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?

• Anonymous Penguin: Welcome to Arduino SE! Have you verified the Arduino code outputs the right data with the serial monitor? If so, I think this question should be migrated to another site to address the C++ code. Thanks!
• user3388: I think your C++ program needs to know what data rate (baud) to use.
• JRobert: Can you also confirm that the receiver program works if you send some "correct" data with a terminal program? The answers to those two questions will tell you whether to look at the Arduino, the receiving CPU, or the serial line.
• Amirullah S.: I want to send keypad data from Arduino to Code Composer Studio or CCS and i'm using Texas instruments (TMS320F8069). When I send the data, I hope that CCS will show the Arduino data that i already send. And the CCS will send it back. It's like a TR > RX program. But, it's not work. I mean the CCS console (serial monitor) doesn't show the data. So, can you guys help me solve this problem? Here my CCS code... const int LEN = 1; char msgg[LEN + 1]; int numBytes = 0; for(;;) { msgg [numBytes] = 0; scia_msg(msg); // Wait for inc character while(SciaRegs.SCIFFRX.bit.RXFFST !=1) { } // wait for XRDY

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円'

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);
}

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