A serial port is a serial communication interface through which information transfers in or out one bit at a time. This article explains implementation serial port communication application in different platforms android, Linux and Windows.

Serial UART test application

The below section explains basic steps in writing serial port applications in Linux, Windows and android (command line application).

Linux and Android

Like all devices, UNIX provides access to serial ports via device files. To access a serial port you simply open the corresponding device file. The serial ports are named as ttyS0, ttyS1, etc. in UNIX and LINUX. Since Android relies on Linux for core system, this is applicable for android platform too.Each serial port on a UNIX/Linux system has one or more device files (files in the /dev directory) associated with it.


In Windows ,Serial ports are named as COM1,COM2 ,COM3.. etc .COM1 and COM2.This article assumes you are familiar with the fundamentals of multiple threading ,synchronization in Windows.,Application programming interfaces (APIs) that control user interface features of windows and dialogue boxes. Since the samples provided blow in windows application uses them and they are not explained here.

Opening a Serial Port


A serial port is a file, so the open(2) function is used to access it. Sometimes Workarounds like changing the access permissions to the file(s) , running your program as the super-user (root), or making your program set-userid as the owner of the device file are needed. Below is example code block for opening serial port.

char *_cl_port = NULL;

_cl_port = strdup(optarg);//Command line arguments like /dev/ttyTHS1

fd = open(_cl_port, O_RDWR | O_NONBLOCK);
if (_fd < 0) {
 printf("Error opening serial port \n");


The CreateFile function opens a communications port. There are two ways to call CreateFile to open the communications port: overlapped and nonoverlapped. The following code block shows how to open COM port in windows.

hComm = CreateFile( gszPort,
                    GENERIC_READ | GENERIC_WRITE,
   // error opening port; abort

Writing Data to the Port


Writing data to the port is done by write(2) system call to send data into the previously open serial port.The write function returns the number of bytes sent or -1 if an error occurred.

ssize_t c = write(_fd, _write_data, _write_size);

if (c < 0) {
 printf("write failed (%d)\n", errno);
 c = 0;


Transmitting data out the communications port is done by writefile. The code snippet below shows the example of writing date in the serial port

// Issue write.
   if (!WriteFile(hComm, lpBuf, dwToWrite, &dwWritten, &osWrite)) {
      if (GetLastError() != ERROR_IO_PENDING) {
         // WriteFile failed, but isn't delayed. Report error and abort.
         fRes = FALSE;
         // Write is pending; Need to handle with synchronous objects

Reading Data from the Port

Linux and Android

Similar to writing, Reading data can be done by read(2) system call.

int c = read(_fd, &rb, sizeof(rb));

If no characters are available, the call will block (wait) until characters come in, an interval timer expires, or an error occurs. The read function can be made to return immediately by doing the following:

 fcntl(fd, F_SETFL, FNDELAY);


The ReadFile function issues a read operation. ReadFileEx also issues a read operation,Here is a code snippet that details how to issue a read request. Notice that the function calls a function to process the data if the ReadFile returns TRUE. This is the same function called if the operation becomes overlapped. Note the fWaitingOnRead flag that is defined by the code; it indicates whether or not a read operation is overlapped. It is used to prevent the creation of a new read operation if one is outstanding.

if (!ReadFile(hComm, lpBuf, READ_BUF_SIZE, &dwRead, &osReader)) {
      if (GetLastError() != ERROR_IO_PENDING)     // read not delayed?
         // Error in communications; report it.
         fWaitingOnRead = TRUE;
   else {
      // read completed immediately


Configuring a Serial Port

Linux and Android

Before reading and writing, it is important to configure(like baud rate, flow control, parity ) the serial port appropriately to that transmitters and receivers work correctly. Below function shows the configuration of serial ports.

void setup_serial_port(int baud)
 struct termios newtio;

 _fd = open(_cl_port, O_RDWR | O_NONBLOCK);

 if (_fd < 0) {   printf("Error opening serial port \n");   free(_cl_port);   exit(1);  }  bzero(&newtio, sizeof(newtio)); /* clear struct for new port settings */  /* man termios get more info on below settings */  newtio.c_cflag = baud | CS8 | CLOCAL | CREAD;  if (_cl_rts_cts) {   newtio.c_cflag |= CRTSCTS;  }  if (_cl_2_stop_bit) {   newtio.c_cflag |= CSTOPB;  }  if (_cl_parity) {   newtio.c_cflag |= PARENB;   if (_cl_odd_parity) {    newtio.c_cflag |= PARODD;   }   if (_cl_stick_parity) {    newtio.c_cflag |= CMSPAR;   }  }  newtio.c_iflag = 0;  newtio.c_oflag = 0;  newtio.c_lflag = 0;  // block for up till 128 characters  newtio.c_cc[VMIN] = 128;  // 0.5 seconds read timeout  newtio.c_cc[VTIME] = 5;  /* now clean the modem line and activate the settings for the port */  tcflush(_fd, TCIOFLUSH);  tcsetattr(_fd,TCSANOW,&newtio);  /* enable rs485 direction control */  if (_cl_rs485_delay >= 0) {
  struct serial_rs485 rs485;
  if(ioctl(_fd, TIOCGRS485, &rs485) < 0) {
   printf("Error getting rs485 mode\n");
  } else {
   rs485.flags |= SER_RS485_ENABLED | SER_RS485_RTS_ON_SEND | SER_RS485_RTS_AFTER_SEND;
   rs485.delay_rts_after_send = _cl_rs485_delay;
   rs485.delay_rts_before_send = 0;
   if(ioctl(_fd, TIOCSRS485, &rs485) < 0) {
    printf("Error setting rs485 mode\n");


There are number of factors effecting read and write behaviours baud-rate, time-outs,Software flow control etc. These all values should be properly configured before read and write operations.Below code blocks shows setting serial ports in windows. Setting time-outs


timeouts.ReadIntervalTimeout = 20;
timeouts.ReadTotalTimeoutMultiplier = 10;
timeouts.ReadTotalTimeoutConstant = 100;
timeouts.WriteTotalTimeoutMultiplier = 10;
timeouts.WriteTotalTimeoutConstant = 100;

if (!SetCommTimeouts(hComm, &timeouts))
   // Error setting time-outs.

//Getting a current buad rate
   DCB dcb = {0};

   if (!GetCommState(hComm, &dcb))
      // Error getting current DCB settings
      // DCB is ready for use.

//Setting a new baud rate
   DCB dcb;

   FillMemory(&dcb, sizeof(dcb), 0);
   if (!GetCommState(hComm, &dcb))     // get current DCB
      // Error in GetCommState
      return FALSE;

   // Update DCB rate.
   dcb.BaudRate = CBR_9600 ;

   // Set new state.
   if (!SetCommState(hComm, &dcb))
      // Error in SetCommState. Possibly a problem with the communications
      // port handle or a problem with the DCB structure itself. 

Closing a Serial Port

Linux and Android

To close the serial port, just use the close system call.Closing a serial port will also usually set the DTR signal low which causes most MODEMs to hang up.



To close the serial port here, CloseHandle function is used.


Sample/Test application source code

Use the below link for serial port test application in Linux, android and windows platform.

  1. For Android:
  2. For Linux:
  3. For Windows:

If have any queries, or suggestions please add it in comment sections.