Linux Serial Ports Using C/C++

Overview

Unluckily, using serial ports in Linux is not the easiest thing in the world. When dealing with the termios.h header, there are many finicky settings buried within multiple bytes worth of bitfields. This page is an attempt to help explain these settings and show you how to configure a serial port in Linux correctly.

Everything Is A File

In typical UNIX style, serial ports are represented by files within the operating system. These files usually pop-up in /dev/, and begin with the name tty*.

Common names are:

• /dev/ttyACM0 - ACM stands for the ACM modem on the USB bus. Arduino UNOs (and similar) will appear using this name.

• /dev/ttyPS0 - Xilinx Zynq FPGAs running a Yocto-based Linux build will use this name for the default serial port that Getty connects to.

• /dev/ttyS0 - Standard COM ports will have this name. These are less common these days with newer desktops and laptops not having actual COM ports.

• /dev/ttyUSB0 - Most USB-to-serial cables will show up using a file named like this.

• /dev/pts/0 - A pseudo terminal. These can be generated with socat.

A listing of the /dev/ directory in Linux with a connected Arduino. The Arduino serial port is present as /dev/ttyACM0.

To write to a serial port, you write to the file. To read from a serial port, you read from the file. Of course, this allows you to send/receive data, but how do you set the serial port parameters such as baud rate, parity, e.t.c? This is set by a special tty configuration struct.

Basic Setup In C

This code is also applicable to C++.

First we want to include a few things:

// C library headers
#include <stdio.h>
#include <string.h>

// Linux headers
#include <fcntl.h> // Contains file controls like O_RDWR
#include <errno.h> // Error integer and strerror() function
#include <termios.h> // Contains POSIX terminal control definitions
#include <unistd.h> // write(), read(), close()

Then we want to open the serial port device (which appears as a file under /dev/), saving the file descriptor that is returned by open():

int serial_port = open("/dev/ttyUSB0", O_RDWR);

// Check for errors
if (serial_port < 0) {
    printf("Error %i from open: %s\n", errno, strerror(errno));
}

One of the common errors you might see here is errno = 2, and strerror(errno) returns No such file or directory. Make sure you have the right path to the device and that the device exists!

Another common error you might get here is errno = 13, which is Permission denied. This usually happens because the current user is not part of the dialout group. Add the current user to the dialout group with:

$ sudo adduser $USER dialout

You must log out and back in before these group changes come into effect.

At this point we could technically read and write to the serial port, but it will likely not work, because the default configuration settings are not designed for serial port use. So now we will set the configuration correctly.

When modifying any configuration value, it is best practice to only modify the bit you are interested in, and leave all other bits of the field untouched. This is why you will see below the use of &= or |=, and never = when setting bits.

Configuration Setup

We need access to the termios struct in order to configure the serial port. We will create a new termios struct, and then write the existing configuration of the serial port to it using tcgetattr(), before modifying the parameters as needed and saving the settings with tcsetattr().

/ Create new termios struct, we call it 'tty' for convention
// No need for "= {0}" at the end as we'll immediately write the existing
// config to this struct
struct termios tty;

// Read in existing settings, and handle any error
// NOTE: This is important! POSIX states that the struct passed to tcsetattr()
// must have been initialized with a call to tcgetattr() overwise behaviour
// is undefined
if(tcgetattr(serial_port, &tty) != 0) {
    printf("Error %i from tcgetattr: %s\n", errno, strerror(errno));
}

We can now change tty’s settings as needed, as shown in the following sections. Before we get onto that, here is the definition of the termios struct if you’re interested (pulled from termbits.h):

struct termios {
	tcflag_t c_iflag;		/* input mode flags */
	tcflag_t c_oflag;		/* output mode flags */
	tcflag_t c_cflag;		/* control mode flags */
	tcflag_t c_lflag;		/* local mode flags */
	cc_t c_line;			/* line discipline */
	cc_t c_cc[NCCS];		/* control characters */
};

Control Modes (c_cflags)

The c_cflag member of the termios struct contains control parameter fields.

PARENB (Parity)

If this bit is set, generation and detection of the parity bit is enabled. Most serial communications do not use a parity bit, so if you are unsure, clear this bit.

tty.c_cflag &= ~PARENB; // Clear parity bit, disabling parity (most common)
tty.c_cflag |= PARENB;  // Set parity bit, enabling parity

CSTOPB (Num. Stop Bits)

If this bit is set, two stop bits are used. If this is cleared, only one stop bit is used. Most serial communications only use one stop bit.

tty.c_cflag &= ~CSTOPB; // Clear stop field, only one stop bit used in communication (most common)
tty.c_cflag |= CSTOPB;  // Set stop field, two stop bits used in communication

Number Of Bits Per Byte

The CS<number> fields set how many data bits are transmitted per byte across the serial port. The most common setting here is 8 (CS8). Definitely use this if you are unsure, I have never used a serial port before which didn’t use 8 (but they do exist). You must clear all of the size bits before setting any of them with &= ~CSIZE.

tty.c_cflag &= ~CSIZE; // Clear all the size bits, then use one of the statements below
tty.c_cflag |= CS5; // 5 bits per byte
tty.c_cflag |= CS6; // 6 bits per byte
tty.c_cflag |= CS7; // 7 bits per byte
tty.c_cflag |= CS8; // 8 bits per byte (most common)

Flow Control (CRTSCTS)

If the CRTSCTS field is set, hardware RTS/CTS flow control is enabled. The most common setting here is to disable it. Enabling this when it should be disabled can result in your serial port receiving no data, as the sender will buffer it indefinitely, waiting for you to be “ready”.

tty.c_cflag &= ~CRTSCTS; // Disable RTS/CTS hardware flow control (most common)
tty.c_cflag |= CRTSCTS;  // Enable RTS/CTS hardware flow control

CREAD and CLOCAL

Setting CLOCAL disables modem-specific signal lines such as carrier detect. It also prevents the controlling process from getting sent a SIGHUP signal when a modem disconnect is detected, which is usually a good thing here. Setting CREAD allows us to read data (we definitely want that!).

tty.c_cflag |= CREAD | CLOCAL; // Turn on READ & ignore ctrl lines (CLOCAL = 1)

Local Modes (c_lflag)

Disabling Canonical Mode

UNIX systems provide two basic modes of input, canonical and non-canonical mode. In canonical mode, input is processed when a new line character is received. The receiving application receives that data line-by-line. This is usually undesirable when dealing with a serial port, and so we normally want to disable canonical mode.

Canonical mode is disabled with:

tty.c_lflag &= ~ICANON;

Also, in canonical mode, some characters such as backspace are treated specially, and are used to edit the current line of text (erase). Again, we don’t want this feature if processing raw serial data, as it will cause particular bytes to go missing!

Echo

If this bit is set, sent characters will be echoed back. Because we disabled canonical mode, I don’t think these bits actually do anything, but it doesn’t harm to disable them just in case!

tty.c_lflag &= ~ECHO; // Disable echo
tty.c_lflag &= ~ECHOE; // Disable erasure
tty.c_lflag &= ~ECHONL; // Disable new-line echo

Disable Signal Chars

When the ISIG bit is set, INTR, QUIT and SUSP characters are interpreted. We don’t want this with a serial port, so clear this bit:

tty.c_lflag &= ~ISIG; // Disable interpretation of INTR, QUIT and SUSP

Input Modes (c_iflag)

The c_iflag member of the termios struct contains low-level settings for input processing. The c_iflag member is an int.

Software Flow Control (IXOFF, IXON, IXANY)

Clearing IXOFF, IXON and IXANY disables software flow control, which we don’t want:

tty.c_iflag &= ~(IXON | IXOFF | IXANY); // Turn off s/w flow ctrl

Disabling Special Handling Of Bytes On Receive

Clearing all of the following bits disables any special handling of the bytes as they are received by the serial port, before they are passed to the application. We just want the raw data thanks!

tty.c_iflag &= ~(IGNBRK|BRKINT|PARMRK|ISTRIP|INLCR|IGNCR|ICRNL); // Disable any special handling of received bytes

Output Modes (c_oflag)

The c_oflag member of the termios struct contains low-level settings for output processing. When configuring a serial port, we want to disable any special handling of output chars/bytes, so do the following:

tty.c_oflag &= ~OPOST; // Prevent special interpretation of output bytes (e.g. newline chars)
tty.c_oflag &= ~ONLCR; // Prevent conversion of newline to carriage return/line feed
// tty.c_oflag &= ~OXTABS; // Prevent conversion of tabs to spaces (NOT PRESENT IN LINUX)
// tty.c_oflag &= ~ONOEOT; // Prevent removal of C-d chars (0x004) in output (NOT PRESENT IN LINUX)

Both OXTABS and ONOEOT are not defined in Linux. Linux however does have the XTABS field which seems to be related. When compiling for Linux, I just exclude these two fields and the serial port still works fine.

VMIN and VTIME (c_cc)

VMIN and VTIME are a source of confusion for many programmers when trying to configure a serial port in Linux.

An important point to note is that VTIME means slightly different things depending on what VMIN is. When VMIN is 0, VTIME specifies a time-out from the start of the read() call. But when VMIN is > 0, VTIME specifies the time-out from the start of the first received character.

Let’s explore the different combinations:

VMIN = 0, VTIME = 0: No blocking, return immediately with what is available

VMIN > 0, VTIME = 0: This will make read() always wait for bytes (exactly how many is determined by VMIN), so read() could block indefinitely.

VMIN = 0, VTIME > 0: This is a blocking read of any number of chars with a maximum timeout (given by VTIME). read() will block until either any amount of data is available, or the timeout occurs. This happens to be my favourite mode (and the one I use the most).

VMIN > 0, VTIME > 0: Block until either VMIN characters have been received, or VTIME after first character has elapsed. Note that the timeout for VTIME does not begin until the first character is received.

VMIN and VTIME are both defined as the type cc_t, which I have always seen be an alias for unsigned char (1 byte). This puts an upper limit on the number of VMIN characters to be 255 and the maximum timeout of 25.5 seconds (255 deciseconds).

“Returning as soon as any data is received” does not mean you will only get 1 byte at a time. Depending on the OS latency, serial port speed, hardware buffers and many other things you have no direct control over, you may receive any number of bytes.

For example, if we wanted to wait for up to 1s, returning as soon as any data was received, we could use:

tty.c_cc[VTIME] = 10;    // Wait for up to 1s (10 deciseconds), returning as soon as any data is received.
tty.c_cc[VMIN] = 0;

Baud Rate

Rather than use bit fields as with all the other settings, the serial port baud rate is set by calling the functions cfsetispeed() and cfsetospeed(), passing in a pointer to your tty struct and a enum:

// Set in/out baud rate to be 9600
cfsetispeed(&tty, B9600);
cfsetospeed(&tty, B9600);

If you want to remain UNIX compliant, the baud rate must be chosen from one of the following:

B0,  B50,  B75,  B110,  B134,  B150,  B200, B300, B600, B1200, B1800, B2400, B4800, B9600, B19200, B38400, B57600, B115200, B230400, B460800

Some implementation of Linux provide a helper function cfsetspeed() which sets both the input and output speeds at the same time:

cfsetspeed(&tty, B9600);

Custom Baud Rates

As you are now fully aware that configuring a Linux serial port is no trivial matter, you’re probably unfazed to learn that setting custom baud rates is just as difficult. There is no portable way of doing this, so be prepared to experiment with the following code examples to find out what works on your target system.

GNU/Linux Method

If you are compiling with the GNU C library, you can forgo the standard enumerations above just specify an integer baud rate directly to cfsetispeed() and cfsetospeed(), e.g.:

// Specifying a custom baud rate when using GNU C
cfsetispeed(&tty, 104560);
cfsetospeed(&tty, 104560);

ermios2 Method

This method relied on using a termios2 struct, which is like a termios struct but with sightly more functionality. I’m unsure on exactly what UNIX systems termios2 is defined on, but if it is, it is usually defined in termbits.h (it was on the Xubuntu 18.04 with GCC system I was doing these tests on):

struct termios2 {
	tcflag_t c_iflag;		/* input mode flags */
	tcflag_t c_oflag;		/* output mode flags */
	tcflag_t c_cflag;		/* control mode flags */
	tcflag_t c_lflag;		/* local mode flags */
	cc_t c_line;			/* line discipline */
	cc_t c_cc[NCCS];		/* control characters */
	speed_t c_ispeed;		/* input speed */
	speed_t c_ospeed;		/* output speed */
};

Which is very similar to plain old termios, except with the addition of the c_ispeed and c_ospeed. We can use these to directly set a custom baud rate! We can pretty much set everything other than the baud rate in exactly the same manner as we could for termios, except for the reading/writing of the terminal attributes to and from the file descriptor — instead of using tcgetattr() and tcsetattr() we have to use ioctl().

Let’s first update our includes, we have to remove termios.h and add the following:

// #include <termios.h> This must be removed! 
// Otherwise we'll get "redefinition of ‘struct termios’" errors
#include <sys/ioctl.h> // Used for TCGETS2/TCSETS2, which is required for custom baud rates

struct termios2 tty;

// Read in the terminal settings using ioctl instead
// of tcsetattr (tcsetattr only works with termios, not termios2)
ioctl(fd, TCGETS2, &tty);

// Set everything but baud rate as usual
// ...
// ...

// Set custom baud rate
tty.c_cflag &= ~CBAUD;
tty.c_cflag |= CBAUDEX;
// On the internet there is also talk of using the "BOTHER" macro here:
// tty.c_cflag |= BOTHER;
// I never had any luck with it, so omitting in favour of using
// CBAUDEX
tty.c_ispeed = 123456; // What a custom baud rate!
tty.c_ospeed = 123456;

// Write terminal settings to file descriptor
ioctl(serial_port, TCSETS2, &tty);

Please read the comment above about BOTHER. Perhaps on your system this method will work!

Not all hardware will support all baud rates, so it is best to stick with one of the standard BXXX rates above if you have the option to do so. If you have no idea what the baud rate is and you are trying to communicate with a 3rd party system, try B9600, then B57600 and then B115200 as they are the most common rates.

Saving termios

After changing these settings, we can save the tty termios struct with tcsetattr():

// Save tty settings, also checking for error
if (tcsetattr(serial_port, TCSANOW, &tty) != 0) {
    printf("Error %i from tcsetattr: %s\n", errno, strerror(errno));
}

Reading And Writing

Now that we have opened and configured the serial port, we can read and write to it!

Writing

Writing to the Linux serial port is done through the write() function. We use the serial_port file descriptor which was returned from the call to open() above.

unsigned char msg[] = { 'H', 'e', 'l', 'l', 'o', '\r' };
write(serial_port, msg, sizeof(msg));

Reading

Reading is done through the read() function. You have to provide a buffer for Linux to write the data into.

// Allocate memory for read buffer, set size according to your needs char read_buf [256]; // Read bytes. The behaviour of read() (e.g. does it block?, // how long does it block for?) depends on the configuration // settings above, specifically VMIN and VTIME int n = read(serial_port, &read_buf, sizeof(read_buf)); // n is the number of bytes read. n may be 0 if no bytes were received, and can also be negative to signal an error.

Closing

This is a simple as:

close(serial_port)

Full Example (Standard Baud Rates)

// C library headers
#include <stdio.h>
#include <string.h>

// Linux headers
#include <fcntl.h> // Contains file controls like O_RDWR
#include <errno.h> // Error integer and strerror() function
#include <termios.h> // Contains POSIX terminal control definitions
#include <unistd.h> // write(), read(), close()

int main() {
  // Open the serial port. Change device path as needed (currently set to an standard FTDI USB-UART cable type device)
  int serial_port = open("/dev/ttyUSB0", O_RDWR);

  // Create new termios struct, we call it 'tty' for convention
  struct termios tty;

  // Read in existing settings, and handle any error
  if(tcgetattr(serial_port, &tty) != 0) {
      printf("Error %i from tcgetattr: %s\n", errno, strerror(errno));
      return 1;
  }

  tty.c_cflag &= ~PARENB; // Clear parity bit, disabling parity (most common)
  tty.c_cflag &= ~CSTOPB; // Clear stop field, only one stop bit used in communication (most common)
  tty.c_cflag &= ~CSIZE; // Clear all bits that set the data size 
  tty.c_cflag |= CS8; // 8 bits per byte (most common)
  tty.c_cflag &= ~CRTSCTS; // Disable RTS/CTS hardware flow control (most common)
  tty.c_cflag |= CREAD | CLOCAL; // Turn on READ & ignore ctrl lines (CLOCAL = 1)

  tty.c_lflag &= ~ICANON;
  tty.c_lflag &= ~ECHO; // Disable echo
  tty.c_lflag &= ~ECHOE; // Disable erasure
  tty.c_lflag &= ~ECHONL; // Disable new-line echo
  tty.c_lflag &= ~ISIG; // Disable interpretation of INTR, QUIT and SUSP
  tty.c_iflag &= ~(IXON | IXOFF | IXANY); // Turn off s/w flow ctrl
  tty.c_iflag &= ~(IGNBRK|BRKINT|PARMRK|ISTRIP|INLCR|IGNCR|ICRNL); // Disable any special handling of received bytes

  tty.c_oflag &= ~OPOST; // Prevent special interpretation of output bytes (e.g. newline chars)
  tty.c_oflag &= ~ONLCR; // Prevent conversion of newline to carriage return/line feed
  // tty.c_oflag &= ~OXTABS; // Prevent conversion of tabs to spaces (NOT PRESENT ON LINUX)
  // tty.c_oflag &= ~ONOEOT; // Prevent removal of C-d chars (0x004) in output (NOT PRESENT ON LINUX)

  tty.c_cc[VTIME] = 10;    // Wait for up to 1s (10 deciseconds), returning as soon as any data is received.
  tty.c_cc[VMIN] = 0;

  // Set in/out baud rate to be 9600
  cfsetispeed(&tty, B9600);
  cfsetospeed(&tty, B9600);

  // Save tty settings, also checking for error
  if (tcsetattr(serial_port, TCSANOW, &tty) != 0) {
      printf("Error %i from tcsetattr: %s\n", errno, strerror(errno));
      return 1;
  }

  // Write to serial port
  unsigned char msg[] = { 'H', 'e', 'l', 'l', 'o', '\r' };
  write(serial_port, msg, sizeof(msg));

  // Allocate memory for read buffer, set size according to your needs
  char read_buf [256];

  // Normally you wouldn't do this memset() call, but since we will just receive
  // ASCII data for this example, we'll set everything to 0 so we can
  // call printf() easily.
  memset(&read_buf, '\0', sizeof(read_buf));

  // Read bytes. The behaviour of read() (e.g. does it block?,
  // how long does it block for?) depends on the configuration
  // settings above, specifically VMIN and VTIME
  int num_bytes = read(serial_port, &read_buf, sizeof(read_buf));

  // n is the number of bytes read. n may be 0 if no bytes were received, and can also be -1 to signal an error.
  if (num_bytes < 0) {
      printf("Error reading: %s", strerror(errno));
      return 1;
  }

  // Here we assume we received ASCII data, but you might be sending raw bytes (in that case, don't try and
  // print it to the screen like this!)
  printf("Read %i bytes. Received message: %s", num_bytes, read_buf);

  close(serial_port)
  return 0; // success
};

For Linux serial port code examples see https://github.com/gbmhunter/CppLinuxSerial.

Issues With Getty

Getty can cause issues with serial communication if it is trying to manage the same tty device that you are attempting to perform serial communications with.

To Stop Getty:

Getty can be hard to stop, as by default if you try and kill the process, a new process will start up immediately.

These instructions apply to older versions of Linux, and/or embedded Linux.

1. Load /etc/inittab in your favourite text editor.

2. Comment out any lines involving getty and your tty device.

3. Save and close the file.

4. Run the command ~$ init q to reload the /etc/inittab file.

5. Kill any running getty processes attached to your tty device. They should now stay dead!

Exclusive Access

It can be prudent to try and prevent other processes from reading/writing to the serial port at the same time you are.

One way to accomplish this is with the flock() system call (note this example is in C++, easy to change to C if needed!):

#include <sys/file.h>

int main() {
    
    // ... get file descriptor here

    // Acquire non-blocking exclusive lock
    if(flock(fd, LOCK_EX | LOCK_NB) == -1) {
        throw std::runtime_error("Serial port with file descriptor " + 
            std::to_string(fd) + " is already locked by another process.");
    }

    // ... read/write to serial port here
}

Getting The Number Of RX Bytes Available

You can use FIONREAD along with ioctl() to see if there are any bytes available in the OS input (receive) buffer for the serial port1. This can be useful in a polling-style method in where the application regularly checks for bytes before trying to read them.

#include <unistd.h>
#include <termios.h>

int main() {

  // ... get file descriptor here

  // See if there are bytes available to read
  int bytes;
  ioctl(fd, FIONREAD, &bytes);
}

The provided pointer to integer bytes gets written by the ioctl() function with the number of bytes available to be read from the serial port.

Changing Terminal Settings Are System Wide

Although getting and setting terminal settings are done with a file descriptor, the settings apply to the terminal device itself and will effect all other system applications that are using or going to use the terminal. This also means that terminal setting changes are persistent after the file descriptor is closed, and even after the application that changed the settings is terminated2.

Examples

For Linux serial port code examples see https://github.com/gbmhunter/CppLinuxSerial (note that this library is written in C++, not C).

External Resources

See http://www.gnu.org/software/libc/manual/html_node/Terminal-Modes.html for the official specifications of the termios struct configuration parameters.

References

---

1. Michael R. Sweet (1999). Serial Programming Guide for POSIX Operating Systems. Retrieved 2022-02-12, from https://www.cmrr.umn.edu/~strupp/serial.html. ↩︎

2. http://www.gnu.org/software/libc/manual/html_node/Mode-Functions.html ↩︎