How to read character using serial port in Linux - c

Question

I trying to read serial raw bytes from using serial port "/dev/ttyS0".In my program, What i want to do is to press the letter and immediately see the the letter I introduced repeated without pressing ENTER. For example, if I press the letter 'a' I want to see an other 'a' next to it.
My code is here:
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <termios.h>
#include <unistd.h>
#include <sys/select.h>
#include <sys/time.h>
#include <sys/types.h>
int main()
{
int n = 0, fd = 0;
struct termios term,trm;
printf("%d\n",getpid());
fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY | O_NDELAY);
if (fd == -1)
{
perror("open");
return 1;
}
else
{
fcntl(fd, F_SETFL, 0);
perror("Port");
}
if (n = tcgetattr(fd, &term) == -1)
{
perror("tcgetattr");
return 1;
}
if (n = cfsetispeed(&term, B115200) == -1)
{
perror("cfsetispeed");
return 1;
}
if (n = cfsetospeed(&term, B115200) == -1)
{
perror("cfsetospeed");
return 1;
}
term.c_cflag |= (CLOCAL | CREAD);
term.c_cflag &= ~PARENB;
term.c_cflag &= ~CSTOPB;
term.c_cflag &= ~CSIZE;
term.c_cflag |= CS8;
term.c_cflag &= ~CRTSCTS;
term.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG);
term.c_iflag &= ~(IXON | IXOFF | IXANY);
term.c_iflag |= (INPCK | ISTRIP);
term.c_oflag &= ~OPOST;
unsigned char c,d;
ssize_t s=0;
tcflush(fd, TCIOFLUSH);
system("/bin/stty raw");
while((c=getchar()) != 'q')
{
write(fd, &c,1);
term.c_cc[VMIN] = 1;
term.c_cc[VTIME] = 0;
tcsetattr(fd, TCSANOW, &term);
if((s=read(fd, &d,1)) != -1)
{
perror("read");
printf("%c",d);
}
}
system("/bin/stty cooked");
close(fd);
return 0;
}

Answer 1

I don't know if it is the solution you are looking for, but you must disable stdin buffering to make getchar exits each char user enter.
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <termios.h>
#include <unistd.h>
#include <sys/select.h>
#include <sys/time.h>
#include <sys/types.h>
int main()
{
int n = 0, fd = 0;
struct termios term, old_stdin, new_stdin;
printf("%d\n",getpid());
fd = open("/dev/ttyUSB0", O_RDWR | O_NOCTTY | O_NDELAY);
if (fd == -1)
{
perror("open");
return 1;
}
else
{
fcntl(fd, F_SETFL, 0);
perror("Port");
}
if (n = tcgetattr(fd, &term) == -1)
{
perror("tcgetattr");
return 1;
}
if (n = cfsetispeed(&term, B115200) == -1)
{
perror("cfsetispeed");
return 1;
}
if (n = cfsetospeed(&term, B115200) == -1)
{
perror("cfsetospeed");
return 1;
}
term.c_cflag |= (CLOCAL | CREAD);
term.c_cflag &= ~PARENB;
term.c_cflag &= ~CSTOPB;
term.c_cflag &= ~CSIZE;
term.c_cflag |= CS8;
term.c_cflag &= ~CRTSCTS;
term.c_lflag &= ~(ECHO | ECHONL | ICANON | IEXTEN | ISIG);
term.c_iflag &= ~(IXON | IXOFF | IXANY);
term.c_iflag |= (INPCK | ISTRIP);
term.c_oflag &= ~OPOST;
unsigned char c,d;
ssize_t s=0;
tcflush(fd, TCIOFLUSH);
term.c_cc[VMIN] = 1;
term.c_cc[VTIME] = 0;
tcsetattr(fd, TCSADRAIN, &term);
// get the terminal settings for stdin
tcgetattr(STDIN_FILENO,&old_stdin);
new_stdin = old_stdin;
// disable canonical mode (buffered i/o) and local echo
new_stdin.c_lflag &=(~ICANON & ~ECHO);
// set the new settings
tcsetattr(STDIN_FILENO,TCSANOW,&new_stdin);
while((c=getchar()) != 'q')
{
write(fd, &c,1);
if((s=read(fd, &d,1)) != -1)
{
perror("read");
printf("%c",d);
}
}
close(fd);
// Restore the old stdin setup
tcsetattr(STDIN_FILENO,TCSANOW,&old_stdin);
return 0;
}

Answer 2

You should change the terminal device's mode to "raw", by using
tty.setraw(file_descriptor)