Perl

1
Perl

• functions, pipes, and client side networking.

2
perl and functions

• Perl starts at the top of scripts and executes
  statements as it finds them. functions are
  skipped over until they are called and all
  functions are global.
• subroutines and functions are the same thing in
  perl
• example
• !/usr/bin/perl
• statements
• function
• statements
• call to function
• statements

3
syntax of functions

• basics
• sub NAME BLOCK
• example
• sub helloworld
• print "Hello World! \n"
• calling the function
• helloworld()
• a arguments to helloworld and return values are
  ignored by this call.
• function calls have the following syntax
• LIST FunctionName (LIST)
• () are "mostly" optional.

4
parameter list

• You don't "declare" the parameter to a function
  and you can send 0 or more parameters to any
  function
• Inside a function the parameter list is located
  in _at__
• example
• sub mysub
• my (parm1, parm2, parm3) _at__
• statements
• Call
• mysub (val1, val2, val3)

5
parameter list

• Since perl doesn't check the parameter, it is up
  to the function
• example
• sub ex2
• foreach par (_at__ )
• print "par \n"
• _at_arr (1, 2, 3, 4)
• ex2(_at_arr)
• _at_arr ("one", "two")
• ex2(_at_arr)

6
Pass by Value and Pass by Reference

• _at__ is a pass by reference value
• So, changing _at__ in the function changes the
  calling variables
• In the last example, _at__ was copied into
  variables. That is pass by value.
• Example
• sub mysub
• my (parm1, parm2, parm3) _at__
• parm1 2 _at__1 12
• Call
• val1 200 val2 0 val3 30
• mysub (val1, val2, val3)
• val1 200, val2 12, val3 30

7
return values

• use the return statement
• sub test
• print "Hi \n"
• return "Hi"
• v test()
• return values can be a scalar data and lists,
  including undef (normally used for error )

8
scoping issues and rules

• my var delcares a "private" variable
• The scope of the variables depends on where is
  declared.
• It is local to the BLOCK it is declared in and to
  any function declared with that BLOCK.
• NOTE if declared outside of BLOCK, then it is
  global. functions are always declared as global.
• Example
• my i 1
• sub iblock print "i\n" output "1 \n"
• iblock()
• print "i \n" output " \n"

9
scoping issues and rules (2)

• my i 1
• sub iblock print "i \n" prints 1
• iblock()
• print "i \n" prints 1
• Note the difference here.
• iblock()
• print "i \n" prints "\n"
• my i 1
• sub iblock print "i \n" prints "\n"
• i has not be declared yet, since my i has not
  executed until after the print statement.

10
my vs local

• my declares a variable private to it's block (or
  global if there is no block)
• local takes a global variable and gives it a
  temporary value for the block it is was
  "declared" in.
• example
• i 1
• print "i \n" output "1 \n"
• local i 2
• print "i \n" output "2 \n"
• print "i \n" output "1 \n"

11
my vs local (2)

• take the global variable / (read marker for
  file) and set it to undef inside the block.
• local /
• _ ltFILEHANDLEgt
• test()
• NOTE
• i1
• sub test print "i \n"
• test() output "1 \n"
• local i2
• test() output "2 \n"

12
Exercise 12

• Using the grading program from previous exercises
• Write a function that determines the grade based
  on the numerical value. It will return the
  letter grade.
• The "main" code calls the function with a
  numerical grade and print the letter grade.

13
built-in functions

• file system functions
• chmod LIST such as chmod 755 'file1'
• opendir, readdir, closedir
• opendir(DIRHANDLE, NAME) like file open, but
  for directories
• opendir(THISDIR, ".")
• LIST readdir DIRHANDLE
• _at_allfiles readdir THISDIR all the filesnames
  are put in _at_allfiles
• nextfile readdir THISDIRthe next filename is
  put in nextfile
• closedir DIRHANDLE
• close THISDIR

14
built-in functions (2)

• do FILE executes the contents of the file as a
  Perl script
• do 'stat.pl'
• chdir DIRECTORY change current working
  directory
• chdir "/usr/local/" change to /usr/local
• chdir change to user home directory
• mkdir FILENAME , MASK
• mkdir "dir"
• rmdir FILENAME
• rmdir "dir" if the directory is empty
• unlink FILENAME deletes the FILENAME
• unlink "filename"

15
built-in functions (3)

• file test (operators)
• -e "file" true if "file" exists
• -r "file" true if "file" is readable
• -w "file" true if "file" is writable
• -d "dir" true if "dir" is a directory
• -T "file" true if "file" is a text file
• Not a binary file
• -f "file" true if "file is a regular file
• Not a directory, device file, etc.
• Examples
• if ( -e "filename") print "filename exists"
• -r file and open FP, file

16
Walking a filesystem

• !/usr/bin/perl
• use Cwd module for finding the current working
  directory
• sub WalkDir
• my (workdir) shift or shift _at__
• my (startdir) cwd() keep track of where
  we began
• if (-r workdir) print "startdir/workdir
  \n"
• else print "Unable to enter
  startdir/workdir \n" return
• chdir(workdir) or die "Unable to enter dir
  startdir workdir!\n"
• opendir(DIR, ".") or die "Unable to open
  workdir!\n"
• my _at_names readdir(DIR) or die "Unable to
  read workdir!\n"
• closedir(DIR)
• foreach my name (_at_names)
• next if (name eq ".") current
  directory
• next if (name eq "..") parent
  directory
• next if (-l name) linked file or
  directory, next
• if (-d name) is this a directory?

17
built-in functions (4)

• time functions
• time returns the number of nonleap seconds
  since "The epoch"
• UNIX since Jan 1, 1970 000000 UTC
• gmtime returns as greenwich time zone
• (sec, min, hour, mday, mon,year,wday,yday,
  isdst) gmtime
• (min,hour) gmtime1,2
• localtime returns the local time zone
• (sec, min, hour, mday, mon,year,wday,yday,
  isdst) localtime
• (min,hour) localtime1,2
• NOTE months 0 through 11, instead of 1 12
• and year is starts at 1900, so 2002 is 102
• so year 1900 month for corrections.

18
built-in functions (5)

• couple other functions
• srand EXPR seeds random number generator
• rand returns a floating-point number from 0 to
  1 (excluding 1)
• rand EXPR return a floating-point number from 0
  to EXPR (excluding EXPR)
• sleep EXPR
• puts the script to sleep for EXPR seconds
• return how many sleeps the script slept.
• x system("COMMAND")
• system("rm rf /tmp/") execute the command
  specified. output is not returned to the script.

19
built-in functions (6)

• back quotes
• perl will execute a command (passing command to
  the shell/DOS prompt to be executed).
• The output of the command is returned.
• returnoutput command
• _at_outputarr command
• example
• out ls -la for UNIX
• out dir for windows
• for both, output is return to out, newline
  between each line out put from the command.
• _at_arr dir or _at_arr ls la
• Each line is placed in an array element (starting
  at 0)

20
Unix built-in functions

• Access to the password file (even shadow password
  file and NIS passwords)
• Perl has a number of function for sysadmin on
  UNIX
• getpwuid(NUMBER)
• returns username for uid number
• getpwnam("string")
• return uid number for the usename
• Each return an array when set to an array value
• (usrname, passwd, uid, gid, quota, comment,
  gcos, dir, shell) getpwnam("seker")
• For on the home home directory
• dir (getpwnam("seker"))7

21
Unix built-in functions (2)

• To access and process the entire "password" file
• setpwent()
• initialize access to the "password file"
• _at_list getpwent()
• fetch next entry of password file
• returns undef, for end of file.
• endpwent()
• "close" password file.
• For the "group file"
• getgrgid(id) and getgrnam("string")
• get group id (returns name) and get group name
  (returns id)
• setgrent, getgrent, and endgrent
• like processing pasword file
• return an array (name, passwd, gid, members)

22
Unix built-in functions (3)

• There are others. With the open, system, and
  backquote commands, most UNIX admin can be
  handled easily.
• I'll cover windows admin functions (win32 module)
  in lecture 7

23
Interprocess Communication

• IPC
• Two or more processes communicating
• The processes must agree on a common protocol
• We'll cover process concurrency, pipes, and
  client network sockets.
• next lecture, server side network sockets.

24
Process Concurrency

• fork command
• The fork command creates a child process, which
  is duplicate copy of the parent process.
• Duplicates all resources and data
• Example
• a 12
• if (pid fork)
• Parent process
• print "Parent a a\n"
• wait wait for the child process to end
• elsif (defined pid) pid contains 0
• child process
• print "Child a a\n"
• else fork failed
• die "Can't fork ! \n"

25
Process Concurrency (2)

• In older versions of perl, win32 machines had to
  use perlfork.
• Use fork very carefully, a "out of control" fork
  can cause systems to crash.
• such as the parent and children process both
  continue to fork.
• The parent process should always use the wait
  command for child process. Child processes on
  exit return information to the parent process.
  If the child process is unable to do so, it can
  be left as zombie process (a process attempting
  to terminate, but unable to).

26
Pipes

• We've looked at pipes with the open command. But
  we were unable to use two way communication.
• command pipe READHANDLE, WRITEHANDLE
• allows two related processes to talk to each
  other.
• With one pipe, 1 process is the reader, the other
  is the writer (doesn't matter which one, so long
  as the roles are known.)
• With two pipes, you have two way communication.
• Process 1 reads on one pipe and writes to the
  second.
• Process 2 reads on the second pipe and writes to
  the first.
• We also use the autoflush command (in the
  IOHandle module), so data does not get "stuck"
  in a pipe.

27
one way pipe example

• !/usr/bin/perl
• use IOHandle
• pipe(READER,WRITER)
• WRITER-gtautoflush(1)
• if (pid fork)
• Parent process
• print WRITER "HI\n"
• waitpid(pid,0) wait for the child process to
  end.
• elsif (defined pid) pid contains 0
• child process
• chomp(line ltREADERgt)
• print "Child line \n"
• else fork failed
• die "Can't fork ! \n"
• close READER
• close WRITER

28
Two way pipes example

• !/usr/bin/perl
• use IOHandle
• pipe(CREADER,PWRITER) create pipe 1 child reads
  from, parent writes to.
• pipe(PREADER,CWRITER) create pipe 2 parent
  reads from, child writes to.
• PWRITER-gtautoflush(1)
• CWRITER-gtautoflush(1)
• if (pid fork)
• Parent process
• print PWRITER "Hi child\n"
• chomp(line ltPREADERgt)
• print "Parent line \n"
• waitpid(pid,0) wait for the child process to
  end.
• elsif (defined pid) pid contains 0
• child process
• print CWRITER "Hi parent\n"
• chomp(line ltCREADERgt)
• print "Child line \n"
• else fork failed

29
two way communication

• Opening two pipes is complicated. To make it
  simpler, we can open a socket-pair instead.
• NOTE socket-pair function is broken in
  ActivePerl
• Uses the Socket module
• More on the Socket module later on.
• Eliminates the need for multiple pipes.
• socketpair(CHILD,PARENT, AF_UNIX, SOCK_STREAM,
  PF_UNSPEC)
• The parent uses the CHILD to read and write
• The child uses the PARENT to read and write
• think of "reading from the PARENT" and "writing
  to the PARENT"

30
Socket-pair example

• !/usr/bin/perl
• use Socket
• use IOHandle
• socketpair(CHILD,PARENT, AF_UNIX, SOCK_STREAM,
  PF_UNSPEC)
• CHILD-gtautoflush(1)
• PARENT-gtautoflush(1)
• if (pid fork)
• parent code
• print CHILD "Hi from Parent\n"
• chomp(line ltCHILDgt)
• print "Parent line\n"
• close CHILD close PARENT
• waitpid(pid,0)
• else
• die "cannot fork !" unless defined pid
• child code
• chomp(line ltPARENTgt)
• print "Child line \n"
• print PARENT "Hi back from child\n"

31
Network Sockets

• Some quick definitions
• Server Offers it's services and "listens" for a
  request
• Client Requests services from a server by
  initiating a "connection" to the server.
• Socket (refers both the Sockets API and a
  communication endpoint.)
• stream (SOCK_STREAM), which provide a
  bidirectional, sequenced, and reliable channel of
  communication
• Used in major network applications
• TCP connection
• datagram (SOCK_DGRAM), which do not guarantee
  sequenced or reliable delivery, but are
  lightweight
• Used as broadcast applications, and "steaming
  video/audio" applications
• Think about as radio or TV
• UDP connection
• Port numbers are used to determine which program
  you want to talk to. Web servers usually use
  port 80, mail port 25, ftp port 23. RFC 1700
  lists well know port numbers.

32
Network Sockets(2)

• There are two modules used to make a connection
• IOSocket or Socket modules
• IOSocket is simpler of the two to establish a
  network connection
• Socket allows more control over the creation of
  the socket.
• Both create a connection to remote machine via a
  socket.

33
IOSocket module

• IOSocket for a client connection
• socket IOSocketINET-gtnew (PeerAddr gt
  remote_host, PeerPort gt remote_port, Proto gt
  "tcp", Type gt SOCK_STREAM)
• or die "Couldn't connect to remove_hostremote_
  port _at_\n"
• PeerAddr gt remote_host
• Where remote_host is either a IP address or
  Name, such as asuwlink.uwyo.edu or 129.72.60.2
• PeerPort gt remote_port
• Where remote_port is a port number, such port 80
  (http port), port 25 (smtp port), etc
• Proto gt "tcp", Type gt SOCK_STREAM
• Connection type
• OR Proto gt "udp", Type gt SOCK_DGRAM
• If these two parameters are left off, the default
  is "tcp", SOCK_STREAM.

34
IOSocket module (2)

• Once it's open use the socket variable as you
  would a FILEHANDLE to read and write the socket.
• sending
• print socket "bah bah\n"
• Receiving
• line ltsocketgt
• To close the connection
• close (socket)

35
IOsocket client example

• !/usr/bin/perl
• use IOSocket
• Read the index page from asuwlink's web server
• remote_host "asuwlink.uwyo.edu"
• remote_port "80"
• socket IOSocketINET-gtnew (PeerAddr gt
  remote_host, PeerPort gt remote_port, Proto gt
  "tcp", Type gt SOCK_STREAM)
• or die "Couldn't connect to remote_hostremote_
  port _at_\n"
• print socket "GET / HTTP/1.0\n\n" http
  protocol
• normally, we would write the next line.
• local / http sends entire file, then closes
  connection, so we'll just read the entire file
• answer ltsocketgt
• print "answer \n"
• close socket

36
Socket module

• More complex and multi stepped.
• create a socket to use
• socket(SERVER, PF_INET, SOCK_STREAM,
  getprotobyname('tcp'))
• SERVER is the HANDLE
• PF_INET it the domain type, with IOSocket it
  was part of the new call.
• getprotobyname('tcp') get the O/S number for tcp
• should allows use this function, since the number
  has been known to change and for portability
  issues.

37
Socket module (2)

• Next build address of remote machine
• internet_addr inet_aton(remote_host)
• paddr sockaddr_in(remote_port,
  internet_addr)
• inet_aton function converts IP number/name to
  machine usable format.
• sockaddr_in for PF_INET function (socket_un for
  PF_UNIX) "packs" the port and name together to
  form a remote socket name
• Now connect to remote machine
• connect(SERVER, paddr)
• SERVER is the handle to use for read/writing to
  server, same as IOSocket socket

38
Socket client example

• /usr/bin/perl
• use Socket
• use IOHandle
• remote_host "asuwlink.uwyo.edu"
• remote_port 80
• socket(SERVER, PF_INET, SOCK_STREAM,
  getprotobyname('tcp'))
• SERVER-gtautoflush(1) socket doesn't have
  autoflush turned on.
• internet_addr inet_aton(remote_host) or die
  "Can't convert !\n"
• paddr sockaddr_in(remote_port,
  internet_addr)
• connect (SERVER, paddr) or die "can't connect
  !\n"
• print SERVER "GET / HTTP/1.0\n\n",flags
• local / again, same thing, normally won't
  set this.
• answer ltSERVERgt
• print "answer \n"
• close SERVER

39
Exercise 13

• Write a client app to connect to a remote
  machine. Use machine name giving to you as the
  host and port 3012
• It should send the message "Connection from YOUR
  NAME"
• and then wait for a response and print the
  response to STDOUT

40
Q
A