我的 gsm 调制解调器在 mini2440 上回显我的命令

Question

我有 mini2440 和 GSM 调制解调器，它们通过 USB 串行端口（ttyUSB0）相互连接。这是我用 C++ 发送 AT 的代码：

#include <QCoreApplication>
#include <stdio.h>   /* Standard input/output definitions */
#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 */
#include <string>    /*To use string type*/
#include <iostream>
#include <string>
using namespace std;

//                 Definations
int fd; /* File descriptor for the port */
string wr;
int rd;
char buffer[100];  /* Input buffer */
int openport(void);
void closeport(void);
void configport(void);
string WriteRead(void);
//-------------------------------------
int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    openport();
    if(fd>-1)
    {
        configport();

        string str=WriteRead();
        qDebug(str.c_str());
    }
    return a.exec();
}
//-------------------------------------
int openport(void)
{
    fd=open("/dev/ttyUSB0",O_RDWR|O_NOCTTY|O_NDELAY);
    if (fd==-1)
    {
        perror("open_port: unable to open port /dev/ttyUSB0\n");
        return -1;
    }
    else
    {
        printf("open_port: succesfully open port /dev/ttyUSB0\n");
        fcntl(fd,F_SETFL,0);
        return 1;
    }
}
//-------------------------------------
void closeport(void)
{
    close(fd);
}
//-------------------------------------
void configport(void)
{
   struct termios options;
   tcgetattr(fd,&options);
   cfsetispeed(&options,B9600);
   cfsetospeed(&options,B9600);
   options.c_cflag |=  (CLOCAL | CREAD);

   options.c_cflag &= ~CSIZE;
   options.c_cflag |= CS8;
   options.c_cflag &= ~ PARENB;
   options.c_cflag &= ~CSTOPB;
   options.c_cflag &= ~CSIZE;
   options.c_cflag |= CS8;
   options.c_iflag &= ~(IXON|IXOFF|IXANY);
   tcsetattr(fd,TCSANOW,&options);
}
//------------------------------------
string WriteRead(void)
{
    char buffer[255];  /* Input buffer */
    char *bufptr;      /* Current char in buffer */
    int  nbytes;       /* Number of bytes read */
    int  tries;        /* Number of tries so far */

    for (tries = 0; tries < 3; tries ++)
      {

        if (write(fd, "AT\r", 3) < 3)// send an AT command followed by a CR
          continue;

       // read characters into our string buffer until we get a CR or NL
        bufptr = buffer;
        while ((nbytes = read(fd, bufptr, buffer + sizeof(buffer) - bufptr
- 1)) > 0)
        {
          bufptr += nbytes;
          if (bufptr[-1] == '\n' || bufptr[-1] == '\r')
            break;
        }

       /* nul terminate the string and see if we got an OK response */
        *bufptr = '\0';
        string s(buffer);
        if (s.find("OK"))
          {

            return s;
          }
        else
            return "not answer";
      }
}

但在发送 AT 之后，我说如果找到“OK”，则将缓冲区设置为字符串 s。然后在输出中写入字符串 s 。但它只写AT。没有 OK，但我的 (if(s.find("OK"))) 返回 true。那么会发生什么呢？为什么回显AT？