c - 如何避免阻塞读取功能？

Question

我正在尝试从 cygwin 命令中心的 USB 端口写入和读取数据。我已经设法在连接设备时写入和读取数据，但我想要一种方法来检测其他设备是否不存在或无法发回任何数据。我当前的测试代码如下所示(我尝试了很多不同的东西，但似乎没有任何效果)。

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <termios.h>
#include <stdlib.h>
#include <strings.h>
#include <string.h>
#include <stdio.h>
#include "USB_com.h"
#include "unistd.h"
void main()
{
  int fd, value, bytes_read, bytes_written, nbytes, i, j;
  char buffR[20];
  char buffS[20];
  fd = USB_init("/dev/com1");
  printf("enter a message (write exit to terminate the session): ");
  fgets(buffS, 19, stdin);
  while (strncmp("exit", buffS, 4) != 0)
  {
    bytes_written = write(fd, buffS, 19);
    sleep(1);
    bytes_read = read(fd, buffR, 19);
    printf("string recieved : %s\n", buffR);
    memset(buffS, '\0', 19);
    printf("enter a message (write exit to terminate the session): ");
    fgets(buffS, 19, stdin);
  }
  USB_cleanup(fd);
}

我的 USB_init.c 用于写入和读取 USB 设备如下所示。

#include "USB_init.h"

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <termios.h>
#include <stdlib.h>
#include <strings.h>
#include <stdio.h>


/* baudrate settings are defined in <asm/termbits.h>, which is
 * included by <termios.h> */
#ifndef BAUDRATE
#define BAUDRATE B9600
#endif

#define _POSIX_SOURCE 1     /* POSIX compliant source */

static int fd, c, res;
static struct termios oldtio, newtio;
static char *device;

int USB_init(char *modemdevice)
{
    /* 
     * Open modem device for reading and writing and not as controlling tty
     * because we don't want to get killed if linenoise sends CTRL-C.
     **/
    device = modemdevice;
    //fd = open (device, O_RDWR | O_NOCTTY | O_NDELAY);
    fd = open (device, O_RDWR | O_NOCTTY );
    if (fd < 0)
      {
      perror (device);
      exit(-1);
      }

    tcgetattr (fd, &oldtio);    /* save current serial port settings */
    bzero (&newtio, sizeof (newtio));   /* clear struct for new port settings */

    /* 
     *BAUDRATE: Set bps rate. You could also use cfsetispeed and cfsetospeed.
     *CRTSCTS : output hardware flow control (only used if the cable has
     *all necessary lines. )
     *CS8     : 8n1 (8bit,no parity,1 stopbit)
     *CLOCAL  : local connection, no modem contol
     *CREAD   : enable receiving characters
     **/
    newtio.c_cflag = BAUDRATE | CS8 | CLOCAL | CREAD;

    /*
     *IGNPAR  : ignore bytes with parity errors
     *ICRNL   : map CR to NL (otherwise a CR input on the other computer
     *          will not terminate input)
     *          otherwise make device raw (no other input processing)
     **/
    newtio.c_iflag = IGNPAR | ICRNL;

    /*
     * Map NL to CR NL in output.
     *                  */
#if 0
    newtio.c_oflag = ONLCR;
#else
    newtio.c_oflag = 0;
#endif


    /*
     * ICANON  : enable canonical input
     *           disable all echo functionality, and don't send signals to calling program
     **/
#if 1
    newtio.c_lflag = ICANON;
#else
    newtio.c_lflag = 0;
#endif

    /* 
     * initialize all control characters 
     * default values can be found in /usr/include/termios.h, and are given
     * in the comments, but we don't need them here
     *                                       */
    newtio.c_cc[VINTR] = 0; /* Ctrl-c */
    newtio.c_cc[VQUIT] = 0; /* Ctrl-\ */
    newtio.c_cc[VERASE] = 0;    /* del */
    newtio.c_cc[VKILL] = 0; /* @ */
    newtio.c_cc[VEOF] = 4;  /* Ctrl-d */
    newtio.c_cc[VTIME] = 0; /* inter-character timer unused*/
    newtio.c_cc[VMIN] = 1;  /* blocking read until 1 character arrives*/
    newtio.c_cc[VSWTC] = 0; /* '\0' */
    newtio.c_cc[VSTART] = 0;    /* Ctrl-q */
    newtio.c_cc[VSTOP] = 0; /* Ctrl-s */
    newtio.c_cc[VSUSP] = 0; /* Ctrl-z */
    newtio.c_cc[VEOL] = 0;  /* '\0' */
    newtio.c_cc[VREPRINT] = 0;  /* Ctrl-r */
    newtio.c_cc[VDISCARD] = 0;  /* Ctrl-u */
    newtio.c_cc[VWERASE] = 0;   /* Ctrl-w */
    newtio.c_cc[VLNEXT] = 0;    /* Ctrl-v */
    newtio.c_cc[VEOL2] = 0; /* '\0' */

    /* 
     * now clean the modem line and activate the settings for the port
     **/
    tcflush (fd, TCIFLUSH);
    tcsetattr (fd, TCSANOW, &newtio);

    /*
     * terminal settings done, return file descriptor
     **/

    return fd;
}

void USB_cleanup(int ifd){
    if(ifd != fd) {
        fprintf(stderr, "WARNING! file descriptor != the one returned by serial_init()\n");
    }
    /* restore the old port settings */
    tcsetattr (ifd, TCSANOW, &oldtio);
}

有人能告诉我如何做类似 read(fd,buffR,19) 的事情，但如果我没有收到任何数据并在一段时间后中止它并打印类似printf("没有接触到设备")?

我非常感谢有关如何解决此问题的任何建议!

Answer 1

在处理连续剧时，您有两个选择:使用带超时的select/poll，或使用termios 配置端口。

对于select 和poll，方法是先等待描述符上的事件，然后 读取成功结果。这种方法的优点是它也适用于网络套接字和其他一些描述符类型:

int fd = ...
fd_set fds;
stuct timeval timeout;

timeout.tv_sec = 10; /* timeout in secs */
timeout.tv_usec = 0;
FD_ZERO(&fds)
FD_SET(fd, &fds)
if (select(fd, fds, NULL, NULL, &timeout) > 0)
  read(...)
else ... timeout

poll 示例与上面非常相似。

但是，在使用任何类型的串行设备时，还有另一种方法可以使读取正确超时。您需要使用 VMIN 和 VTIME 属性:

newtio.c_cc[VTIME] = timeout; /* timeout in 1/10 of second  1==100ms, 10==1sec*/
newtio.c_cc[VMIN] = 0;

就是这样。