How do I read data from serial port in Linux using C?

Question

I am new to serial programming in Linux using C. I have found a small piece of code to write data on serial port which I am sharing here. After running this code I may assume that data has written on a specific port. Now I would like to open another terminal and using separate code want to read the data written on that specific port - how do I do that?

#include <stdio.h>   /* Standard input/output definitions */
#include <string.h>  /* String function definitions */
#include <unistd.h>  /* UNIX standard function definitions */
#include <fcntl.h>   /* File control definitions */
#include <errno.h>   /* Error number definitions */
#include <termios.h> /* POSIX terminal control definitions */

/*
 * 'open_port()' - Open serial port 1.
 *
 * Returns the file descriptor on success or -1 on error.
 */
int
open_port(void)
{
  int fd; /* File descriptor for the port */

  fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY | O_NDELAY);
  if (fd == -1)
  {
   /* Could not open the port. */
    perror("open_port: Unable to open /dev/ttyS0 - ");
  }
  else
    fcntl(fd, F_SETFL, 0);

  n = write(fd, "ATZ\r", 4);
  if (n < 0)
    fputs("write() of 4 bytes failed!\n", stderr);

  return (fd);
}

The code above will write the data on a specific port.

Answer 1

In theory, all you have to do is open the relevant port for reading, and use read() to get the data.

int
read_port(void)
{
    int fd = open("/dev/ttyS0", O_RDONLY | O_NOCTTY);
    if (fd == -1)
    {
        /* Could not open the port. */
        perror("open_port: Unable to open /dev/ttyS0 - ");
    }

    char buffer[32];
    int n = read(fd, buffer, sizeof(buffer));
    if (n < 0)
        fputs("read failed!\n", stderr);
    return (fd);
}

There are differences; notably, the read needs a buffer to put the data in. The code shown discards the first message read. Note that a short read simply indicates that there was less data available than requested at the time when the read completed. It does not automatically indicate an error. Think of a command line; some commands might be one or two characters (ls) where others might be quite complex (find /some/where -name '*.pdf' -mtime -3 -print). The fact that the same buffer is used to read both isn't a problem; one read gives 3 characters (newline is included), the other 47 or so.

Answer 2

The program posted makes a lot of assumptions about the state of the port. In a real world application you should do all the important setup explicitly. I think the best source for learning serial port programming under POSIX is the

Serial Programming Guide for POSIX Operating Systems

I'm mirroring it here: https://www.cmrr.umn.edu/~strupp/serial.html