Survey of Advanced Perl Topics

1
Survey of Advanced Perl Topics

• Database access
• "OpSys"-like functions
• Signal Handling
• Tieing

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Database Access

• Standardized through the DBI module
• not core module, but installed on CS system
• Differing Database systems have their own DB
  driver module
• DBDmysql, DBDOracle, DBDInformix, etc
• By simply changing which DBD you use, your
  existing code can work with a new database.
• Check the FAQ at http//www.cs.rpi.edu/lab/labmanu
  al.html for generic instructions on setting up a
  DB on CSNet

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General Format of Perl DBI

• declare a "DataBase Handler" to connect to the DB
• define SQL statement(s)
• prepare the SQL, returning "STatement Handler(s)"
• execute the statement handlers
• fetch the results from the statement handlers, if
  apporpriate

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DBI Example

• use DBImy dsn 'DBImysqldatabaselallip_db
  hostbarbara.cs.rpi.eduport1234'my dbh
  DBI-gtconnect(dsn,user,password)
• my sql "SELECT FROM players "sql .
  "WHERE score gt ? AND score lt ?"
• my sth dbh-gtprepare(sql)sth-gtexecute(mins
  core, maxscore)
• while (my r sth-gtfetchrow_hashref) print
  "r-gtname's score r-gtscore\n"
• - You can use the same sth to now execute the
  prepared SQL with different values

5
Fetching Methods

• fetchrow_hashref
• fetch "next" row, as a hash reference (key
  column name, value field value)
• fetchrow_arrayref
• fetch next row, as an array reference (in order
  defined by the table)
• fetchrow_array
• fetch next row as an array
• fetchall_arrayref
• fetch entire results
• no parameters - entire table as arrayref of
  arrayrefs
• pass array ref - select numbers of which columns
  of entire table to return
• pass hashref - select names of which columns to
  return, and return as an array of hash referneces
• pass empty hashref - return entire table as array
  of hash references
• fetchall_hashref(key)
• fetch entire results, as a hashref of hashrefs.
  Key is index of table

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DBI errata

• if you don't need to fetch (an UPDATE, INSERT, or
  DELETE statement),
• just use do(). No need to prepare or execute
• dbh-gtdo('DELETE FROM class WHERE drop 1')
• sth-gtrows returns the number of rows inserted,
  updated, or deleted.
• DOES NOT return number of rows that are selected!
• SQL NULL gt Perl undef
• dbh-gtRaiseError 1, Perl will die on any SQL
  error.
• (Otherwise, must check return value of every db
  call, and then check DBIerr)
• http//dbi.perl.org perldoc DBI

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"OpSys-like" functions

• touched on these earlier in the semester
• basically said "Don't do that".
• please, take extreme caution when using these
  functions
• listing of Perl equivalents only
• for more information about the internals of Unix,
  take OpSys

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fork()

• split off a separate process, duplicating the
  code and environment
• return value in parent is child's new pid
• return value in child is 0
• my pid fork()if (pid) parent print
  "Child pid just forked off\n"
  do_parent_stuff() else print "I'm the
  child, just spawned\n" do_child_stuff()

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wait()

• wait for one of the child processes to finish
• returns the PID of the child that just finished
• ? is set to the exit status of the child that
  was just found with wait
• waitpid(pid)
• wait for a specific child to exit

10
exec(cmd, _at_args)

• Execute cmd, passing _at_args to that command
• executes IN THE CURRENT PROCESS, wiping out
  anything else this code was going to do
• Example I have an if statement in hw_submit.pl
  so that if you tried using it to submit hw4, it
  exec'ed hw_submit4.pl
• therefore, has no return value
• any code below the exec (other than a warning
  that the exec failed) is meaningless.
• retval system(cmd, _at_args) is equivalent to
• if (my pid fork()) waitpid(pid) retval
  ? else exec (cmd, _at_args)

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Signal Sending

• Processes can send signals to one another.
• Most common use is to tell a process to die
• Therefore, function to do this is kill
• kill(pid, signal)
• to see which signals are available, run kill -l
  on your system
• By default, most (all?) signals result in program
  termination
• Most shells respond to a CTRL-C by sending the
  current process a SIGINT

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Signal Handling

• With the exception of SIGKILL (9) and SIGSTOP
  (23), all signals can be caught and processed.
• If your program receives a certain signal, you
  can decide what to do about it.
• Assign a reference to the handler subroutine to
  the SIG hash, where the key is the 'name' of the
  signal
• signal name also passed as first argument to the
  subroutine.
• SIGINT sub print "Bwaha, your CTRL-C
  doesn't scare me!"
• Now, if the user tries to CTRL-C your program,
  the message will be printed out instead of the
  program dieing.
• (To actually kill this script, find out its pid
  from ps -u ltrcsidgt, and then send it a SIGKILL
  kill -9 ltpidgt)

13
Tieing

• a method by which you can make your objects hide
  all their functionality, pretending to be
  ordinary variables
• Example TieFile
• hides functionality of a class in an ordinary
  array
• "tie" an array to a file.
• accessing elements of the array is now equivalent
  to accessing lines of the file
• push, pop, splice, foon, etc all Do The Right
  Thing

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Tieing your own classes

• perldoc perltie
• constructor TIESCALAR, TIEARRAY, TIEHASH
• what type of variable should your object pretend
  to be?
• Create your object, just as you would in any
  other constructor.
• Define your object however you like, probably as
  a hashref
• User will call your object's constructor using
  the tie() function
• tie my sca, 'TieMyObj'
• tie my _at_arr, 'TieMyObj', 'alpha', 'beta'
• tie my hsh, 'TieMyObj'

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Tied methods

• For scalars FETCH and STORE
• define FETCH to take one argument - the object,
  and return whatever value should be returned when
  the user "reads" the object
• define STORE to take two args - the object, and
  the value to store, make appropriate changes to
  the object when user "writes" to it
• For arrays FETCH and STORE take additional
  argument - index user wants to retrieve or write
  to.
• Also FETCHSIZE and STORESIZE for when array used
  in scalar context or the arr variable is used
• Also PUSH, POP, SHIFT, UNSHIFT, SPLICE subs
  that define what happens if user uses these
  functions on the array

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Tied Hash Methods

• For hashes FETCH/STORE also take a key
• EXISTS, DELETE
• also each take a key. EXISTS should return true
  if the key is "in" the hash DELETE should remove
  the key from the hash
• FIRSTKEY
• Called the first time Perl processes the keys()
  or each() function. Should return the "first"
  key from the list of keys of the hash.
• NEXTKEY
• Called each subsequent time Perl processes the
  keys() or each() function (within a loop). Takes
  the previous key as an argument, and should
  return the "next" key.
• SCALAR
• called when hash is used in a scalar context

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Recommended Further Reading

• Advanced Perl Programming, Simon Cozens
• Introspection, parsing beyond Regexps,
  Templating, Databases, Unicode, Event Driven
  Programming, Embedding C in Perl
• Higher Order Perl, Mark Jason Dominus
• Recursion Callbacks, Dispatch Tables,
  Iterator-based programs, Parsing Infinite
  Streams, "Higher-Order" functions, Program
  transformations
• Perl Best Practices, Damian Conway
• Tips and Advice for writing clean, elegant,
  maintainable Perl program