write() to serial port fails when called twice

Question

I'm trying to write a char array to the serial port twice. For some reason the second time I call write() it fails and I have no idea why. The weird part is that the first write succeeds.

Here's my write function:

void write_port()
{
  unsigned char writeBuffer[5];
  int i;
  size_t len;

  writeBuffer[0] = 0x00;
  writeBuffer[1] = 0x01;
  writeBuffer[2] = 0x02;
  writeBuffer[3] = 0x03;
  writeBuffer[4] = 0x04;  

  len = sizeof(writeBuffer);

  for (i=0; i < len; i++)
  {
    printf("Write: %x\n", writeBuffer[i]);
  }

  //First Write:
  int n = write(fd, writeBuffer, len);
  if (n < 0)
    fputs("Serial Port Write Failed!\n", stderr);
  else
    printf("Wrote %0d bytes to serial port\n", n);

  //Second Write:
  n = write(fd, writeBuffer, len);
  if (n < 0)
    fputs("Serial Port Write Failed!\n", stderr);
  else
    printf("Wrote %0d bytes to serial port\n", n);  
}

Here's where I configure the port:

int open_port(void)
{
  struct termios options;

  fd = open("/dev/ttyUSB0", O_RDWR | O_NOCTTY | O_NDELAY | O_NONBLOCK);

  bzero(&options, sizeof(options));   // set everything to 0 

  if (fd != -1)
  {
    printf("Serial Port Open\n");

    tcgetattr(fd, &options_original);
    tcgetattr(fd, &options);
    cfsetispeed(&options, B460800);
    cfsetospeed(&options, B460800);
    options.c_cflag |= (CLOCAL | CREAD); /* Enable the receiver and set local mode */
    options.c_cflag &= ~ECHO; // Disable echoing of input characters
    options.c_cflag &= ~ECHOE;
    options.c_cflag &= ~PARENB; // parity disabled
    options.c_cflag &= ~CSTOPB; // 1 stop bit
    options.c_cflag &= ~CSIZE;  // Mask the character size bits
    options.c_cflag |=  CS8;    // 8 data bits    
    options.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG);  /* Raw Input Mode */

    if (tcsetattr(fd, TCSANOW, &options))    /* Set the new options for the port */
    {
      perror("could not set the serial settings!");
      return -1;
    }
  }
  else
  {
    /* Could not open the port */
    perror("open_port: Unable to open /dev/ttyUSB0 - ");
  }

  return (fd);
}

Finally here's my console output.

Serial Port Open
Flushing IO Buffers
Write: 0
Write: 1
Write: 2
Write: 3
Write: 4
Wrote 5 bytes to serial port
Serial Port Write Failed!
Serial Port Closed

So for some unknown reason I get -1 back from the second write() and have absolutely no idea why, what in the world could cause that?

Answer 1

I think that your send buffer is getting full and you are using a non-blocking socket which means that instead of waiting for the buffer to clear like in a blocking socket, the process just returns the "Resource temporarily unavailable error". See this thread for more info: What can cause a “Resource temporarily unavailable” on sock send() command

Answer 2

The write system call if fails set errno variable. You can check that to get exact details of why write failed. Additionally you may refer this project's native code for linux to see how write is implemented https://github.com/RishiGupta12/serial-communication-manager