Chapter 5.Connection Controls

Device == File ?

• Filenames• Properties• permission bits• system call– open, read, write, lseek, close, stat

Device == File ?

• Example: tty

modelink

owner Last_modified filenameinode

* Directory(d), Symbolic Link(l), Character Device(c), Block Device(b)

group

major # : Device Driver

minor # : Device

mode owner group size Last_modified filenamelink

(chapter 2)

Device == File ?

• Device Files and Inodes

Device != File ?

• Disk Files vs Terminal Files– Devices have their own attributes.• Connection attributes

- we can control attributes in a different way.

System call: fcntl

fcntl

PUR-POSE

Control file descriptors

INCLUDE #include <fcntl.h>#include <unistd.h>#include <sys/types.h>

USAGE int result = fcntl(int fd, int cmd);int result = fcntl(int fd, int cmd, long arg);int result = fcntl(int fd, int cmd, struct flock *lockp);

ARGS fdcmdarglock

the file descriptor to controlthe operation to performarguments to the operationlock information

RE-TURNS

-1 Other

if errordepends on operation

Change settings -Disk

• 3-steps for controlling filedesciptors

#include <fcntl.h>int s;s = fcntl(fd, F_GETFL); //step1. get flagss |= O_SYNC; //step2. modify flagsresult = fcntl(fd,F_SETFL,s); //step3. set flagsif(result == -1) perror(“setting SYNC”);

Change settings -Disk

• If two people log in at the same time, how can this Race Condition be avoided?

• Auto Append#include <fcntl.h>int s;s = fcntl(fd, F_GETFL);s |= O_APPEND;result = fcntl(fd,F_SETFL,s); if(result == -1) perror(“setting APPEND”);

Controlling File Descriptors

• fd = open(file_name, flags)– fd = open(fn, O_WRONLY|O_APPEND|O_SYNC);

• Flags– O_CREAT: Create the file if it does not exist.– O_TRUNC: If the file exists, truncate the file to

length zero.– O_EXCL: This flag is intended to prevent wo

processes from creating the same file. The second call returns -1.

Attribute of Terminal• A simple example

#include <stdio.h>main(){ int c, n = 0;

while((c = getchar()) != 'Q') printf("char %3d is %c code %d\n", n++, c, c);}$ ./a.outhellochar 0 is h code 104char 1 is e code 101char 2 is l code 108char 3 is l code 108char 4 is o code 111char 5 is code 10Q$

Change settings -Terminal

• The stty command

$stty erase X #make ‘X’ the erase key$stty –echo #type invisibly$stty erase @ echo #multiple requests

Change settings -Terminal

• 3-steps for changing terminl driver

#include <termios.h>

struct termios attribs;

tcgetattr(fd, &attribs); //step1. get attribssettings.c_lflag |= O_ECHO; //step2. modify attribstcsetattr(fd, TCSANOW, &attribs); //step3. send attribs

☞ TCSANOW, TCSADRAIN, TCSAFLUSH

function: tcgetattr

tcgetattr

PUR-POSE Read attributes from tty driver

INCLUDE #include <termios.h>#include <unistd.h>

USAGE int result = tcgetattr(int fd, struct termios *info);

ARGS fdinfo

the file descriptor connected to a terminalpointer to a struct termios

RE-TURNS

-1 0

if errorIf success

function: tcsetattr

tcsetattr

PURPOSE Set attributes in tty driver

INCLUDE #include <termios.h>#include <unistd.h>

USAGE int result = tcsetattr(int fd, int when, struct termios *info);

ARGSfdwheninfo

the file descriptor connected to a terminalwhen to change the settingspointer to a struct termios

RETURNS -1 0

if errorIf success

struct termios

struct termios {

tcflag_t c_iflag; /*input mode flags*/ tcflag_t c_oflag; /*output mode flags*/

tcflag_t c_cflag; /*control mode flags*/tcflag_t c_lflag; /*local mode flags*/cc_t c_cc[NCCS]; /*control charters*/speed_t c_ispeed; /*input speed*/speed_t c_ospeed; /*output speed*/

};

Example – Echo State

Example – change state#include <stdio.h>#include <termios.h>

#define oops(s,x) { perror(s);exit(x);}

main(int ac, char *av[]){

struct termios info;if(ac==1)

exit(0);if(tcgetattr(0, &info) == -1) //get attribs

oops(“tcgetattr”, 1);if(av[1][0] == ‘y’)

info.c_lflag |= ECHO; //turn on bitelse

info.c_lflag &= ~ECHO; //turn off bitif(tcsetattr(0,TCSANOW, &info) == -1) //set attribs

oops(“tcsetattr”, 2);}

Example – display attribs

Other Devices?

• System call: ioctlioctl

PUR-POSE Control a device

INCLUDE #include <sys/ioctl.h>

USAGE int result = ioctl(int fd, int operation (, arg…) );

ARGS

fdopera-tioninfo

the file descriptor connected to deviceOperation to performAny args required for the operation

RE-TURNS

-1 other

if errorDepends on device

Example

#include <stdio.h>#include <sys/ioctl.h>

int main(){ struct winsize wbuf;

if(ioctl(0, TIOCGWINSZ, &wbuf) != -1) printf("%d rows x %d cols\n", wbuf.ws_row, wbuf.ws_col); return 0;}

Chapter 6.Programming for Humans

Software tools• Programs that see no difference between disk

files and devices Ex) who, ls, sort, uniq, grep, tr, du, …

• Read stdin or files, Write to stdout and stderr.– $ sort > outputfile– $ sort x > /dev/lp

Device-Specific Program

• Programs that interact with specific devices

Ex) scanner, camera, cd-rom, …

• In this chapter, we make terminal-specific program. user-program !

User-Program

• Consider how the keystrokes are handled and output is processed.

• Common concerns of user program(a) immediate response to keys(b) limited input set(c) timeout on input(d) resistance to Ctrl-C

Modes of the terminal driver

• Raw mode- Sends entire sequence of characters.

• Canonical mode- Erase characters before sending the revised sequence to the reading process.

- Processes edit and delete.

Canonical mode

Modes of the terminal driver

• Example input Hello data DEL DEL DEL DEL world

• RAW mode output Hello data DEL DEL DEL DEL world

• Canonical mode output Hello world

Example#include <stdio.h>#include <ctype.h>int main( ){

int c;while((c = getchar( )) != EOF) {

if(c == ‘z’) c = ‘a’;else if(islower(c)) c++;putchar(c);

}}

$ ./a.outabxcd????efg <ctrl-C>

$ stty -icannon$ ./a.out????

$ stty icannon

* In Canonical Mode * In Non-Canonical Mode

Writing a User Program#!/bin/sh

while truedo

do_a_transactionif play_again # our targetthen

continuefibreak

done

Accept input

if “y” : return 0if “n” : return 1

y : loop backn : break• What play_again does is

• Prompt User with Question• If “y”, return 0• If “n”, return 1

First play_again#include <stdio.h>#include <termios.h>

#define QUESTION "Do you want another transaction“

int get_response(char *q){

printf("%s (y/n)?", q);while(1) {

switch(getchar()) {case 'y':case 'Y': return 0;case 'n':case 'N':case EOF: return 1;

}}

}

int main(){

int response;response = get_response(QUESTION);return response;

}

Second play_again: immediate re-sponse

int set_crmode(){ struct termios ttystate;

tcgetattr(0, &ttystate); ttystate.c_lflag &= ~ICANON; ttystate.c_cc[VMIN] = 1; tcsetattr(0, TCSANOW, &ttystate);}

int tty_mode(int how){ static struct termios original_mode; if(how == 0) { tcgetattr(0, &original_mode); return 0; } return tcsetattr(0, TCSANOW, &original_mode);}

int main(){ int response; tty_mode(0); set_crmode(); response = get_response(QUESTION); tty_mode(1); return response;}

Non-blocking mode

• Blocking– getchar() or read() wait for input

• How to set non-blocking– Use O_NDELAY in open or fcntl

• New play_again– Timeout feature.– Telling the terminal drive not to wait– No input found then sleep for few seconds and

look again for input– After three tries give up

New play_again/* play_again.c

purpose: Ask if you want another transactionMethod : set tty into chr-by-chr, no-echo mode

set tty into no-delay mode read char, return resultReturns: 0=> yes, 1=> no, 2=> timeoutBetter : rest the terminal mode on interrupt

*/

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

#define ASK "Do you want another transaction“#define TRIES 3 /* maximum tries*/#define SLEEPTIME 2 /* time per try */#define BEEP putchar('\a') /* alert user */

/* play_again.c continued */

main( ){

int response;tty_mode(0); /* save current mode*/set_cr_noecho_mode(); /* set –icanon, echo */set_nodelay_mode(); /* noinput => EOF */

/* get some answer */response = get_response(ASK, TRIES);tty_mode(1); /* restore orig mode */

return response;}

/* purpose : ask a question and wait for y/n answer or maxtries * method : use getchsr and complain about non –y/n input * returns : 0=> yes, 1=> no, 2=> timeout */

get_response(char* question, int maxtries) {

int input;printf ("%s (y/n)?", question); /* ask*/fflush(stdout); /* force output */while(1)

{ sleep(SLEEPTIME); /* wait a bit*/input = tolower(get_ok_char()); /* get next char*/if (input == 'y') return 0; if (input == 'n') return 1;if (maxtries-- == 0) /* out time? */

return 2; /* say no */BEEP;

}}

/* skip over ilegal chars and return y,Y,n,N or EOP */

get_ok_char( ){

int c;while ((c = getchar())!=EOF && strchr("yYnN",c) == NULL) ;return c;

}

/* how == 0 => save current mode, how ==> 1 restore mode *//* this version handles termios and fcnt1 flags */

tty_mode(int how){

static struct termios original_mode;static int original_flags;if (how == 0) {

tcgetattr(0, &original_mode);orginal_flags= fcntl(0, F_GETFL);

} else {return tcsetattr(0, TCSANOW, &original_mode);fcnt1 (0, F_SETFL, orginal_flags);

}}

set_nodelay_mode( ){

int termflags;termflags = fcntl(0, F_GETFL); /* read current setting */termflags |= O_NDELAY; /* flip on nodelay bit */fcntl(0, F_SETFL, termflags); /* and install em */

}

set_cr_noecho_mode( ){

struct termios ttystate;tcgetattr(0, &ttystate); /* read current settings */ttystate.c_lflag &= ~ICANON; /* no buffering */ttystate.c_lflag &= ~ECHO; /* no echo either */ttystate.c_cc[VMIN] = 1; /* get 1 char at a time */tcsetattr(0, TCSANOW, &ttystate); /* install settings */

}

Others methods to implement timeouts

• What happens when Ctrl–C is pressed?$ make play_again3cc play_again3.c -o play_again3$ ./ play_again3Do you want another transaction (y/n)?Press Ctrl-C now$ logoutConnection to host closed bash$

• What is the problem of the sequence above?• Couldn’t restore ttys!

Ctrl-C kills process

User-Program

• Consider how the keystrokes are handle and output is processed.

• Common concerns of user program(a) immediate response to keys(b) limited input set(c) timeout on input(d) resistance to Ctrl-C

Signals

• What does Ctrl-C do?- Ctrl-C interrupts program

It generates a signal !

• Signal is a kernel mechanism• Simple and powerful

What does Ctrl-C do ?

What is signal ?

• Signal : One-word message Ex) go, stop, out, green light

• Each signal has a numerical code.– Interrupt signal is No. 2.

Where do signals come from?

• User, Kernel, other process

List of signals

/usr/include/signal.h

When a signal comes …

• Process have 3 choices:1. Accept

- signal(SIGINT, SIG_DFL)2. Ignore

- signal(SIGINT, SIG_IGN)3. Call a function

How to call a function

signal(signum, functionname);

signal

PURPOSE Simple signal handling

INCLUDE #include <signal.h>

USAGE result = signal(int signum, void(*action)(int))

ARGS signum : the signal to respond toaction : how to respond

RETURNS -1 if errorprevaction if success

Examples of signal handling

• sigdemo1.c : catching a signal#include <stdio.h>#include <signal.h>

main(){void f(int); int i;signal(SIGINT, f);for (i=0;i<5;i++){

printf("hello\n");sleep(1);

}}void f(int signum){ printf("OUCH!\n"); }

How sigdemo1 works

Run sigdemo1

$ ./sigdemo1hellohello (press Ctrl-C !)OUCH!hellohellohello

Examples of signal handling

• sigdemo2.c : ignoring a signal

#include <stdio.h>#include <signal.h>main(){

signal(SIGINT, SIG_IGN);printf("you can't stop me!\n");while(1){sleep(1);printf("haha\n");}

}

Run sigdemo2

$ ./sigdemo2you can’t stop me!hahahaha (press Ctrl-C !) (SIGINT)hahahaha (press Ctrl-\ !) (SIGQUIT)$

• What count is the number of printed “haha”s!• Why only four times?