The horrorsNezer (T.A) and Evyatar (Bodek)

This ex – what we learned

planned Stream sockets – TCP

Stream sockets – UDS

Select, blocking and non blocking system calls

fork(2)

syslog(3)

getenv(3)

time(2)

signal(3)/sigaction(2)

waitpid(2)

For some of you vi or other unix editors

make/makefile

Debugging code in UNIX (gdb/ddd and core dumps)

Opendir/readdir/dirent

grep(1) and other basic UNIX functions

Strace(1)

Working on schedule

Ps(1)/kill(1) etc.

Some of you were lucky….

The lucky encountered enough bugs to learn something….

(The extremely lucky ones lost sleep over some of the bugs so they will remember them!)

This is a collection of common errors we encountered for the less fortunate ones

We also bring some style errors so you can learn for future ex.

Busy waitingThe code

While (timeToRun>time(null) ;

// that is call time infinite number of times

Better ways to actTimeToSleep=atoi(argv[2])-

time(NULL);

sleep(TimeToSleep); // or

alarm(TimeToSleep);

pause(); // or

struct timeval tv;

tv.tv_sec=TimeToSleep;

tv.tv_usec=0;

select (1, MULL, NULL, NULL, &tv);

The chicken and the eggWhat comes first > or

=?If (pid = fork() < 0)….

How it should be done

If ((pid = fork()) < 0)

< is calculated before =!!!

Logical assumptions and checking error codes

A team comes to me with the following input…

They didn’t change the server

The server used to work

Now the server does not appear in ps(1)

It also doesn’t accept clients

Reaching the only logical conclusion

The server becomes invisible

And UNIX hates them

After de-mysitification….

1. UNIX doesn’t hate them

2. The server crashes

After further investigation we found the crash in following line

Myport = atoi(getenv(“My port”));

What lesson have we learned?

The OS is my bitch 1Fopen mania

x=fopen (“log.txt”,”a”);

fprintf(x,”something”);

fclose(x);

// some lines later

x=fopen (“log.txt”,”a”);

fprintf(x,”something”);

fclose(x);

How it should be done

At main start : fopen/fopen

Each time we log we write/fprint (and possible fflush)

At finish we fclose/close

The OS is my bitch 2Stupid sleeping

While (timeToRun>time(null) sleep(1)

What is wrong

sleep the time difference

Don’t use magic numbers

No points were deducted for this (this is not busy waiting, just stupid)

Garbage is my friendSome of you just had to :

Send (sockfd, buf, 100, 0)

Resulting in sending 100-strlen(buf) bytes of garbage.

Unlucky perhaps but the garbage also contained null so your printing didn’t detect it

MY PRECIOUS BUFFER!!!! MINE! MINE!!!

Using buffers returned by dirent structure is perfectly legal…. Yet some of you had to copy everything to YOUR OWN buffer…..

Some of you even malloced the buffer and freed it at every iteration

Some of you forgot to free

Unless there was also no free no points were deducted.

The peakNot even 8.3

Memcpy(buf,dp->name,10);

Send(sockfd,s,10,0);

How it should work

Use original dirent data. (no need to copy)

Use dirent struct to tell you how much to send

Some things just hurt our eyes

Functions should be described in one sentence with no and/or.

If you can’t describe your functions with one sentence – break them to two.

How to learn something new!THE KISS METHOD

Common errorsDaemon process under SVR4 UNIX (Linux, Solaris,

HP-UX) need to fork() twice. (13.3 Advanced programming in the UNIX environment.)

For those that seen other sources in the internet: Daemon process under BSD UNIX (xxxBSD, OSX, AIX) can fork() only once. (but forking twice is not a mistake)

Environment parameters ARE NOT command line argument. (read the UNIX process presentation)

UDS socket was relative path name. (./smt) this will fail if the client is run from different directory! Use absolute path (/tmp/smt or ~/smt) to avoid this.

Using Syslog in TAU /* works between processes (such as between bzip

and main thread in the ex. Not between threads) */

Int main()

{

openlog(“scipio”, 0, LOG_LOCAL5);

syslog(LOG_ERR, “%s, %d : hello world”, __FILE__, __LINE__);

closelog();

}

Fcntl file locking example

#include <stdio.h>

#include <stdlib.h>

#include <errno.h>

#include <fcntl.h>

#include <unistd.h>

int main(int argc, char *argv[])

{

/* l_type l_whence l_start l_len l_pid */

struct flock fl = { F_WRLCK, SEEK_SET, 0, 0, 0 };

Fcntl example cont’d int fd;

fl.l_pid = getpid();

if (argc > 1) fl.l_type = F_RDLCK;

fd = open("lockdemo.c", O_RDWR)

fcntl(fd, F_SETLKW, &fl);

fl.l_type = F_UNLCK;

fcntl(fd, F_SETLK, &fl) ;

close(fd);

}

GCC versionsChar s[200];

Sizeof(s)???

In tau that returns 200;

We have tried on other servers and since sizeof char *=4 and char [] is actually char*…. That returned 4.

To avoid problems in the future don’t do sizeof array but sizeof (element) * array size.

No points were deducted.

(Evyatar apologizes for causing several undue heart attacks).

Request from System Avoid running your jobs on NOVA if you can

You are Running lots of jobs Busy waiting Taking all the CPU

Instead once you login, ssh to any doppler or Euler workstation and work there.

NOVA is the only host accessible from outside TAU. So if you login to NOVA and load it with unbearable load you are screwing your fellow students

IF NOVA DOESN’T REPLY Don’t wait for the last minute to submit the ex.

Check your timeouts. NOVA was very stable during the ex. Submission but also very loaded and several students failed to login due to short timeouts

REFERENCES FOR USERLAND

MAN PAGES

ADVANCED PROGRAMMING IN THE UNIX ENVIRONMENT/Stevens & Rago (chapters 4,7,8,9,10,13,14,(15 = bonus), 16, 17 were covered in ex. 1 + chapters 3+8+10+11+12+14 are covered in ex. 2)

KR (The C programming language – Brian Keringhan and Dennis Ritchie)

Other good books

Solaris System programming / Rich teer (run the examples to make sure they work on Linux as well) 1,4,6,10,13,14,15,16,17,18,19,21 – Almost identical arrangement to Stevens with some Solaris specific code

Advanced UNIX programming (Warren W. Gay) 1-7, 9-11, 15-19,22,26

The Costsend(3) with fixed buflen parameter: -3

Segmentation fault when running client with missing/no parameters: -5

UDS only connects when client and server are in the same folder: -10

Sending only 10 chars of file name. -10

Not using environment variables at all: -10

Using printf() in the server daemon: -2

Having us kick-start your project (i.e. project that would crash) – variable

More cost Not printing daemon getppid(): -10

(first line in the server description!)

Not setting the session ID and letting me use ctrl+c to kill your server: -10

Client hangs on a past/illegal time index: -5

Server crashes when UDS connects: -30

Not logging to syslog() OR file: -10

Not running as Daemon at all. -20

Not logging child termination: -10

A Silver LiningLeaving defunct processes (zombies): just a

warning

Printing getpid() instead of getppid(): just a notice

Non-functional FIFO with effort: +[1,9]

Doing well on the oral exam: +[1,10]

FYIWith the exception of one team (who had some

problems) all teams submitted on time.

AVG grade was about 85.

No students that was tested Wednesday copied his work in our opinion

Most students were very successful in the oral exam. (nobody was mistaken in more then 1 question. And 1 was rare)

Regarding ex. 2 Here is what I meant for you to do

1. I am a thread

2. The file to be sent is X, last position sent was Y, chunk size is Z

3. If the file isn't locked: 

     3.1. lock 

     3.2 create chunk from Y to Y+Z  

3.3 Increase Y by Z 

     3.4 Release lock

3.5 send (Send can also be done in 3.2 but this is more accurate)

Thanks to Dor Levi that put it clear.

Defining protocol If you want to support only 1 client you only need

to define the last packet and the packet number (As they may arrive out of order). You may want to transfer the filesize and offset instead.

If you want to support several clients you need to add mutex for file name received (or to start different set of connection handling threads for each client, with different conds) and make sure files don’t overwrite each other.

Exact design it up to you. You may use temp files but you have to clean your work

SyncingWhile it is possible to do no syncing AT ALL

(this ex. Can work even as single thread sending or compressing. (since we work on the same host no sending is even necessary)

BUT you are required to sync the file reading and sending process