VMIN] = mcount ? 1 : 0; 
    tty.c_cc[VTIME] = 5;  /* half second timer */ 

    if (tcsetattr(fd, TCSANOW, &tty) < 0) 
     printf("Error tcsetattr: %s\n", strerror(errno)); 
} 


int main() 
{ 
    char *portname = "/dev/ttyUSB0"; 
    int fd; 
    int wlen; 

    fd = open(portname, O_RDWR | O_NOCTTY | O_SYNC); 
    if (fd < 0) { 
     printf("Error opening %s: %s\n", portname, strerror(errno)); 
     return -1; 
    } 
    /*baudrate 115200, 8 bits, no parity, 1 stop bit */ 
    set_interface_attribs(fd, B115200); 
    //set_mincount(fd, 0);    /* set to pure timed read */ 

    /* simple output */ 
    wlen = write(fd, "Hello!\n", 7); 
    if (wlen != 7) { 
     printf("Error from write: %d, %d\n", wlen, errno); 
    } 
    tcdrain(fd); /* delay for output */ 


    /* simple noncanonical input */ 
    do { 
     unsigned char buf[80]; 
     int rdlen; 

     rdlen = read(fd, buf, sizeof(buf) - 1); 
     if (rdlen > 0) { 
#ifdef DISPLAY_STRING 
      buf[rdlen] = 0; 
      printf("Read %d: \"%s\"\n", rdlen, buf); 
#else /* display hex */ 
      unsigned char *p; 
      printf("Read %d:", rdlen); 
      for (p = buf; rdlen-- > 0; p++) 
       printf(" 0x%x", *p); 
      printf("\n"); 
#endif 
     } else if (rdlen < 0) { 
      printf("Error from read: %d: %s\n", rdlen, strerror(errno)); 
     } 
     /* repeat read to get full message */ 
    } while (1); 
} 

爲了使程序將接收到的數據作爲ASCII碼處理，使用符號DISPLAY_STRING（例如，

cc -DDISPLAY_STRING demo.c