Why can't this C++ program read my arduino's Serial.write()?

Question

I made a super simple arduino uno sketch to send a serial byte once every second:

void setup(){
  Serial.begin(9600);
}

void loop(){
   Serial.write(12); // send a byte with the value 12
   delay(1000);
}

My arduino is hooked up to com 3.

On the other end of the line, I have a C++ program with the following read function:

int Serial::ReadData(char *buffer, unsigned int nbChar)
{
    //Number of bytes we'll have read
    DWORD bytesRead;
    //Number of bytes we'll really ask to read
    unsigned int toRead;

    //Use the ClearCommError function to get status info on the Serial port
    ClearCommError(this->hSerial, &this->errors, &this->status);

    //Check if there is something to read
    if(this->status.cbInQue>0)
    {
        //If there is we check if there is enough data to read the required number
        //of characters, if not we'll read only the available characters to prevent
        //locking of the application.
        if(this->status.cbInQue>nbChar)
        {
            toRead = nbChar;
        }
        else
        {
            toRead = this->status.cbInQue;
        }

        //Try to read the require number of chars, and return the number of read bytes on success
        if(ReadFile(this->hSerial, buffer, toRead, &bytesRead, NULL) && bytesRead != 0)
        {
            return bytesRead;
        }

    }

    //If nothing has been read, or that an error was detected return -1
    return -1;

}

This seems to check only one time if data is available, so obviously I have to loop through it until data comes in. I did that in my main program:

#include "Serial.h"
#include <iostream>

using namespace std;

int main(void)
{
    Serial serial("COM3");

    char* c = "";
    int len = strlen(c);
    while(c == "")
    {
        serial.ReadData(c, len);
    }

    cout << "\n\n";
    system("PAUSE");
}

When I do this, my C++ program gets stuck in an infinite loop waiting for data from the arduino, and I cannot figure out why this is happening. My program never sees the data the arduino is sending.

This Serial class works and is configured properly because I can SEND all the data I want to the arduino without any problems. My issues are only with reading back from the arduino.

Can someone please help me figure out why this is not working?

Answer 1

The problem is in your C++ program at the other end of the line (not the Arduino side):

char* c = "";
int len = strlen(c);
while(c == "")
{
    serial.ReadData(c, len);
}

Here, len is zero because that is the length of "" , the empty string! Thus, in your loop, you ask ReadData to read 0 bytes. Hence, the condition of your while loop is always true as, c never changes.

Also, the while condition seems incorrect as it is not recommended in C++ to compare 2 strings (defines as char * or char [...]) with ==. In your situation, what is important is the number of characters read by ReadData:

char c[LEN + 1];
int numBytes = 0;
while(true)
{
    numBytes = serial.ReadData(c, LEN);
    if (numBytes != -1)
    {
        // Terminate the string if we want to use c variable as a string
        c[numBytes] = 0;
        break;
    }
}

in the code above, LEN is the number of characters you want to read at one time, in you example it should be set to 1, either as a #define or as a const variable:

// either:
#define LEN 1
// or:
const int LEN = 1;

The code above also ensures that c can be used as a normal string, by adding a 0 at the end. This is mandatory only if you want to use string functions on c such as strlen, strcpy, strchr... otherwise numBytes is enough to manage the number of characters read into c.

Answer 2

char* c = "";
while(c == "")

The variable named c is a misnomer, because it is a pointer to a string, not a character. For example c = 0x123456.

You should use a more explicit name for your variable.

This first line assigns a pointer to an empty string to the c pointer. So, the value stored at address 0x123456 == whatever pointer to a static memory address containing '\0'

Note that no memory has been allocated to hold characters. Only a pointer to char has been allocated. Thus you cannot store anything in your string!!

You could write something like: char data[2] = ""; That would allocate a pointer, and a string to hold 2 characters

---

The second line, while(c == "") compares a real memory pointer (e.g. 0x123456) with a compiler pointer. "" is not a real application pointer, you should not manipulate it.

So that second line relies on a compiler behavior.

---

If I were you I would wrap the multi-char routine into a single-char routine. For example:

int ReadChar(char *c)
{
  return ReadData(c, 1);
}

Then:

main()
{
  char c;
  ReadChar(&c);
}

---

On a different note, I think the compiler should catch such error. The line c == "" would never do anything useful and should be caught.