Transcript Portable perl How to write code that will work everywhere Some common fallacies • Perl is portable, my code is written in perl –

Portable perl
How to write code that will work
everywhere
Some common fallacies
• Perl is portable, my code is written in perl
– Therefore my code is portable.
Some common fallacies
• Perl is portable, my code is written in perl
– Therefore my code is portable.
• WRONG!
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.
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!
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
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
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
Operating system platforms
• Unix
Operating system platforms
• Unix
• MS Win32
Operating system platforms
• Unix
• MS Win32
• Apple Mac
Operating system platforms
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Unix
MS Win32
Apple Mac
VMS
Other (see perldoc perlport for the full list)
What are the main issues
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File names and paths
Shell commands
Environment variables
User interaction
Communications
Multitasking
Internationalisation
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
POSIX filenames
• A platform independent standard
/foo/bar/module.pm
lib/Foo/Bar/module.pm
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
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.TXTNO!!
The alternative
• Use native syntax throughout
• Test $^O for the operating system
• Do what is necessary to construct a filename
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));
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
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
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
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
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
Environment Variables
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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.
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.
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);
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
A better way
• Refer to perlfaq8:
– How do I ask the user for a password?
use Term::ReadKey;
ReadMode('noecho');
my $password = ReadLine(0);
• Unfortunately, the POD for Term::ReadKey says:
– ReadLine MODE [, Filehandle]
• This call is currently not available under Windows.
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');
Other nice features of Term::ReadLine
• Allows command recall from the history if
the O/S supports it
• Can operate inside Tk
Communications
• File sharing with a different architecture
• Exchanging packets over a network
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
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)
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
Line termination
• Remember:
"\n" ne "\012"
| Unix | DOS | Mac |
--------------------------\n
| LF | LF | CR |
\r
| CR | CR | LF |
\n * | LF | CRLF | CR |
\r * | CR | CR | LF |
--------------------------* text-mode STDIO
• perldoc perlport goes into more detail.
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.
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
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
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.
Choices for the perl admin
• Download and install a pre-built binary
• Roll your own
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.
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
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
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.
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
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