Portable perl

1. Portable perl How to write code that will work everywhere

2. Some common fallacies • Perl is portable, my code is written in perl • Therefore my code is portable.

3. Some common fallacies • Perl is portable, my code is written in perl • Therefore my code is portable. • WRONG!

4. Some common fallacies • Perl is portable, my code is written in perl • Therefore my code is portable. • WRONG! • My code uses no XS, calls no XS, • Therefore portable.

5. Some common fallacies • Perl is portable, my code is written in perl • Therefore my code is portable. • WRONG! • My code uses no XS, calls no XS, • Therefore portable. • WRONG!

6. Who cares about portability • Portability is only an issue if you genuinely want your code to work everywhere. • Portability is not a requirement for ad-hoc scripts written for a single environment

7. Who cares about portability • Portability is only an issue if you genuinely want your code to work everywhere. • Portability is not a requirement for ad-hoc scripts written for a single environment • But… Think of CPAN. • You publish your module • You want it to work on as many platforms as possible

8. Nearly all of Perl is portable • Perl tries to bridge the gaps between different platforms • It is usually worth the mileage to make modules portable. • Module authors should be interested in making/keeping their modules portable

9. Operating system platforms • Unix

10. Operating system platforms • Unix • MS Win32

11. Operating system platforms • Unix • MS Win32 • Apple Mac

12. Operating system platforms • Unix • MS Win32 • Apple Mac • VMS • Other (see perldoc perlport for the full list)

13. What are the main issues • File names and paths • Shell commands • Environment variables • User interaction • Communications • Multitasking • Internationalisation

14. File names • Unix • /home/me/my_dir/my_file • Windows • C:\My documents\my_dir\My file.txt • VMS • MY_DEVICE:[ME.MY_DIR]MYFILE.TXT;1

15. POSIX filenames • A platform independent standard /foo/bar/module.pm lib/Foo/Bar/module.pm

16. POSIX filenames • A platform independent standard /foo/bar/module.pm lib/Foo/Bar/module.pm • Works on Windows: C:/perl/lib/test.pm • Even works on VMS /my_device/me/my_dir/myfile.txt

17. Problems with POSIX filenames • No provision for a 'volume' or 'device' • (in Unix terminology, a mount point) • Variations in character set • E.g. Windows allows spaces • Is underscore allowed? Dash? Dollar? More than 1 dot? Punctuation characters? • Case sensitivity • On Unix, FOO, Foo and foo are different files • Accented letters • Problems if you mix native and POSIX syntax C:/mydir\myfile.txt OK sometimes MYDEV:[MYDIR]SUBDIR/FILE.TXT NO!!

18. The alternative • Use native syntax throughout • Test $^O for the operating system • Do what is necessary to construct a filename

19. The alternative • Use native syntax throughout • Test $^O for the operating system • Do what is necessary to construct a filename • Fortunately, this has been done for you • use File::Spec; • my $file = File::Spec->catfile( • qw( mydev mydir file.txt));

20. File::Spec and File::Spec::Functions • File::Spec provides a class methods API • File::Spec::Functions provides equivalent functions for export. • The following are exported by default: • canonpath catdir catfile curdir rootdir updir no_upwards file_name_is_absolute path • There are also other functions available: • devnull tmpdir splitpath splitdir catpath abs2rel rel2abs case_tolerant

21. A word of warning • File::Spec merely manipulates strings • curdir returns ‘.’ on Unix • canonpath merely removes /. and /foo/.. etc. • use Cwd to handle real paths • cwd gives you what curdir really is • abs_path does what you would expect canonpath to do

22. Other things to watch out for with files • Symbolic links • Are Unix specific • Many O/S don't even support hard links • Security and permissions • (rwx rwx rwx) is a Unix convention • Some O/S don’t have the same concept of a user and a group • VMS ends directories with a .DIR extension

23. Calling the shell • Don't do it! • (if you want your code to be portable) • Thus, avoid pipe files, backticks and system • Unless providing a mechanism for your interactive user to type in commands. • If you absolutely must • Stick to commands with a common syntax • Test $^O and provide variants

24. Shell globbing • Not all command interpreters expand wildcards • (i.e. not all shells glob). Unix shells do glob. perl myprog.pl *.txt • Perl on MS-DOS does not glob • Perl on VMS does glob! • Though the VMS command line interpreter does not

25. Environment Variables • Unix scripters are used to having standard ones: • HOME TERM SHELL USER etc. • These are all Unix specific. • $ENV{PATH} is common to most O/S's • But not all, VMS for example. • VMS does not need PATH, as you are required to enter the command "RUN" in order to run a program, or @ for a script.

26. Interacting with the user Problem: • You really need to talk to the user. • stdin, stdout and stderr could be redirected • You want to prompt him for a password. • You don't want to proceed until you have received the password.

27. Non-portable solution open TTYIN,'/dev/tty' or die "Failed to open terminal"; open TTYOUT,'>/dev/tty' or die "Failed to open terminal"; # Autoflush on for TTYOUT select TTYOUT; $| = 1; # Echo off system "stty -echo"; print "Enter password:"; my $pass = <TTYIN>; chomp $pass; # Echo on again system "stty echo"; validate_pass($pass);

28. What was non-portable? • Use of "system" to control echoing • /dev/tty - the terminal will not be called this • A minor point: echoing might already be off

29. A better way • Refer to perlfaq8: • How do I ask the user for a password? • Unfortunately, the POD for Term::ReadKey says: • ReadLine MODE [, Filehandle] • This call is currently not available under Windows. use Term::ReadKey; ReadMode('noecho'); my $password = ReadLine(0);

30. Combining Term::ReadKey and Term::ReadLine use Term::ReadKey; use Term::ReadLine; my $term = Term::ReadLine->new; ReadMode('noecho'); my $password = $term->readline ("Enter Password:"); ReadMode('restore');

31. Other nice features of Term::ReadLine • Allows command recall from the history if the O/S supports it • Can operate inside Tk

32. Communications • File sharing with a different architecture • Exchanging packets over a network

33. Communications • File sharing with a different architecture • Exchanging packets over a network Look to standards • Each side changes the data to comply to the standard format

34. Line termination • How exactly is "\n" stored or transmitted? • On Unix, a single ASCII \012 (line feed) • On Windows, \015 \012 (carriage return, line feed) • On Mac OS, just plain \015 (carriage return)

35. Line termination • How exactly is "\n" stored or transmitted? • On Unix, a single ASCII \012 (line feed) • On Windows, \015 \012 (carriage return, line feed) • On Mac OS, just plain \015 (carriage return) • This means that "\n" can stand for two characters • This is why chomp is better than chop • This is important when reading foreign files • This is also important when sending and receiving packets

36. Line termination • Remember: "\n" ne "\012" • perldoc perlport goes into more detail. | Unix | DOS | Mac | --------------------------- \n | LF | LF | CR | \r | CR | CR | LF | \n * | LF | CRLF | CR | \r * | CR | CR | LF | --------------------------- * text-mode STDIO

37. Use binmode on non-standard files(i.e. other than local text files) • binmode makes no difference on Unix ASCII files. • Other O/S need to differentiate between text files and binary files. • binmode is needed for UTF-8 locale differences • "seek" and "tell" will produce spurious results if binmode is not correctly set.

38. Number representation • You will often be unable to read another system's binary numbers • Differing endianness • Different floating point representations • Use a "network" standard • pack and unpack format 'n' and 'N' • IEEE floats are standard and portable

39. Multitasking • Beware forks or threads • If you want to be portable, you are best off avoiding fork altogether. • But you can probably use a multitasking module if it has been written with portability in mind. • Threads support is far from brilliant on many platforms. • Not all O/S are that good at non-blocking I/O

41. Portability and XS modules • For the module writer, this is a question of writing portable C code, not portable perl. • XS code can be a pain for the installer as it needs a C compiler. • Not only does it need a C compiler; it needs the same one (and the same options) that the perl you are running was built with.

42. Choices for the perl admin • Download and install a pre-built binary • Roll your own

43. Choices for the perl admin • Download and install a pre-built binary • Roll your own • Rolling your own is usually better, but you always need a working ANSI C compiler • Even with pre-built binaries, you need a C compiler to extend perl (i.e. add XS modules) - the same one perl was built with.

44. Choices for Win32 • Download ActiveState perl • Module binaries available through ppm • MVC can be used to build CPAN modules • Roll your own with MVC and DOS • Good luck! • But it's do-able. Use nmake to make • Roll your own with Cygwin and MingW • You might need to spend time getting the C compiler to work first

45. Choices for most Unix O/S • O/S may come with perl pre-installed • But it's probably a really old version • Roll your own, to get the perl build you want • You may need to build and install gcc first • Or pay for a licensed ANSI C compiler

46. C compilers on Unix • HP-UX: /bin/cc not suitable for much apart from building kernels. gcc works fine. • Solaris: native cc is fine. Pre-installed perls were built with gcc though.

47. C compiler for VMS • Buy a licence for DECC - tough. • Get a test drive account and do all your building there, for free :) • gcc not recommended on this platform according to the README.VMS

48. Reporting portability problems in other people's code • Core modules (are meant to work everywhere) • Port problems in the core are bugs • Report to p5p via perlbug • Discuss on platform mailing lists e.g. vmsperl • Other CPAN modules • Consider whether the module was intended to be portable • Check reports from the CPAN testers • Use RT (rt.cpan.org) • Or mail the module author