Lec16) Programming Terminal Devices

Character Special Files (c)

represents hardware devices that reads or writes a serial stream of data bytes. sound cards , terminal devices(tty)

 

Block speical files (b)

represents hardware devices that raeds or wirtes data in fixed size blocks. HDD, SSD, cdrom

 

 

colored boxes are device drvierers , work as an interface so that OS can communciate with devices more formally.

regular file is container , inode block points to its data blocks.

device file is conenction , inode block points to function inside kernel called device driver.

 

 

linux identify devices using a 16 bit number divided into two parts

• Major nubmer (8bits) = identifies the driver program
• Minor number (8bits) = used by driver program to identify the instance

 

 

  we can write something into other terminal by redirection.

 

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char *argv[]){
   if (argc != 2 ){
	fprintf(stderr,"usage: ./a.out ttyname\n");
	exit(1);
   }
   int fd = open(argv[1], 1);
   if (fd == -1){
	perror("open() failed"); 
        exit(1);
   }
   char	buf[512];	/* loop until EOF on input */
   while(fgets(buf, 512, stdin) != NULL )
      if (write(fd, buf, strlen(buf)) == -1 )
	break;
   close(fd);
   return 0;
}

we open terminal file (character special file) with write mode.

and get input from keyboard and write it back to given fd (terminal file).

 

Modes of terminal driver

Canonical Mode = input is made available line by line, and the line goest to the process only after the user presses the enter key (buffered inside the tty driver program)

Non Canonical Mode = input is made available immediately as a key is pressed, without the need to press the Enter key (no buffering is done by the driver program)

 

 

Attribute of Terminal Driver

Input Processing

Processing performed by tty driver on the characters received via key board, sending them to the prcoess. icrnl

 

Output Processing

Processing performed by tty driver on the characters received from process, before sending them to the display unit. onclr

 

Control Processing

How characters are represented? cs8

 

Local Processing

What the driver do while the characters are inside the driver? icanon, echo

 

 

Stty

change and print terminal line settings

there are lots of flags(options for input and output) that we can manage.

 

tcgetattr , tcsetattr

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

int main(int argc, char *argv[]){
	char passwd[50];
	printf("Password:");
//get attributes
   struct termios info;
   tcgetattr(0, &info); 
//save a copy of original attributes
   struct termios save = info;
//make the echo bit off and set attributes
   info.c_lflag &= ~ECHO;        
   tcsetattr(0, TCSANOW, &info);
//get password from user
   fgets(passwd, 50, stdin);
   char *q;
   q = strchr(passwd, '\n');
   *q = '\0';
//now set the attributes back to original
   tcsetattr(0, TCSANOW, &save);
//compare password and print appropriate message
   int rv = strcmp(passwd, "pucit");
   if (rv == 0)
		printf("\nThe password is correct\n");
	else
		printf("\nThe password is incorrect\n");
	return 0;
}

by making the echo bit off , we can hide our password because terminal won't echo what we wrote in keyboard.

tcgetattr's first argument is keyboard because file descriptor zero is standard input.

 

to open disk file = fcntl system call

to open device file = ioctl system call