how to open, read, and write from serial port in C
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?
I wrote this a long time ago, and just copy and paste the bits needed into each project.
#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;
memset (&tty, 0, sizeof 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 00 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.
...
*/
For demo code that conforms to POSIX standard as described in Setting Terminal Modes Properly and Serial Programming Guide for POSIX Operating Systems, the following is offered.
It's essentially derived from the other answer, but inaccurate and misleading comments have been corrected.
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <termios.h>
#include <unistd.h>
int set_interface_attribs(int fd, int speed)
{
struct termios tty;
if (tcgetattr(fd, &tty) < 0) {
printf("Error from tcgetattr: %sn", strerror(errno));
return -1;
}
cfsetospeed(&tty, (speed_t)speed);
cfsetispeed(&tty, (speed_t)speed);
tty.c_cflag |= (CLOCAL | CREAD); /* ignore modem controls */
tty.c_cflag &= ~CSIZE;
tty.c_cflag |= CS8; /* 8-bit characters */
tty.c_cflag &= ~PARENB; /* no parity bit */
tty.c_cflag &= ~CSTOPB; /* only need 1 stop bit */
tty.c_cflag &= ~CRTSCTS; /* no hardware flowcontrol */
/* setup for non-canonical mode */
tty.c_iflag &= ~(IGNBRK | BRKINT | PARMRK | ISTRIP | INLCR | IGNCR | ICRNL | IXON);
tty.c_lflag &= ~(ECHO | ECHONL | ICANON | ISIG | IEXTEN);
tty.c_oflag &= ~OPOST;
/* fetch bytes as they become available */
tty.c_cc[VMIN] = 1;
tty.c_cc[VTIME] = 1;
if (tcsetattr(fd, TCSANOW, &tty) != 0) {
printf("Error from tcsetattr: %sn", strerror(errno));
return -1;
}
return 0;
}
void set_mincount(int fd, int mcount)
{
struct termios tty;
if (tcgetattr(fd, &tty) < 0) {
printf("Error tcgetattr: %sn", strerror(errno));
return;
}
tty.c_cc[VMIN] = mcount ? 1 : 0;
tty.c_cc[VTIME] = 5; /* half second timer */
if (tcsetattr(fd, TCSANOW, &tty) < 0)
printf("Error tcsetattr: %sn", strerror(errno));
}
int main()
{
char *portname = "/dev/ttyUSB0";
int fd;
int wlen;
fd = open(portname, O_RDWR | O_NOCTTY | O_SYNC);
if (fd < 0) {
printf("Error opening %s: %sn", portname, strerror(errno));
return -1;
}
/*baudrate 115200, 8 bits, no parity, 1 stop bit */
set_interface_attribs(fd, B115200);
//set_mincount(fd, 0); /* set to pure timed read */
/* simple output */
wlen = write(fd, "Hello!n", 7);
if (wlen != 7) {
printf("Error from write: %d, %dn", wlen, errno);
}
tcdrain(fd); /* delay for output */
/* simple noncanonical input */
do {
unsigned char buf[80];
int rdlen;
rdlen = read(fd, buf, sizeof(buf) - 1);
if (rdlen > 0) {
#ifdef DISPLAY_STRING
buf[rdlen] = 0;
printf("Read %d: "%s"n", rdlen, buf);
#else /* display hex */
unsigned char *p;
printf("Read %d:", rdlen);
for (p = buf; rdlen-- > 0; p++)
printf(" 0x%x", *p);
printf("n");
#endif
} else if (rdlen < 0) {
printf("Error from read: %d: %sn", rdlen, strerror(errno));
}
/* repeat read to get full message */
} while (1);
}
To make the program treat the received data as ASCII codes, compile the program with the symbol DISPLAY_STRING, eg
cc -DDISPLAY_STRING demo.c
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