c - 为什么 select 函数在 linux 中没有被屏蔽

Question

我使用 select() 来阻止串口。然后我向串行端口输入一条消息，然后我收到这样的。在我看来，我认为它会打印一次，但实际上它是连续打印的。那么我哪里做错了？ serial port

#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <errno.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <termios.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/shm.h>
#include <pthread.h>

int fd_se;
fd_set readfs;

int set_opt(int fd,int nSpeed, int nBits, char nEvent, int nStop)
{
    struct termios newtio,oldtio;
    if  ( tcgetattr( fd,&oldtio)  !=  0)
    {
        perror("SetupSerial 1");
        return -1;
    }
    bzero( &newtio, sizeof( newtio ) );
    newtio.c_cflag  |=  CLOCAL | CREAD;
    newtio.c_cflag &= ~CSIZE;

    switch( nBits )
    {
    case 7:
        newtio.c_cflag |= CS7;
        break;
    case 8:
        newtio.c_cflag |= CS8;
        break;
    }

    switch( nEvent )
    {
    case 'O':                     
        newtio.c_cflag |= PARENB;
        newtio.c_cflag |= PARODD;
        newtio.c_iflag |= (INPCK | ISTRIP);
        break;
    case 'E':                     
        newtio.c_iflag |= (INPCK | ISTRIP);
        newtio.c_cflag |= PARENB;
        newtio.c_cflag &= ~PARODD;
        break;
    case 'N':                   
        newtio.c_cflag &= ~PARENB;
        break;
    }

switch( nSpeed )
{
    case 2400:
        cfsetispeed(&newtio, B2400);
        cfsetospeed(&newtio, B2400);
        break;
    case 4800:
        cfsetispeed(&newtio, B4800);
        cfsetospeed(&newtio, B4800);
        break;
    case 9600:
        cfsetispeed(&newtio, B9600);
        cfsetospeed(&newtio, B9600);
        break;
    case 38400:
        cfsetispeed(&newtio, B38400);
        cfsetospeed(&newtio, B38400);
        break;
    case 115200:
        cfsetispeed(&newtio, B115200);
        cfsetospeed(&newtio, B115200);
        break;
    default:
        cfsetispeed(&newtio, B9600);
        cfsetospeed(&newtio, B9600);
        break;
}
if( nStop == 1 )
{
    newtio.c_cflag &=  ~CSTOPB;
}
else if ( nStop == 2 )
{
    newtio.c_cflag |=  CSTOPB;
}
newtio.c_cc[VTIME]  = 0;
newtio.c_cc[VMIN] = 0;
tcflush(fd,TCIFLUSH);
if((tcsetattr(fd,TCSANOW,&newtio))!=0)
{
    perror("com set error");
    return -1;
}
printf("set done!\n");
return 0;
}

int open_port(int i)
{
    char *dev[]={"/dev/ttySAC0","/dev/ttySAC1","/dev/ttySAC2","/dev/ttySAC3"};
    long vdisable;
    int fd;
    fd = open( dev[i], O_RDWR|O_NOCTTY|O_NDELAY);
    if (-1 == fd)
    {
        printf("Can't Open Serial Port %s\n",dev[i]);
        return(-1);
    }
    else
        printf("Open Serial Port %s success!\n",dev[i]);
    if(fcntl(fd, F_SETFL, 0)<0)
    {
        printf("fcntl failed!\n");
        exit(0);
    }
    else
         printf("fcntl id is %d\n",fcntl(fd, F_SETFL,0));
    if(isatty(STDIN_FILENO)==0)
    {
        printf("standard input is not a terminal device\n");
        exit(0);
    }
    else
        printf("This is a terminal device\n");
        return fd;
    }

void *serial_function(void *args)
{



    while(1)
    {
        FD_SET(fd_se, &readfs);  /* set testing for source 1 */
        select(fd_se+1, &readfs, NULL, NULL, NULL);

        if (FD_ISSET(fd_se,&readfs))
        {
            printf("serial_function is running\n");

        }   
    }
       close(fd_se);
}

int main(void)
{
    int i;
    if((fd_se=open_port(1))<0)
    {
        perror("open_port error");
        return;
    }
    if((i=set_opt(fd_se,38400,8,'N',1))<0)
    {
        perror("set_opt error");
        return;
    }

    pthread_t thread1;
    if(pthread_create(&thread1, NULL, serial_function, NULL) != 0)
    {
         perror("Thread create failed!");
         exit(EXIT_FAILURE);
     }
    while(1)
    {
        sleep(1);
    }
    return;
}

Answer 1

好吧，您被告知该文件有数据可供读取，但是您没有读取它。当然，下次您询问时，文件仍然有数据可供读取。

select() 函数不跟踪实际数据以了解它是否已经收到通知。只要文件可读，它就会报告为可读。

要清除它，读出数据。

此外，正如另一个答案中所指出的，您应该在使用前清除文件描述符集。