Serial Programming/termios

Introduction

termios is the newer (now already a few decades old) Unix API for terminal I/O. The anatomy of a program performing serial I/O with the help of termios is as follows:

• Open serial device with standard Unix system call open(2)

• Configure communication parameters and other interface properties (line discipline, etc.) with the help of specific termios functions and data structures.

• Use standard Unix system calls read(2) and write(2) for reading from, and writing to the serial interface. Related system calls like readv(2) and writev(2) can be used, too. Multiple I/O techniques, like blocking, non-blocking, asynchronous I/O (select(2) or poll(2), or signal-driven I/O (SIGIO signal)) are also possible. The selection of the I/O technique is an important part of the application's design. The serial I/O needs to work well with other kinds of I/O performed by the application, like networking, and must not waste CPU cycles.

• Close device with the standard Unix system call close(2) when done.

An important part when starting a program for serial I/I is to decide on the I/O technique to deploy.

The necessary declarations and constants for termios can be found in the header file <termios.h>. So code for serial or terminal I/O will usually start with

#include <termios.h>

Some additional functions and declarations can also be found in the <stdio.h>, <fcntl.h>, and <unistd.h> header files.

The termios I/O API supports two different modes: doesn't old termio do this too? if yes, move paragraphs up to the general section about serial and terminal I/O in Unix).

1. Canonical mode.
This is most useful when dealing with real terminals, or devices that provide line-by-line communication. The terminal driver returns data line-by-line.

2. Non-canonical mode.
In this mode, no special processing is done, and the terminal driver returns individual characters.

On BSD-like systems, there are three modes:

1. Cooked Mode.
Input is assembled into lines and special characters are processed.

2. Raw mode.
Input is not assembled into lines and special characters are not processed.

3. Cbreak mode.
Input is not assembled into lines but some special characters are processed.

Unless set otherwise, canonical (or cooked mode under BSD) is the default. The special characters processed in the corresponding modes are control characters, such as end-of-line or backspace. The full list for a particular Unix flavor can be found in the corresponding termios man page. For serial communication it is often advisable to use non-canonical, (raw or cbreak mode under BSD) to ensure that transmitted data is not interpreted by the terminal driver. Therefore, when setting up the communication parameters, the device should also be configured for raw/non-canonical mode by setting/clearing the corresponding termios flags. It is also possible to enable or disable the processing of the special characters on an individual basis.

This configuration is done by using the struct termios data structure, defined in the termios.h header. This structure is central to both the configuration of a serial device and querying its setup. It contains a minimum of the following fields:

struct termios {
  tcflag_t c_iflag;    /* input specific flags (bitmask) */
  tcflag_t c_oflag;    /* output specific flags (bitmask) */
  tcflag_t c_cflag;    /* control flags (bitmask) */
  tcflag_t c_lflag;    /* local flags (bitmask) */
  cc_t     c_cc[NCCS]; /* special characters */
};

It should be noted that real struct termios declarations are often much more complicated. This stems from the fact that Unix vendors implement termios so that it is backward compatible with termio and integrate termio and termios behavior in the same data structure so they can avoid to have to implement the same code twice. In such a case, an application programmer may be able to intermix termio and termios code.

There are more than 45 different flags that can be set (via tcsetattr()) or got (via tcgetattr()) with the help of the struct termios. The large number of flags, and their sometimes esoteric and pathologic meaning and behavior, is one of the reasons why serial programming under Unix can be hard. In the device configuration, one must be careful not to make a mistake.

Opening/Closing a Serial Device

open(2)

A few decisions have to be made when opening a serial device. Should the device be opened for reading only, writing only, or both reading and writing? Should the device be opened for blocking or non-blocking I/O (non-blocking is recommended)? And should the device be opened in exclusive mode so other programs cannot access the device once opened?

While open(2) can be called with quite a number of different flags controlling these and other properties, the following as a typical example:

#include <fcntl.h>
...

const char device[] = "/dev/ttyS0";
fd = open(device, O_RDWR | O_NOCTTY | O_NDELAY);
if(fd == -1) {
  printf( "failed to open port\n" );
}

Where:

device

The path to the serial port (e.g. /dev/ttyS0)

fd

The returned file handle for the device. -1 if an error occurred

O_RDWR

Opens the port for reading and writing

O_NOCTTY

The port never becomes the controlling terminal of the process.

O_NDELAY

Use non-blocking I/O. On some systems this also means the RS232 DCD signal line is ignored.

Note well: if present, the O_EXCL flag is silently ignored by the kernel when opening a serial device like a modem.

On modern Linux systems programs like ModemManager will sometimes read and write to your device and possibly corrupt your program state. To avoid problems with programs like ModemManager you should set TIOCEXCL on the terminal after associating a terminal with the device. You cannot open with O_EXCL because it is silently ignored.

close(2)

Given an open file handle fd you can close it with the following system call

close(fd);

Serial Device Configuration

After a serial device has been opened you often perform two or three tasks to configure the device. First, you verify the device is indeed a serial device. Second, you configure terminal settings for the specific hardware. This step includes settings like baud rate or line discipline. And third, you optionally set exclusive mode on the terminal. Configuration can be a challenging task because the interface supports many hardware devices and there are over 60 termios flags. The example code below demonstrates the most important flags.

TTY Device

The first step in configuration is verifying a device is a tty. You can verify a device is a tty using isatty as shown below.

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

//
// Check if the file descriptor is pointing to a TTY device or not.
//
if(!isatty(fd)) { ... error handling ... }

Terminal Configuration

The second step in configuration is settings terminal attributes like baud rate or line discipline. This is done with a rather complex data structure using the tcgetattr(3) and tcsetattr(3) functions.

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

struct termios  config;

//
// Get the current configuration of the serial interface
//
if(tcgetattr(fd, &config) < 0) { ... error handling ... }

//
// Input flags - Turn off input processing
//
// convert break to null byte, no CR to NL translation,
// no NL to CR translation, don't mark parity errors or breaks
// no input parity check, don't strip high bit off,
// no XON/XOFF software flow control
//
config.c_iflag &= ~(IGNBRK | BRKINT | ICRNL |
                    INLCR | PARMRK | INPCK | ISTRIP | IXON);

//
// Output flags - Turn off output processing
//
// no CR to NL translation, no NL to CR-NL translation,
// no NL to CR translation, no column 0 CR suppression,
// no Ctrl-D suppression, no fill characters, no case mapping,
// no local output processing
//
// config.c_oflag &= ~(OCRNL | ONLCR | ONLRET |
//                     ONOCR | ONOEOT| OFILL | OLCUC | OPOST);
config.c_oflag = 0;

//
// No line processing
//
// echo off, echo newline off, canonical mode off,
// extended input processing off, signal chars off
//
config.c_lflag &= ~(ECHO | ECHONL | ICANON | IEXTEN | ISIG);

//
// Turn off character processing
//
// clear current char size mask, no parity checking,
// no output processing, force 8 bit input
//
config.c_cflag &= ~(CSIZE | PARENB);
config.c_cflag |= CS8;

//
// One input byte is enough to return from read()
// Inter-character timer off
//
config.c_cc[VMIN]  = 1;
config.c_cc[VTIME] = 0;

//
// Communication speed (simple version, using the predefined
// constants)
//
if(cfsetispeed(&config, B9600) < 0 || cfsetospeed(&config, B9600) < 0) {
    ... error handling ...
}

//
// Finally, apply the configuration
//
if(tcsetattr(fd, TCSAFLUSH, &config) < 0) { ... error handling ... }

Exclusive Access

If you wish ensure exclusive access to the serial device then use ioctl to set TIOCEXCL. If your system includes programs like ModemManager then you should set this attribute.

if (ioctl(fd, TIOCEXCL, NULL) < 0) {
    ... error handling ...
}

Note well: once you set TIOCEXCL other programs will not be able to open the serial device. If your program architecture includes a separate reader and writer, then you should fork/exec unique instances that inherit the file descriptor.

Line-Control Functions

termios contains a number of line-control functions. These allow a more fine-grained control over the serial line in certain special situations. They all work on a file descriptor fildes, returned by an open(2) call to open the serial device. In the case of an error, the detailed cause can be found in the global errno variable (see errno(2)).

tcdrain

#include <termios.h>
int tcdrain(int fildes);

Wait until all data previously written to the serial line indicated by fildes has been sent. This means, the function will return when the UART's send buffer has cleared.
If successful, the function returns 0. Otherwise it returns -1, and the global variable errno contains the exact reason for the error.

without tcdrain()

Today's computer are fast, have more cores and code is subject to a lot of optimizations. Together they can cause strange results. In the example below:

set_rts();
write();
clr_rts();

You would expect a signal going up, the write and then a signal going down. But that did not happen though the programmer intended it. Perhaps optimization causes the kernel to report success to write before the data are really written.

with tcdrain()

Now the same code using tcdrain():

set_rts();
write();
tcdrain();
clr_rts();

Now the code behaves as expected, because the clr_rts(); is executed only when data really is written. Several programmers solve the problem by using sleep()/usleep() though it may be not exactly what you want.

tcflow

#include <termios.h>
int tcflow(int fildes, int action);

This function suspends/restarts transmission and/or reception of data on the serial device indicated by fildes. The exact function is controlled by the action argument. action should be one of the following constants:

TCOOFF

Suspend output.

TCOON

Restarts previously suspended output.

TCIOFF

Transmit a STOP (xoff) character. Remote devices are supposed to stop transmitting data if they receive this character. This requires the remote device on the other end of the serial line to support this software flow-control.

TCION

Transmit a START (xon) character. Remote devices should restart transmitting data if they receive this character. This requires the remote device on the other end of the serial line to support this software flow-control.

If successful, the function returns 0. Otherwise it returns -1, and the global variable errno contains the exact reason for the error.

tcflush

#include <termios.h>
int tcflush(int fildes, int queue_selector);

Flushes (discards) not-sent data (data still in the UART send buffer) and/or flushes (discards) received data (data already in the UART receive buffer). The exact operation is defined by the queue_selector argument. The possible constants for queue_selector are:

TCIFLUSH

Flush received, but unread data.

TCOFLUSH

Flush written, but not send data.

TCIOFLUSH

Flush both.

If successful, the function returns 0. Otherwise it returns -1, and the global variable errno contains the exact reason for the error.

tcsendbreak

#include <termios.h>
int tcsendbreak(int fildes, int duration_flag);

Sends a break for a certain duration. The duration_flag controls the duration of the break signal:

0

Send a break of at least 0.25 seconds, and not more than 0.5 seconds.

any other value

For other values than 0, the behavior is implementation defined. Some implementations interpret the value as some time specifications, others just let the function behave like tcdrain().

A break is a deliberately generated framing (timing) error of the serial data – the signal's timing is violated by sending a series of zero bits, which also encompasses the start/stop bits, so the framing is explicitly gone.

If successful, the function returns 0. Otherwise it returns -1, and the global variable errno contains the exact reason for the error.

Reading and Setting Parameters

The Unix and Linux serial interface have more than 60 parameters due to the different hardware supported by the interface. This plethora of parameters and the resulting different interface configuration is what make serial programming in Unix and Linux challenging. Not only are there so many parameters, but their meanings are often rather unknown to contemporary hackers, because they originated at the dawn of computing, where things were done differently and are no longer known or taught in Little-Hacker School.

Nevertheless, most of the parameters of a serial interface in Unix are controlled via just two functions:

tcgetattr()

For reading the current attributes.

and

tcsetattr()

For setting serial interface attributes.

All information about the configuration of a serial interface is stored in an instance of the struct termios data type. tcgetattr() requires a pointer to a pre-allocated struct termios where it will write to. tcsetattr() requires a pointer to a pre-allocated and initialized struct termios where it reads the values from.

Further, speed parameters are set via a separate set of functions:

cfgetispeed()

Get line-in speed.

cfgetospeed()

Get line-out speed.

cfsetispeed()

Set line-in speed.

cfsetospeed()

Set line-out speed.

The following sub-section explain the mentioned functions in more detail.

Attribute Changes

50+ attributes of a serial interface in Unix can be read with a single function: tcgetattr(). Among these parameters are all the option flags and, for example, information about which special character handling is applied. The signature of that function is as it follows:

#include <termios.h>
int tcgetattr(int fd, struct termios *attribs);

Where the arguments are:

fd

A file handle pointing to an opened terminal device. The device has typically be opened via the open(2) system call. However, there are several more mechanisms in Unix to obtain a legitimate file handle (e.g. by inheriting it over a fork(2)/exec(2) combo). As long as the handle points to an opened terminal device things are fine.

*attribs

A pointer to a pre-allocated struct termios, where tcgetattr() will write to.

tcgetattr() returns an integer that either indicates success or failure in the way typical for Unix system calls:

0

Indicates successful completion

-1

Indicates failure. Further information about the problem can be found in the global (or thread-local) errno variable. See the errno(2), intro(2), and/or perror(3C) man page for information about the meaning of the errno values.

Note; it is a typical beginner and hacker error to not check the return value and assume everything will always work.

The following is a simple example demonstrating the use of tcgetattr(). It assumes that standard input has been redirected to a terminal device:

#include <stdio.h>
#include <stdlib.h>
#include <termios.h>

int main(void) {
    struct termios attribs;
    speed_t speed;
    if(tcgetattr(STDIN_FILENO, &attribs) < 0) {
        perror("stdin");
        return EXIT_FAILURE;
    }

    /*
     * The following mess is to retrieve the input
     * speed from the returned data. The code is so messy,
     * because it has to take care of a historic change in
     * the usage of struct termios. Baud rates were once
     * represented by fixed constants, but later could also
     * be represented by a number. cfgetispeed() is a far
     * better alternative.
     */

    if(attribs.c_cflag & CIBAUDEXT) {
        speed = ((attribs.c_cflag & CIBAUD) >> IBSHIFT)
                 + (CIBAUD >> IBSHIFT) + 1;
    }
    else
    {
        speed = (attribs.c_cflag & CIBAUD) >> IBSHIFT;
    }
    printf("input speed: %ul\n", (unsigned long) speed);

    /*
     * Check if received carriage-return characters are
     * ignored, changed to new-lines, or passed on
     * unchanged.
     */
    if(attribs.c_iflag & IGNCR) {
        printf("Received CRs are ignored.\n");
    }
    else if(attribs.c_iflag & ICRNL)
    {
        printf("Received CRs are translated to NLs.\n");
    }
    else
    {
        printf("Received CRs are not changed.\n");
    }
    return EXIT_SUCCESS;
}

Once the above program is compiled and linked, let's say under the name example, it can be run as it follows:

 ./example < /dev/ttya

Assuming, /dev/ttya is a valid serial device. One can run stty to verify of the output is correct.

tcsetattr()

#include <termios.h>
tcsetattr( int fd, int optional_actions, const struct termios *options );

Sets the termios struct of the file handle fd from the options defined in options. optional_actions specifies when the change will happen:

TCSANOW

the configuration is changed immediately.

TCSADRAIN

the configuration is changed after all the output written to fd has been transmitted. This prevents the change from corrupting in-transmission data.

TCSAFLUSH

same as above but any data received and not read will be discarded.

---

Baud-Rate Setting

Reading and setting the baud rates (the line speeds) can be done via the tcgetattr() and tcsetattr() function. This can be done by reading or writing the necessary data into the struct termios. The previous example for tcgetattr() show how messy it can be when directly accessing the structure members.

Instead of directly accessing the structure members, it is recommended you use one of the following functions:

cfgetispeed()

Get line-in speed.

cfgetospeed()

Get line-out speed.

cfsetispeed()

Set line-in speed.

cfsetospeed()

Set line-out speed.

Which have the following signatures:

#include <termios.h>
speed_t cfgetispeed(const struct termios *attribs);

speed

The input baud rate.

attribs

The struct termios from which to extract the speed.

#include <termios.h>
speed_t cfgetospeed(const struct termios *attribs);

speed

The output baud rate.

attribs

The struct termios from which to extract the speed.

#include <termios.h>
int cfsetispeed(struct termios *attribs, speed_t speed);

attribs

The struct termios in which the input baud rate should be set.

speed

The input baud rate that should be set.

The speed parameter should be one of the predefined values, like B115200, B57600, or B9600.

The function returns

0

If the speed could be set (encoded).

-1

If the speed could not be set (e.g. if it is not a valid or supported speed value).

#include <termios.h>
int cfsetospeed(struct termios *attribs, speed_t speed);

attribs

The struct termios in which the output baud rate should be set.

speed

The output baud rate that should be set.

The function returns

0

If the speed could be set (encoded).

-1

If the speed could not be set (e.g. if it is not a valid or supported speed value).

Here is a simple example for cfgetispeed(). cfgetospeed() works very similar:

#include <stdio.h>
#include <stdlib.h>
#include <termios.h>

int main(void) {
    struct termios attribs;
    speed_t speed;

    if(tcgetattr(STDIN_FILENO, &attribs) < 0) {
        perror("stdin");
        return EXIT_FAILURE;
    }

    speed = cfgetispeed(&attribs);
    printf("input speed: %lu\n", (unsigned long) speed);

    return EXIT_SUCCESS;
}

cfsetispeed() and cfsetospeed() work straight-forward, too. The following example sets the input speed of stdin to 9600 baud. Note, the setting will not be permanent, since the device might be reset at the end of the program:

#include <stdio.h>
#include <stdlib.h>
#include <termios.h>

int main(void) {
    struct termios attribs;
    speed_t speed;

    /*
     * Get the current settings. This saves us from
     * having to initialize a struct termios from
     * scratch.
     */
    if(tcgetattr(STDIN_FILENO, &attribs) < 0)
    {
        perror("stdin");
        return EXIT_FAILURE;
    }

    /*
     * Set the speed data in the structure
     */
    if(cfsetispeed(&attribs, B9600) < 0)
    {
        perror("invalid baud rate");
        return EXIT_FAILURE;
    }

    /*
     * Apply the settings.
     */
    if(tcsetattr(STDIN_FILENO, TCSANOW, &attribs) < 0)
    {
        perror("stdin");
        return EXIT_FAILURE;
    }

    /* data transmision should happen here */

    return EXIT_SUCCESS;
}

Modes

Canonical Mode

Everything is stored into a buffer and can be edited until a carriage return or line feed is entered. After the carriage return or line feed is pressed, the buffer is sent.

options.c_lflag |= ICANON;

where:

ICANON

Enables canonical input mode

Non-Canonical Mode

This mode will handle a fixed number of characters and allows for a character timer. In this mode input is not assembled into lines and input processing does not occur.Here we have to set two parameters time and minimum number of characters to be received before read is satisfied and these are set by setting VTIME and VMIN characters for example if we have to set Minimum number of characters as 4 and we don't want to use any timer then we can do so as follows-:

options.c_cc[VTIME]=0;
options.c_cc[VMIN]=4;

Misc.

There are a few C functions that can be useful for terminal and serial I/O programming and are not part of the terminal I/O API. These are

#include <stdio.h>
char *ctermid(char *s);

This function returns the device name of the current controlling terminal of the process as a string (e.g. "/dev/tty01"). This is useful for programs who want to open that terminal device directly in order to communicate with it, even if the controlling terminal association is removed later (because, for example, the process forks/execs to become a daemon process).
*s can either be NULL or should point to a character array of at least L_ctermid bytes (the constant is also defined in stdio.h). If *s is NULL, then some internal static char array is used, otherwise the provided array is used. In both cases, a pointer to the first element of the char array is returned

#include <unistd.h>
int isatty(int fildes)

Checks if the provided file descriptor represents a terminal device. This can e.g. be used to figure out if a device will understand the commands send via the terminal I/O API.

#include <unistd.h>
char *ttyname (int fildes);

This function returns the device name of a terminal device represented by a file descriptor as a string.

#include <sys/ioctl.h>
ioctl(int fildes, TIOCGWINSZ, struct winsize *);
ioctl(int fildes, TIOCSWINSZ, struct winsize *);

These I/O controls allow to get and set the window size of a terminal emulation, e.g. an xterm in pixel and character sizes. Typically the get variant (TIOCGWINSZ) is used in combination with a SIGWINCH signal handler. The signal handler gets called when the size has changed (e.g. because the user has resized the terminal emulation window), and the application uses the I/O control to get the new size.

Modems

Modems are still common for many users, like those people who use dialup connections in Central and South America and the Africas. Additionally, users who integrate with telephone company services will use modems for faxing and call blocking. Modems are also common in more modern applications that manage communications over cellular and other wireless networks as there are several wireless module vendors that offer system developers solutions that allow them to easily add wireless access to their product offerings. Those modules are often controlled via modem APIs. This section will provide configuration and operational information on modems.

Modem Configuration

There are usually two ways to configure a modem, give or take. First, you can modify the existing file descriptor. Second, you can use cfmakeraw and apply fresh configuration flags.

Modify an existing file descriptor would use code similar to below. The example is based on Mike Sweet's Serial Programming Guide for POSIX Operating Systems.

int fd;
struct termios options;

/* open the port */
fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY | O_NDELAY);
fcntl(fd, F_SETFL, 0);

/* get the current options */
tcgetattr(fd, &options);

/* set raw input, 1 character trigger */
options.c_cflag     |= (CLOCAL | CREAD);
options.c_lflag     &= ~(ICANON | ECHO | ECHOE | ISIG);
options.c_oflag     &= ~OPOST;
options.c_cc[VMIN]  = 1;
options.c_cc[VTIME] = 0;

/* set the options */
tcsetattr(fd, TCSANOW, &options);

The second way to configure a modem is use cfmakeraw as shown below.

int fd;
struct termios options;

/* open the port */
fd = open("/dev/ttyACM0", O_RDWR | O_NOCTTY | O_SYNC);

/* raw mode, like Version 7 terminal driver */
cfmakeraw(&options);
options.c_cflag |= (CLOCAL | CREAD);

/* set the options */
tcsetattr(fd, TCSANOW, &options);

In both cases you should tune the options to your particular modem. You will have to consult the modem's documentation for settings like line discipline, start and stop bits, UART speed, etc.

Ring Count

Software that operates a modem connected to the telephone network often performs tasks like detect ring, collect Caller ID information, answer a call and play a message. The software will often maintain a state machine that depends upon the ring count.

The ring count can be retrieved from the S1-parameter register by writing ATS1? and then reading the response. The response will be similar to the following with ATE1 (echo=on).

ATS1?
001
OK

A typical workflow sequence using the ring count is shown below.

RING  # unsolicited message from the modem
count = read S1  # program reads ring count
NAME = JOHN DOE
NMBR = 4105551212
DATE = 0101  # month and date
TIME = 1345  # hour and minute
RING
count = read S1
RING
count = read S1
RING
count = read S1
...

Some versions US Robotics modems, like the USR5637, have a firmware bug when reading the S1. The bug is, reading the S1 register destroys the Caller ID message. If you are using an affected USR modem your program will observe the following.

RING  # unsolicited message from the modem
count = read S1  # program reads ring count
RING  # No Caller ID message
count = read S1
...

If you use an affected modem you should not use the S1 register. Rather, maintain an internal timestamp that resets after 8 seconds. The code would look similar to the following.

/* Last activity for ring count. We used to read S1 and the modem would  */
/* return 1, 2, 3, etc. Then we learned reading S1 is destructive on     */
/* USR modems. Reading S1 between Ring 1 and Ring 2 destroys Caller ID   */
/* information. Caller ID is never sent to the DTE for USR modems.       */
static time_t s_last = 0;

/* Conexant modems reset the ring count after 8 seconds of inactivity.  */
/* USR modems reset the ring count after 6 seconds of inactivity.       */
/* We now do this manually to track ring state due to USR modems.       */
#define INACTIVITY 8
static int s_count = 0;

int get_ring_count(const char* msg)
{
    /* Only increment ring count on a RING message */
    /* Otherwise, return the current count         */
    if (strstr(msg, "RING") ==  NULL) {
        return s_count;
    }

    /* Number of seconds since epoch */
    time_t now = time(NULL);
    if (now >= s_last + INACTIVITY) {
        /* 8 seconds have passed since the last ring. */
        /* This is a new call. Set count to 0.        */
        s_count = 0;
    }

    /* Only increment ring count on a RING message */
    s_count++;
    s_last = now;
    return s_count;
}

Common Problems

Mike Sweet provides the following advice in Serial Programming Guide for POSIX Operating Systems:

• don't forget to disable input echoing. Input echoing will cause a feedback loop between the MODEM and computer.
• you must terminate modem commands with a carriage return (CR) and not a newline (NL). The C character constant for CR is \r.
• make sure you use a baud that the MODEM supports. While many MODEMs do auto-baud detection, some have limits (19.2kbps is common) that you must observe.

Example terminal program

A simple terminal program with termios.h can look like this:

Warning: In this program the VMIN and VTIME flags are ignored because the O_NONBLOCK flag is set.

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <termios.h>
#include <string.h> // needed for memset

int main(int argc,char** argv)
{
        struct termios tio;
        struct termios stdio;
        int tty_fd;
        fd_set rdset;

        unsigned char c='D';

        printf("Please start with %s /dev/ttyS1 (for example)\n",argv[0]);
        memset(&stdio,0,sizeof(stdio));
        stdio.c_iflag=0;
        stdio.c_oflag=0;
        stdio.c_cflag=0;
        stdio.c_lflag=0;
        stdio.c_cc[VMIN]=1;
        stdio.c_cc[VTIME]=0;
        tcsetattr(STDOUT_FILENO,TCSANOW,&stdio);
        tcsetattr(STDOUT_FILENO,TCSAFLUSH,&stdio);
        fcntl(STDIN_FILENO, F_SETFL, O_NONBLOCK);       // make the reads non-blocking

        memset(&tio,0,sizeof(tio));
        tio.c_iflag=0;
        tio.c_oflag=0;
        tio.c_cflag=CS8|CREAD|CLOCAL;           // 8n1, see termios.h for more information
        tio.c_lflag=0;
        tio.c_cc[VMIN]=1;
        tio.c_cc[VTIME]=5;

        tty_fd=open(argv[1], O_RDWR | O_NONBLOCK);        // O_NONBLOCK might override VMIN and VTIME, so read() may return immediately.
        cfsetospeed(&tio,B115200);            // 115200 baud
        cfsetispeed(&tio,B115200);            // 115200 baud

        tcsetattr(tty_fd,TCSANOW,&tio);
        while (c!='q')
        {
                if (read(tty_fd,&c,1)>0)        write(STDOUT_FILENO,&c,1);              // if new data is available on the serial port, print it out
                if (read(STDIN_FILENO,&c,1)>0)  write(tty_fd,&c,1);                     // if new data is available on the console, send it to the serial port
        }

        close(tty_fd);
}

termio vs termios

Sometimes you may encounter a struct called termio. This is an old SYSV interface. The structure is smaller as the termios. The interface was replaced in POSIX.1-1990 and should not appear in newer programms. When ever possible use tcsetattr() and friends.