How to open, read, and write from serial port in C?

c, linux, serial-port

Solution

I wrote this a long time ago (from years 1985-1992, with just a few tweaks since then), and just copy and paste the bits needed into each project.

You must call `cfmakeraw` on a `tty` obtained from `tcgetattr`. You cannot zero-out a `struct termios`, configure it, and then set the `tty` with `tcsetattr`. If you use the zero-out method, then you will experience unexplained intermittent failures, especially on the BSDs and OS X. "Unexplained intermittent failures" include hanging in `read(3)`.

#include <errno.h>
#include <fcntl.h> 
#include <string.h>
#include <termios.h>
#include <unistd.h>

int
set_interface_attribs (int fd, int speed, int parity)
{
        struct termios tty;
        if (tcgetattr (fd, &tty) != 0)
        {
                error_message ("error %d from tcgetattr", errno);
                return -1;
        }

        cfsetospeed (&tty, speed);
        cfsetispeed (&tty, speed);

        tty.c_cflag = (tty.c_cflag & ~CSIZE) | CS8;     // 8-bit chars
        // disable IGNBRK for mismatched speed tests; otherwise receive break
        // as \000 chars
        tty.c_iflag &= ~IGNBRK;         // disable break processing
        tty.c_lflag = 0;                // no signaling chars, no echo,
                                        // no canonical processing
        tty.c_oflag = 0;                // no remapping, no delays
        tty.c_cc[VMIN]  = 0;            // read doesn't block
        tty.c_cc[VTIME] = 5;            // 0.5 seconds read timeout

        tty.c_iflag &= ~(IXON | IXOFF | IXANY); // shut off xon/xoff ctrl

        tty.c_cflag |= (CLOCAL | CREAD);// ignore modem controls,
                                        // enable reading
        tty.c_cflag &= ~(PARENB | PARODD);      // shut off parity
        tty.c_cflag |= parity;
        tty.c_cflag &= ~CSTOPB;
        tty.c_cflag &= ~CRTSCTS;

        if (tcsetattr (fd, TCSANOW, &tty) != 0)
        {
                error_message ("error %d from tcsetattr", errno);
                return -1;
        }
        return 0;
}

void
set_blocking (int fd, int should_block)
{
        struct termios tty;
        memset (&tty, 0, sizeof tty);
        if (tcgetattr (fd, &tty) != 0)
        {
                error_message ("error %d from tggetattr", errno);
                return;
        }

        tty.c_cc[VMIN]  = should_block ? 1 : 0;
        tty.c_cc[VTIME] = 5;            // 0.5 seconds read timeout

        if (tcsetattr (fd, TCSANOW, &tty) != 0)
                error_message ("error %d setting term attributes", errno);
}


...
char *portname = "/dev/ttyUSB1"
 ...
int fd = open (portname, O_RDWR | O_NOCTTY | O_SYNC);
if (fd < 0)
{
        error_message ("error %d opening %s: %s", errno, portname, strerror (errno));
        return;
}

set_interface_attribs (fd, B115200, 0);  // set speed to 115,200 bps, 8n1 (no parity)
set_blocking (fd, 0);                // set no blocking

write (fd, "hello!\n", 7);           // send 7 character greeting

usleep ((7 + 25) * 100);             // sleep enough to transmit the 7 plus
                                     // receive 25:  approx 100 uS per char transmit
char buf [100];
int n = read (fd, buf, sizeof buf);  // read up to 100 characters if ready to read

The values for speed are `B115200`, `B230400`, `B9600`, `B19200`, `B38400`, `B57600`, `B1200`, `B2400`, `B4800`, etc. The values for parity are `0` (meaning no parity), `PARENB|PARODD` (enable parity and use odd), `PARENB` (enable parity and use even), `PARENB|PARODD|CMSPAR` (mark parity), and `PARENB|CMSPAR` (space parity).

"Blocking" sets whether a `read()` on the port waits for the specified number of characters to arrive. Setting no blocking means that a `read()` returns however many characters are available without waiting for more, up to the buffer limit.

Addendum:

`CMSPAR` is needed only for choosing mark and space parity, which is uncommon. For most applications, it can be omitted. My header file `/usr/include/bits/termios.h` enables definition of `CMSPAR` only if the preprocessor symbol `__USE_MISC` is defined. That definition occurs (in `features.h`) with

#if defined _BSD_SOURCE || defined _SVID_SOURCE
 #define __USE_MISC     1
#endif

The introductory comments of `<features.h>` says:

/* These are defined by the user (or the compiler)
   to specify the desired environment:

...
   _BSD_SOURCE          ISO C, POSIX, and 4.3BSD things.
   _SVID_SOURCE         ISO C, POSIX, and SVID things.
...
 */

Problem

I am a little bit confused about reading and writing to a serial port. I have a USB device in Linux that uses the FTDI USB serial device converter driver. When I plug it in, it creates: /dev/ttyUSB1. I thought itd be simple to open and read/write from it in C. I know the baud rate and parity information, but it seems like there is no standard for this? Am I missing something, or can someone point me in the right direction?

Original source

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