cannot properly read data string using windows serial in windows.h

i have a microcontroller connected to my usb port which i am reading using the code below `

#include <windows.h>
#include <stdio.h>
#include <conio.h>

int main (void)
{

    int n = 25;
    char szBuff[25 + 1] = {0};

    HANDLE hSerial;
    DCB dcbSerialParams = {0};
    COMMTIMEOUTS timeouts={0};
    DWORD dwBytesRead =25;

    dcbSerialParams.DCBlength=sizeof(DCB);

    hSerial = CreateFile("COM4",
                     GENERIC_READ | GENERIC_WRITE,
                     0,
                     0,
                     OPEN_EXISTING,
                     FILE_ATTRIBUTE_NORMAL,
                     0);


    if(hSerial==INVALID_HANDLE_VALUE)
    {
      if(GetLastError()==ERROR_FILE_NOT_FOUND)
      {
        puts ("cannot open port!");
        return;
      }

      puts ("invalid handle value!");     
      return;                            
    }

    if (!GetCommState(hSerial, &dcbSerialParams)) 
    {
      puts ("error getting state");
      return;
    }

    dcbSerialParams.BaudRate=CBR_57600;
    dcbSerialParams.ByteSize=8;
    dcbSerialParams.StopBits=ONESTOPBIT;
    dcbSerialParams.Parity=NOPARITY;

   if(!SetCommState(hSerial, &dcbSerialParams))
   {
      puts ("error setting port state");
      return;
   }

   timeouts.Read开发者_开发技巧IntervalTimeout = 30;
   timeouts.ReadTotalTimeoutMultiplier = 100;
   timeouts.ReadTotalTimeoutConstant = 100;

   if (!SetCommTimeouts(hSerial, &timeouts))
   {
      puts ("timeouts setting fail!");
   }

   while (1){      
   if(!ReadFile(hSerial, szBuff, n, &dwBytesRead, NULL)){
      puts ("serial read error fail!");
      return;
   } 

   else
   {
      printf ("%s\n" , szBuff);
   }
   }


   getch();     
   return 0;

}
`

i am sending data by this format: $A.B.C$ followed by a newline. so its 7 (or 8, including newline) bytes right? i set the 3rd argument for readfile to 20, greater than 7 bytes so that i can succesfully read all of the data string. however reading sometimes misses a few characters. instead of reading $A.B.C$ i read in one line $A.B.C and in the line after that $ (a hidden'\n'). how can i fix this?

---

This is normal. When the receive buffer contains at least one byte, you'll get back whatever is in the buffer. Which is usually but a fraction of what you expect, serial ports are quite slow. You'll have to keep reading until you get the full response.

---

Just to add to the answer, be sure to use some type of timeout, or you will block waiting a character that maybe never comes.

Maybe one option could be reading byte per byte then just wait the last '$' or '\n' to know that you received the complete string and then process it.