Linux C++ Serial Port Reading/Writing

Question

I'm trying to read data that I've written to the serial port /tty/USBS0. I've opened the port through serial_fd = open(serialport.str().c_str(), O_RDWR | O_NOCTTY | O_NDELAY);, and I am writing data through retVal2 = write(serial_fd, (void *)&msg, length);.

However, I am running into an issue where when I call read(), it stops there; in other words, it continues to read indefinitely.

void cmgGSP::read_thread(){ 

unsigned char msg[256];
unsigned char messagelength;
msg_header msgHeader; //msg_header is defined in cmgCOM.h
msgHeader.startByte = 0;
msgHeader.length = 0;
msgHeader.ID = 0;
int totalBytes;
int retVal;
cout<<"Scope Test -- In Read Thread"<<endl;

while(1) //infinte loop, since we're going to want to always be reading command data

{

cout<<"Scope Test -- In While(1)"<< endl;

    totalBytes = 0;
    while (totalBytes == 0) {
        cout<<"Scope Test -- First While Loop"<<endl;

        retVal = read(serial_fd, (unsigned char*)&msgHeader, 1);
        cout<<"retVal: "<<retVal <<endl; //This does not get printed

        if (retVal == -1)
        {
            printf("read() message failed: %s\n", strerror(errno));
        }

        if (msgHeader.startByte != FROM_CMG) {
            printf("Bad start byte on read()\n");
            totalBytes = 0;

        } else {

            cout<<"scope test 2"<<endl;

            totalBytes+=retVal;

        }

    }



    while (totalBytes < sizeof(msgHeader)) {

retVal = read(serial_fd, (unsigned char*)&msgHeader+totalBytes, sizeof(msgHeader)-totalBytes);

        cout<< "in second while"<<endl;         

        if (retVal == -1){
            printf("read() message failed: %s\n", strerror(errno));
        }
        totalBytes+=retVal;

    }

    if (msgHeader.startByte != START_BYTE) {
        printf("Bad start byte on message\n");
        continue;
    }
    else
    {
        printf("MSG RCV: StartByte=0x%X, Length=0x%X, ID=0x%X\n", msgHeader.startByte, msgHeader.ID, msgHeader.length);

        break;
    }
}
}

Above is the thread that I have created; however, the output never makes it past "Scope Test -- First While Loop". Even when I have explicitly called the thread after I write data, the result is the same: it reads infinitely. I know it's actually reading because if I disconnect the board while it's stuck, it starts printing "read() message failed ..."

Any help is appreciated - thanks!

I was not aware of QtSerialPort, but from the description on the site ("Qt Serial Port provides the basic functionality, which includes configuring, I/O operations, getting and setting the control signals of the RS-232 pinouts.") this wouldn't be an optimal solution since we're connecting USB to RS-422. — Try431
But would that fix the issue with read()? I don't think we're having an issue in opening/connecting to the serial port. — Try431
Well, it would be easy to test by running any of the reading command line example. — lpapp

K.K K.K · Accepted Answer · 2014-07-11T16:00:35

You may need O_NONBLOCK flag to be set in the open system call; that way depending upon the data written to the port, you will exit out of read() and error out/ get the filled buffer.

"If some process has the pipe open for writing and O_NONBLOCK is clear, read() will block the calling thread until some data is written or the pipe is closed by all processes that had the pipe open for writing."

Read about the O_NONBLOCK flag here: http://pubs.opengroup.org/onlinepubs/7908799/xsh/read.html

Linux C++ Serial Port Reading/Writing

1 Answers