[10836] in Perl-Users-Digest
Perl-Users Digest, Issue: 4437 Volume: 8
daemon@ATHENA.MIT.EDU (Perl-Users Digest)
Wed Dec 16 10:07:26 1998
Date: Wed, 16 Dec 98 07:00:29 -0800
From: Perl-Users Digest <Perl-Users-Request@ruby.OCE.ORST.EDU>
To: Perl-Users@ruby.OCE.ORST.EDU (Perl-Users Digest)
Perl-Users Digest Wed, 16 Dec 1998 Volume: 8 Number: 4437
Today's topics:
Re: array initialisation (Joergen W. Lang)
binmode (J.K.)
error reading drive letters on win-nt, need help <ibelgaufts@gfc-net.de>
FMTEYEWTK on open <tchrist@mox.perl.com>
Re: Formatting Numbers (Clay Irving)
Re: How do I change current working directory ? <ghoti@telerama.lm.com>
Re: How do I change current working directory ? <qdtcall@esb.ericsson.se>
Re: How to automate post form submissions to a server t <jamesht@idt.net>
Re: How to extract emails from HTML page (Bart Lateur)
Re: Inconsistent behavior s/// ? <rra@stanford.edu>
Re: Inconsistent behavior s/// ? <ebohlman@netcom.com>
Special: Digest Administrivia (Last modified: 12 Dec 98 (Perl-Users-Digest Admin)
----------------------------------------------------------------------
Date: Wed, 16 Dec 1998 15:09:35 +0100
From: jwl@worldmusic.de (Joergen W. Lang)
Subject: Re: array initialisation
Message-Id: <1dk4via.1mq38p81pc8kjkN@host057-210.seicom.net>
Ala Qumsieh <aqumsieh@matrox.com> wrote:
> Maybe ... I don't know how to operate my new crystal ball yet.
That's almost always the biggest problem in answering those questions.
There's always a good part of guessing involved.
Maybe someday we have modules like Crystal::Ball and Read::Mind which
suggest answers in an associative manner.
(Didn't I see something related to this in an older version of perlfaq8
regarding the conversion of Shell Scripts to Perl ? ;-)
Joergen
--
-------------------------------------------------------------------
"Everything is possible - even sometimes the impossible"
HOELDERLIN EXPRESS - "Touch the void"
-------------------------------------------------------------------
------------------------------
Date: 16 Dec 1998 13:46:13 GMT
From: cljlk@hotmail.com (J.K.)
Subject: binmode
Message-Id: <758dj5$pn1$1@swlab1.msd.ray.com>
Would anyone please explain about binmode function and/or provide a
piece of example codes.
Many thanks
------------------------------
Date: Wed, 16 Dec 1998 09:53:51 +0100
From: "J|rgen Ibelgaufts" <ibelgaufts@gfc-net.de>
Subject: error reading drive letters on win-nt, need help
Message-Id: <3677751F.9732A433@gfc-net.de>
hi folks,
i am using perl on win-nt and need to find all valid drive letters. the following
snippet works fine:
@letters = qw/C D E F G H I J K L M N O P Q R S T U V W X Y Z/;
foreach (@letters) {
$letter = $_;
print "Testing drive $letter ... ";
if ( -e "$letter:\\") {
print "(OK)\n";
}
else {
print "\n";
}
}
it prints all letters and prints "OK" when a valid drive is found.
now, my problem is that if you have removable media, e. g. iomega zip drives with
no drive in it, windows issues a windowed messagebox saying something like "No
media in drive, insert media in drive \Device\Harddisk1\Partition1" (can't tell
the exact message text in english because i have a german winnt version).
does anybody see a chance to omit these messages ?
TIA
juergen ibelgaufts
------------------------------
Date: 16 Dec 1998 14:29:31 GMT
From: Tom Christiansen <tchrist@mox.perl.com>
Subject: FMTEYEWTK on open
Message-Id: <758g4b$p4m$1@csnews.cs.colorado.edu>
=head1 NAME
perlopentut - the basics on opening things in Perl
=head1 DESCRIPTION
Perl has two simple, built-in ways to open files: the shell way for
convenience, and the C way for precision. The choice is yours.
=head1 Open a la shell
Perl's C<open> function was designed to mimic the way command-line
redirection in the shell works. Here are some basic examples
from the shell:
$ myprogram file1 file2 file3
$ myprogram < inputfile
$ myprogram > outputfile
$ myprogram >> outputfile
$ myprogram | otherprogram
$ otherprogram | myprogram
And here are some more advanced examples:
$ otherprogram | myprogram f1 - f2
$ otherprogram 2>&1 | myprogram -
$ myprogram <&3
$ myprogram >&4
Programmers accustomed to constructs like those above can take comfort
in learning that Perl directly supports these familiar constructs using
virtually the same syntax as the shell.
=head2 Simple Opens
The C<open> function takes two arguments: the first is a filehandle,
and the second is a single string comprising both what to open and how
to open it. C<open> returns true when it works, and when it fails,
returns a false value and sets the special variable $! to reflect
the system error.
For example:
open(INFO, "datafile") || die("can't open datafile: $!");
open(INFO, "< datafile") || die("can't open datafile: $!");
open(RESULTS,"> runstats") || die("can't open runstats: $!");
open(LOG, ">> logfile ") || die("can't open logfile: $!");
If you prefer the low-punctuation version, you could write that this way:
open INFO, "< datafile" or die "can't open datafile: $!";
open RESULTS,"> runstats" or die "can't open runstats: $!";
open LOG, ">> logfile " or die "can't open logfile: $!";
A few things to notice. First, the leading less-than is optional.
If omitted, Perl assumes that you want to open the file for reading.
Next, a filehandle not really directly assignable (but you can look
at the FMTEYEWTK on Indirect Filehandles for alternative strategies).
Instead, you select a filehandle by choosing an identifier. This is
a symbol much like a subroutine name, and is in the package namespace.
By convention, we don't use all lower-case for fear of clashing with a
reserved word. For example, if we had used C<log> instead of C<LOG>,
the best we could hope for is some very strange errors as perl replaces
that symbol with the result of calculating the natural logarithm of the
value currently stored in $_. Not a pretty picture.
The other important thing to notice is that, just as in the shell,
any white space before or after the filename is ignored. This is good,
because you wouldn't want these to do different things:
open INFO, "<datafile"
open INFO, "< datafile"
open INFO, "< datafile"
It also helps for when you read a filename in from a different
file, and forget to trim it before opening:
$filename = <INFO>; # oops, \n still there
open(EXTRA, "< $filename") || die "can't open $filename: $!";
This is not a bug, but a feature. Because C<open> mimics the shell in
its style of using redirection arrows to specify how to open the file, or
append (or more -- see below), it follows the Principle of Least Surprise
also does so with respect to extra white space around the filename itself.
For accessing files with strange names, keep reading.
=head2 Pipe Opens
In C, when you want to open a file using the standard I/O library,
you use the C<fopen> function, but when opening a pipe, you use the
C<popen> function. But in the shell, you just use a different redirection
character. That's also the case for Perl. The C<open> call
remains the same -- just its argument differs.
If the leading character is a pipe symbol, you start up a new command and
open a write-only filehandle leading into that command. This lets you
write into that handle and have what you write show up on that command's
standard input. For example:
open(PRINTER, "| lpr -Plp1") || die "cannot fork: $!";
print PRINTER "stuff\n";
close(PRINTER) || die "can't close lpr: $!";
If the trailing character is a pipe, you start up a new command and open a
read-only filehandle leading out of that command. This lets whatever that
command writes to its standard output show up on your handle for reading.
For example:
open(NET, "netstat -i -n |") || die "cannot fork: $!";
while (<NET>) { } # do something with input
close(NET) || die "can't close netstat: $!";
What happens if you try to open a pipe to or from a non-existent command?
In most systems, such an C<open> will not return an error. That's
because in the traditional C<fork>/C<exec> model, running the other
program happens only in the forked child process, which means that
the failed C<exec> can't be reflected in the return value of C<open>.
Only a failed C<fork> shows up there. See L<perlfaq8/"Why doesn't open()
return an error when a pipe open fails?"> to see how to cope with this.
There's also an explanation in L<perlipc>.
If you would like to open a bidirectional pipe, the IPC::Open2
library will handle this for you. Check out L<perlipc/"Bidirectional
Communication with Another Process">
=head2 The Minus File
Again following the lead of the standard shell utilities, Perl's
C<open> function treats a file whose name is a single minus, "-", in a
special way. If you open minus for reading, it really means to access
the standard input. If you open minus for writing, it really means to
access the standard output.
What happens if you open a pipe into or out of minus? What's the
command being run? This is actually stealth C<fork> hidden inside an
C<open> call. See L<perlipc/"Safe Pipe Opens"> for details.
=head2 Mixing Reads and Writes
It is possible to specify both read and write access. All you do is
add a "+" symbol in front of the redirection. But as in the shell,
using a less-than on a file never creates a new file; it only opens an
existing one. On the other hand, using a greater-than always clobbers
(truncates to zero length) an existing file, or creates a brand-new one
if there isn't an old one. Adding a "+" for read-write doesn't affect
whether it only works on existing files or always clobbers existing ones.
open(WTMP, "+< /usr/adm/wtmp")
|| die "can't open /usr/adm/wtmp: $!";
open(SCREEN, "+> /tmp/lkscreen")
|| die "can't open /tmp/lkscreen: $!";
open(LOGFILE, "+>> /tmp/applog"
|| die "can't open /tmp/applog: $!";
The first one won't create a new file, and the second one will always
clobber an old one. The third one will create a new file if necessary
and not clobber and old one, and it will allow you to read at any point
in the file, but all writes will always go to the end. In short, the
first case is substantially more common than the second and third cases,
which are almost always wrong.
In fact, when it comes to updating a file, unless you're working on
a binary file as in the WTMP case above, you probably don't want to
use this approach for updating. Instead, Perl's B<-i> flag comes to
the rescue. The following command takes all the C, C++, or yacc source
or header files and changes all their foo's to bar's, leaving
the old version in the original file name with a ".orig" tacked
on the end:
$ perl -i.orig -pe 's/\bfoo\b/bar/g' *.[Cchy]
This is a short cut for some renaming games that are really
the best way to update textfiles. See the second question in
L<perlfaq5> for more details.
=head2 Filters
One of the most common users for C<open> is one you never
even notice. When you process the ARGV filehandle using
C<E<lt>ARGVE<gt>>, Perl actually does an implicit open
on each file in @ARGV. Thus a program called like this:
$ myprogram file1 file2 file3
Can have all its files opened and processed one at a time
using no more complex a construct than:
while (<>) {
# do something with $_
}
If @ARGV is empty when the loop first begins, Perl pretends you've opened
up minus, that is, the standard input. In fact, $ARGV (the currently
open file during C<E<lt>ARGVE<gt>> processing, is even set to C<"-">
in these circumstances.
You are welcome to pre-process your @ARGV before starting the loop to
make sure it's to your liking. One reason to do this might be to remove
command options beginning with a minus. While you can always roll the
simple ones by hand, the Getopts modules are good for this.
use Getopt::Std;
# -v, -D, -o ARG, sets $opt_v, $opt_D, $opt_o
getopts("vDo:");
# -v, -D, -o ARG, sets $args{v}, $args{D}, $args{o}
getopts("vDo:", \%args);
Or the standard Getopt::Long module to permit named arguments:
use Getopt::Long;
GetOptions( "verbose" => \$verbose, # --verbose
"Debug" => \$debug, # --Debug
"output=s" => \$output ); # --output=somestring or --output somestring
Another reason for preprocessing arguments is to deal make an empty
argument list default to all files:
@ARGV = glob("*") unless @ARGV;
You could even filter out all but plain, text files. This is a bit
silent, of course, and you might prefer to mention them on the way.
@ARGV = grep { -f && -T } @ARGV;
Remember that a normal C<open> has special properties, in that it might
call fopen(3S) or it might called popen(3S), depending on what its
argument looks like; that's why it's sometimes called "magic open".
Here's an example:
$pwdinfo = `domainname` =~ /^(\(none\))?$/
? '< /etc/passwd'
: 'ypcat passwd |';
open(PWD, $pwdinfo)
or die "can't open $pwdinfo: $!";
This sort of thing also comes into play in filter processing. Because
ARGV processing employs the normal, shell-style Perl C<open>, it respects
all the special things we've already seen:
$ myprogram f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile
That program will read from the file F<f1>, the process F<cmd1>, standard
input (F<tmpfile> in this case), the F<f2> file, the F<cmd2> command,
and finally the F<f3> file.
One of the more interesting applications is to change files of a certain
name into pipes. For example, to autoprocess gzipped or compressed
files by decompressing them with I<gzip>:
@ARGV = map { /^\.(gz|Z)$/ ? "gzip -dc $_ |" : $_ } @ARGV;
Or, if you have the I<GET> program installed from LWP,
you can fetch URLs before processing them:
@ARGV = map { m#^\w+://# ? "GET $_ |" : $_ } @ARGV;
It's not for nothing that this is called magic C<E<lt>ARGVE<gt>>.
Pretty nifty, eh?
=head1 Open a la C
If you want the convenience of the shell, then Perl's C<open> is
definitely the way to go. On the other hand, if you want finer precision
than C's simplistic fopen(3S) provides, then you should look to Perl's
C<sysopen>, which is a direct hook into the open(2) system call.
That does mean it's a bit harder to use, but that's the price of
precision.
C<sysopen> takes 3 (or 4) arguments.
sysopen HANDLE, PATH, FLAGS, [MASK]
The HANDLE argument is a filehandle just as with open. The PATH is a
literal path, one that doesn't pay attention to any greater-thans or
less-thans or pipes or minuses. If it's there, it's part of the path.
The FLAGS argument contains one or more values derived from the Fcntl
module that have been or'd together using the bitwise or operator.
The final argument, the MASK, is optional; if present, it is combined with
the user's current umask for the creation mode of the file. You should
usually omit this.
Although the traditional values of read-only, write-only, and read-write
are 0, 1, and 2 respectively, this is known not to hold true on exotic
systems such as VMS. Instead, it's best to load in the appropriate
constants first from the Fcntl module, which supplies the following
common flags:
O_RDONLY Read only
O_WRONLY Write only
O_RDWR Read and write
O_CREAT Create the file if it doesn't exist
O_EXCL Fail if the file already exists
O_APPEND Append to the file
O_TRUNC Truncate the file
O_NONBLOCK Non-blocking access
Less common flags that are sometimes available on some operating systems
include C<O_BINARY>, C<O_TEXT>, C<O_SHLOCK>, C<O_EXLOCK>, C<O_DEFER>,
C<O_SYNC>, C<O_ASYNC>, C<O_DSYNC>, C<O_RSYNC>, C<O_NOCTTY>, C<O_NDELAY>
and C<O_LARGEFILE>. Consult your open(2) manpage or its local equivalent
for details.
Here's how to use C<sysopen> to emulate the simple C<open> calls we had
before. We'll omit the C<|| die $!> checks for clarity, but make sure
you always check the return values in real code. These aren't quite
the same, since C<open> will trim leading and trailing white space,
but you'll get the idea:
=over
=item To open file for reading:
open(FH, "< $path");
sysopen(FH, $path, O_RDONLY);
=item To open file for writing, create new file if needed or else truncate old file:
open(FH, "> $path");
sysopen(FH, $path, O_WRONLY | O_TRUNC | O_CREAT);
=item To open file for appending, create if necessary:
open(FH, ">> $path");
sysopen(FH, $path, O_WRONLY | O_APPEND | O_CREAT);
=item To open file for update, file must exist:
open(FH, "+< $path");
sysopen(FH, $path, O_RDWR);
=back
And here are things you can do with C<sysopen> that you cannot do with
a regular C<open>. As you see, it's just a matter of controlling the
flags in the third argument.
=over
=item To open file for writing, create new file, file must not exist:
sysopen(FH, $path, O_WRONLY | O_EXCL | O_CREAT);
=item To open file for appending, file must exist:
sysopen(FH, $path, O_WRONLY | O_APPEND);
=item To open file for update, create file if necessary:
sysopen(FH, $path, O_RDWR | O_CREAT);
=item To open file for update, file must not exist:
sysopen(FH, $path, O_RDWR | O_EXCL | O_CREAT);
=item To open a file without blocking, creating if necessary:
sysopen(FH, $path, O_WRONLY | O_NONBLOCK | O_CREAT);
=back
=head2 Permissions a la mode
If you omit the MASK argument to C<sysopen>, Perl uses the octal value
C<0666>. The normal MASK to use for executables and directories should
be C<0777>, and for anything else, C<0666>.
Why so permissive? Well, it isn't really. These MASK will be modified
by your process's current C<umask>. A umask is a number representing
I<disabled> permissions bits; that is, bits that will not be turned on
in the created files' permissions field.
For example, if your C<umask> were 027, then the 020 part would
disable the group from writing, and the 007 part would disable others
from reading, writing, or executing. Under these conditions, passing
C<sysopen> 0666 would create a file with mode 0640, since C<0666 &~ 027>
is 0640.
You should seldom use the MASK argument to C<sysopen()>. That takes away
the user's option to have a more permissive umask than you might choose.
One good reason that the option exists is for special cases of privacy
concerns, such as mail files or cookie files.
=head1 Obscure Open Tricks
=head2 Re-Opening Files (dups)
Sometimes you already have a filehandle open, and want to make another
handle that's a duplicate of the first one. In the shell, we place an
ampersand in front of a file descriptor number when doing redirections.
For example, 2>&1 makes descriptor 2 (that's STDERR in Perl) be redirected
into descriptor 1 (which is usually Perl's STDOUT). The same is
essentially true in Perl: a filename that begins with an ampersand
is treated instead as a file descriptor if a number, or as a
filehandle if a string.
open(SAVEOUT, ">&SAVEERR") || die "couldn't dup SAVEERR: $!";
open(MHCONTEXT, "<&4") || die "couldn't dup fd4: $!";
That means that if a function is expecting a filename, but you
don't want to give it a filename because you already have the
file open that you care about, you can just pass it the filehandle
with a leading ampersand. It's best to use a fully qualified
handle though, just case the routine is in a different package:
somefunction("&main::LOGFILE");
This way if somefunction() is planning on opening its argument,
it can just use the already opened handle.
If you one of those tricky I/O objects, and not a proper filehandle,
then this doesn't work. You'll have to use fileno() to pull out
the proper descriptor number, assuming you can:
use IO::Socket;
$handle = IO::Socket::INET->new("www.perl.com:80");
$fd = $handle->fileno;
somefunction("&$fd");
It can be easier (and will certainly be faster) just to use real
filehandles though:
use IO::Socket;
local *REMOTE = IO::Socket::INET->new("www.perl.com:80");
die "can't connect" unless defined(fileno(REMOTE));
somefunction("&main::REMOTE");
If the filehandle or descriptor number is preceded not just with a simple
"&" but rather with a "&=" combination, then Perl will not use to create
a complete new descriptor opened to the same place. Instead, it will
just make something of an alias. This is slightly more parsimonious of
systems resources, although this seldom a concern anymore. Here's an
example of that:
$fd = $ENV{"MHCONTEXTFD"};
open(MHCONTEXT, "<&=$fd") or die "couldn't fdopen $fd: $!";
If you're using magic C<E<lt>ARGVE<gt>>, you could even pass in as a
command line argument in @ARGV something like "&=$MHCONTEXTFD", but
we've never seen anyone actually do this.
=head2 Dispelling the Dweomer
Perl is a DWIMmer language than, say, Java, where DWIM is "do what I
mean". But this principle sometimes leads to more hidden magic than one
knows what to do with. In this way, Perl is also filled with I<dweomer>,
an obscure word meaning an enchantment. Sometimes, Perl's DWIMmer is just
too much like dweomer for comfort.
If magic C<open> is a bit too magical for you, you don't have to turn
to C<sysopen>. To open a file with arbitrary weird characters in
it, it's necessary to protect any leading and trailing whitespace.
Leading whitespace is protected by inserting a C<"./"> in front of a
filename that starts with whitespace. Trailing whitespace is protected
by appending an ASCII NUL byte (C<"\0">) at the end off the string.
$file =~ s#^(\s)#./$1#;
open(FH, "< $file\0") || die "can't open $file: $!";
This assumes, of course, that your system considers dot the current
working directory, slash the directory separator, and disallows ASCII
NULs within a valid filename. Most systems follow these conventions,
including all POSIX systems as well as proprietary Microsoft systems.
The only vaguely system that doesn't work this way is the proprietary
Macintosh system, which uses a colon where the rest of us use a slash.
Maybe C<sysopen> isn't such a bad idea after all.
If you want to use C<E<lt>ARGVE<gt>> processing in a totally boring
and non-magical way, you could do this first:
# "Sam sat on the ground and put his head in his hands.
# 'I wish I had never come here, and I don't want to see
# no more magic,' he said, and fell silent."
for (@ARGV) {
s#^([^./])#./$1#;
$_ .= "\0";
}
while (<>) {
# now process $_
}
But be warned that users will not appreciate being able to use
C<"-"> to mean standard input as per the convention.
=head2 Paths as Opens
You've probably noticed how Perl's C<warn> and C<die> functions can
produce messages like:
Some warning at scriptname line 29, <FH> chunk 7.
That's because you opened a filehandle FH, and had read in seven records
from it. But what was the name of the file, not the handle?
If you aren't running with C<strict refs>, or if you've turn them off
temporarily, then all you have to do is this:
open($path, "< $path") || die "can't open $path: $!";
while (<$path>) {
# whatever
}
Since you're using the pathname of the file as its handle,
you'll get warnings more like
Some warning at scriptname line 29, </etc/motd> chunk 7.
=head2 Single Argument Open
Remember how we said that Perl's open took two arguments? That was a
passive prevarication. You see, it can also take just one argument.
If and only if the variable is a global variable, not a lexical, you
can pass C<open> just one argument, the filehandle, and it will
get the path from the global scalar variable of the same name.
$FILE = "/etc/motd";
open FILE or die "can't open $FILE: $!";
while (<FILE>) {
# whatever
}
Why is this here? Someone has to cater to the hysterical porpoises.
It's something that's been in Perl since the very beginning, if not
before.
=head2 Playing with STDIN and STDOUT
One clever move with STDOUT is to explicitly close it when you're done
with the program.
END { close(STDOUT) || die "can't close stdout: $!" }
If you don't do this, and your program filled up the disk partition due
to a command line redirection, it won't report the error exit with a
failure status.
You don't have to accept the STDIN and STDOUT you were given. You are
welcome to reopen them if you'd like.
open(STDIN, "< datafile")
|| die "can't open datafile: $!";
open(STDOUT, "> output")
|| die "can't open output: $!";
And then these can be read directly or passed on to subprocesses.
This makes it look as though the program were initially invoked
with those redirections from the command line.
It's probably more interesting to connect these to pipes. For example:
$pager = $ENV{PAGER} || "(less || more)";
open(STDOUT, "| $pager")
|| die "can't fork a pager: $!";
And now you automatically get everything paged automatically. This makes
it appear that your program was called already piped into the pager.
You can even use this in conjunction with an implicit fork to yourself
if you would rather handle the post processing in your own program,
but in a different process:
head(100);
while (<>) {
print;
}
sub head {
my $lines = shift || 20;
return unless $pid = open(STDOUT, "|-");
die "cannot fork: $!" unless defined $pid;
while (<STDIN>) {
print;
last if --$lines < 0;
}
exit;
}
This principle can be applied to repeatedly push as many
filters on your output stream as you care to.
=head1 Other I/O Issues
These aren't strictly related to open, but they can affect
how you open files.
=head2 Opening Non-File Files
When is a file not a file? Well, you could say
when it exists but isn't a plain file. We'll check
whether it's a symbolic link first, just in case.
if (-l $file || ! -f _) {
print "$file is not a plain file\n";
}
What other kinds of files are there that, well, files? Directories,
symbolic links, named pipes, Unix-domain sockets, and block and character
devices. Those are all files, too -- just not plain files.
To open a directory, you should use the C<opendir> function, then
process it with C<readdir>, carefully restoring the directory
name if necessary:
opendir(DIR, $dirname) or die "can't opendir $dirname: $!";
while (defined($file = readdir(DIR))) {
# do something with "$dirname/$file"
}
closedir(DIR);
If you want to process directories recursively, it's better
to use the File::Find module. For example, this prints out
all files recursively, add adds a slash to their names if
the file is a directory.
@ARGV = qw(.) unless @ARGV;
use File::Find;
find sub { print $File::Find::name, -d && '/', "\n" }, @ARGV;
To open a symbolic link, you can just pretend its what
it points to. Or, if you want to know I<what> it points
to, then C<readlink> is called for:
if (-l $file) {
if (defined($whither = readlink($file))) {
print "$file points to $whither\n";
} else {
print "$file points nowhere: $!\n";
}
}
Named pipes are a different matter. You pretend they're regular files,
but their opens will normally block until there is both a reader and
a writer. You can read more about them in L<perlipc/"Named Pipes">.
Unix-domain sockets are rather different beasts as well; they're
described in L<perlipc/"Unix-Domain TCP Clients and Servers">.
When it comes to opening devices, it can be easy and it can tricky.
We'll assume that if you're opening up a block device, you know what
you're doing. The character devices are more interesting. These are
typically used for modems, mice, and some kinds of printers. This is
described in L<perlfaq8/"How do I read and write the serial port?">
It's often enough to open them carefully:
sysopen(TTYIN, "+</dev/ttyS1", O_RDWR | O_NDELAY | O_NOCTTY)
or die "can't open /dev/ttyS1: $!";
sysopen(TTYOUT, "+>&TTYIN)
or die "can't dup TTYIN: $!";
$ofh = select(TTYOUT); $! = 1; select($ofh);
print TTYOUT "+++at\015";
$answer = <TTYIN>;
What else can you open? To open a connection using sockets, you won't use
one of Perl's two open functions. See L<perlipc/"Sockets: Client/Server
Communication"> for that. Here's an example. Once you have it,
you can use FH as a bidirectional filehandle.
use IO::Socket;
local *FH = IO::Socket::INET->new("www.perl.com:80");
For opening up a URL, the LWP modules from CPAN are just what
the doctor ordered. There's no filehandle interface, but
it's still easy to get the contents of a document:
use LWP::Simple;
$doc = get('http://www.sn.no/libwww-perl/');
=head2 Binary Files
On certain legacy systems with terminally convoluted (some would say
broken) I/O models, a file isn't a file -- at least, not with respect
to the C standard I/O library. On these old systems that distinguish
between text and binary streams, to get files to behave properly you'll
have to bend over backwards to avoid nasty problems. On such unhappy
systems, sockets and pipes are already opened in binary mode, and there
is currently no way to turn that off. With files, you have more options.
One thing you can do to do only raw I/O on the file using C<sysread>
and C<syswrite>. These are the real system calls and don't do any sneaky
data mutilation behind your back.
while (sysread(STDIN, $buf, 1024)) {
syswrite(STDOUT, $buf, 1024);
}
Your second option is to use the C<binmode> function function on the
appropriate handle before doing regular I/O on them:
binmode(STDIN);
binmode(STDOUT);
while (<STDIN>) { print }
Your third option is to pass C<sysopen> a non-standard flag option:
sysopen(BINDAT, "records.data", O_RDWR | O_BINARY)
|| die "can't open records.data: $!";
And now you can use C<read> and C<print> on that handle without worrying
about the system non-standard I/O library breaking your data. It's not
a pretty picture, but then, legacy systems seldom are. CP/M will be
with us until the end of days, and after.
=head2 File Locking
In a multitasking environment, you may need to be careful not to collide
with other processes who want to do I/O on the same files as others
are working on. You can't You'll often need shared or exclusive locks
on files for reading and writing respectively. (You might do best to
pretend there exist only exclusive locks.)
Never use the existence of a file C<-e $file> as a locking indication,
because there is a race condition between the test for the existence of
the file and its creation. Atomicity is critical.
Perl's most portable locking interface is via the C<flock> function,
whose simplicity is emulated on systems that don't directly support it,
such as SysV or WindowsNT. The underlying semantics may affect how it
all works, so you should learn how C<flock> is implemented on your system.
File locking I<does not> lock out another process that would like to
do I/O. A file lock only locks out others trying to get a lock, not
those trying to do I/O. Because locks are advisory, if one process uses
locking and another doesn't, all bets are off.
By default, the C<flock> call will block until a lock is granted.
A request for a shared lock will be granted as soon as there is no
exclusive locker. A request for a exclusive lock will be granted as
soon as there is no locker of any kind. Locks are on file descriptors,
not file names. You can't lock a file until you open it, and you can't
hold on to a lock once the file has been closed.
Here's how to get a blocking shared lock on a file, typically used
for reading:
use 5.004;
use Fcntl qw(:DEFAULT :flock);
open(FH, "< filename")
or die "can't open filename: $!";
flock(FH, LOCK_SH)
or die "can't lock filename: $!";
# now read from FH
You can get a non-blocking lock by using
flock(FH, LOCK_SH | LOCK_NB)
or die "can't lock filename: $!";
To get an exclusive lock, typically used for writing, do the following.
We C<sysopen> the file so it can be locked before it was emptied.
You can get a nonblocking version using C<LOCK_EX | LOCK_NB>.
use 5.004;
use Fcntl qw(:DEFAULT :flock);
sysopen(FH, "filename", O_WRONLY | O_CREAT)
or die "can't open filename: $!";
flock(FH, LOCK_EX)
or die "can't lock filename: $!";
truncate(FH, 0)
or die "can't truncate filename: $!";
# now write to FH
Finally, because everyone wants to know how to update a file in place
just because they cannot be dissuaded from wasting cycles on useless
hit counters, here's how you increment a number in a file:
use Fcntl qw(:DEFAULT :flock);
sysopen(FH, "numfile", O_RDWR|O_CREAT)
or die "can't open numfile: $!";
# autoflush FH
$ofh = select(FH); $| = 1; select ($ofh);
flock(FH, LOCK_EX)
or die "can't write-lock numfile: $!";
$num = <FH> || 0;
seek(FH, 0, 0)
or die "can't rewind numfile : $!";
print FH $num+1, "\n"
or die "can't write numfile: $!";
truncate(FH, tell(FH))
or die "can't truncate numfile: $!";
close(FH)
or die "can't close numfile: $!";
=head2 POSIX Tty Handling
[* Does this section belong in this document? *]
Rather than losing yourself in a morass of twisting, turning C<ioctl>s,
all dissimilar, if you're going to manipulate ttys, it's best to
make calls out to the stty(1) program if you have it, or else use the
portable POSIX interface. To figure this all out, you'll need to read the
termios(3) manpage, which describes the POSIX interface to tty devices,
and then L<POSIX>, which describes Perl's interface to POSIX.
For example, here we'll set the speed on particular tty:
use POSIX qw(:termios_h);
$term = POSIX::Termios->new;
sysopen(CUA, "+</dev/cua1", O_RDWR | O_NDELAY | O_NOCTTY)
or die "can't open /dev/cua1: $!";
$fd = fileno(CUA);
$term->getattr($fd));
$term->setospeed(B9600);
$term->setispeed(B9600);
$term->setattr($fd, TCSANOW);
Here's a longer demo of manipulating termios to play games with the the
erase and kill characters:
use POSIX qw(:termios_h);
$term = POSIX::Termios->new;
$term->getattr(fileno(STDIN));
$erase = $term->getcc(VERASE);
$kill = $term->getcc(VKILL);
printf "Erase is character %d, %s\n", $erase, uncontrol(chr($erase));
printf "Kill is character %d, %s\n", $kill, uncontrol(chr($kill));
$term->setcc(VERASE, ord('#'));
$term->setcc(VKILL, ord('@'));
$term->setattr(1, TCSANOW);
print("erase is #, kill is @; type something: ");
$line = <STDIN>;
print "You typed: $line";
$term->setcc(VERASE, $erase);
$term->setcc(VKILL, $kill);
$term->setattr(1, TCSANOW);
sub uncontrol {
local $_ = shift;
s/([\200-\377])/sprintf("M-%c",ord($1) & 0177)/eg;
s/([\0-\37\177])/sprintf("^%c",ord($1) ^ 0100)/eg;
return $_;
}
And here's an example for manipulating single character input
without echo and without waiting for a carriage return:
$| = 1;
while (1) {
print "Enter `q' to quit: ";
$key = getone();
last if ($key eq 'q');
print "$key is not it\n";
}
print "\nbye!\n";
BEGIN {
use POSIX qw(:termios_h);
my ($term, $oterm, $echo, $noecho, $fd_tty);
open(TTY, "+< /dev/tty") || die "no /dev/tty access: $!";
$fd_tty = fileno(TTY);
$term = POSIX::Termios->new();
$term->getattr($fd_tty);
$oterm = $term->getlflag();
$echo = ECHO | ECHOK | ICANON;
$noecho = $oterm & ~$echo;
sub cbreak {
$term->setlflag($noecho);
$term->setcc(VTIME, 1);
$term->setattr($fd_tty, TCSANOW);
}
sub cooked {
$term->setlflag($oterm);
$term->setcc(VTIME, 0);
$term->setattr($fd_tty, TCSANOW);
}
sub getone {
my $key = '';
cbreak();
sysread(TTY, $key, 1);
cooked();
return $key;
}
}
END { cooked() }
If you wanted nonblocking input, you would open the tty this way instead:
use Fcntl;
sysopen(TTY, "/dev/tty", O_RDWR | O_NONBLOCK)
|| die "no /dev/tty access: $!";
With descriptors that you haven't opened using C<sysopen>, such as a
socket, you can set them to be non-blocking using C<fcntl>:
use Fcntl;
fcntl(Connection, F_SETFL, O_NONBLOCK)
or die "can't set non blocking: $!";
=head1 AUTHOR and COPYRIGHT
Copyright 1998 Tom Christiansen.
All Rights Reserved.
=head1 HISTORY
None yet. This is a pre-release.
--
"... an initial underscore already conveys strong feelings of
magicalness to a C programmer."
--Larry Wall in <1992Nov9.195250.23584@netlabs.com>
------------------------------
Date: 16 Dec 1998 09:10:18 -0500
From: clay@panix.com (Clay Irving)
Subject: Re: Formatting Numbers
Message-Id: <758f0a$f1n@panix.com>
In <slrn77epha.l27.sholden@pgrad.cs.usyd.edu.au> sholden@pgrad.cs.usyd.edu.au (Sam Holden) writes:
>It would be much better to refer the person to the FAQ. If you must answer the
>question here and now then at least copy the solution from the FAQ...
> sub commify {
> local $_ = shift;
> 1 while s/^(-?\d+)(\d{3})/$1,$2/;
> return $_;
> }
...or a Perl Module:
Number::Format
http://www.perl.com/CPAN/authors/William_R_Ward/
A library for formatting numbers. Functions are provided for converting
numbers to strings in a variety of ways, and to convert strings that
contain numbers back into numeric form. The output formats may include
thousands separators - characters inserted between each group of three
characters counting right to left from the decimal point. The characters
used for the decimal point and the thousands separator come from the
locale information or can be specified by the user.
Example:
use Number::Format;
my $us = new Number::Format(-thousands_sep => ',',
-decimal_point => '.');
my $formatted = $us->format_number($number);
--
Clay Irving
clay@panix.com
------------------------------
Date: Wed, 16 Dec 1998 08:38:28 -0500
From: "Adam Stoller" <ghoti@telerama.lm.com>
Subject: Re: How do I change current working directory ?
Message-Id: <3677b76d.0@news.nauticom.net>
Peter Forsbom wrote in message <36779D06.A00A286F@statoil.com>...
>I want to change the current working directory so when my perl program
>is done my pwd is diffrent from when I started the program.
>
>Peter Forsbom
In general, a process (your script) spawned by another process (your shell),
cannot alter the spawning process (your shell).
The general alternative is to have your script itself spawn a new shell -
and thus your script won't end until that shell is killed.
#!/bin/perl
[do some stuff - set environments, etc.]
chdir("/some/path");
exec($SHELL);
exit 0;
or something like that (the above is very much off the top-o-my-head)
--fish
------------------------------
Date: 16 Dec 1998 14:55:13 +0100
From: Calle Dybedahl <qdtcall@esb.ericsson.se>
Subject: Re: How do I change current working directory ?
Message-Id: <is3e6gry2m.fsf@godzilla.kiere.ericsson.se>
Peter Forsbom <KONPEF@statoil.com> writes:
> I want to change the current working directory so when my perl program
> is done my pwd is diffrent from when I started the program.
The answer to your question can be found in part 8 of the Perl FAQ.
The Perl FAQ is included in your Perl distribution.
--
Calle Dybedahl, qdtcall@esavionics.se, http://www.lysator.liu.se/~calle/
It is by Perl alone I set my mind in motion. It is by the regex of Larry
that the code acquires flexibility, the flexibility enables obscurity, the
obscurity generates a warning. It is by Perl alone I set my mind in motion.
------------------------------
Date: Wed, 16 Dec 1998 09:38:59 -0500
From: jamesht <jamesht@idt.net>
To: Tommy1452 <tommy1452@aol.com>
Subject: Re: How to automate post form submissions to a server that sets cookies?
Message-Id: <3677C603.CF512F54@idt.net>
Hi,
LWP is definately the way to go. There's a book that you might look at,
called "Web client Programming with Perl". It will get you moving in no
time.
James
------------------------------
Date: Wed, 16 Dec 1998 12:28:34 GMT
From: bart.lateur@skynet.be (Bart Lateur)
Subject: Re: How to extract emails from HTML page
Message-Id: <3677a066.938887@news.skynet.be>
Philip Class wrote:
>Contancting potential customers in a discrete way (inculding our own personal
>address data as sender and offering a 'REMOVE ME' feature) is not what I call
>usual cheap spam.
The REMOVE ME feature should be automatic: i.e. contact people at most
ONCE. If they're interested, they'll come to your website. If they
aren't, you should NEVER bother them again.
If you don't do it like this, it IS the "usual cheap spam".
Bart.
------------------------------
Date: 16 Dec 1998 03:58:16 -0800
From: Russ Allbery <rra@stanford.edu>
Subject: Re: Inconsistent behavior s/// ?
Message-Id: <ylbtl41ep3.fsf@windlord.stanford.edu>
Paul Matthijsse <Paul.Matthijsse@wanadoo.fr> writes:
> I want to change the order of (Dutch) words in a text that is read from
> disk.
Okay, so you're matching words....
> When I read in a short file (a couple of lines), the following code works
> well:
> while (<INPUT>) {
> s/om te vormen een (.*) om/om een \1 te vormen om/g;
Whoops! You're using .*. I'm betting that your problems stem from the
fact that . matches *anything* except a newline. That *includes* spaces.
So (.*) isn't going to just grab one word... it's going to grab absolutely
as much as it can up to the entire rest of the line provided it can still
match.
Because of the om, you limited how much it could match, but when you took
it away....
> When I omit the last occurrence of 'om' in both the search and the replace
> part, resulting in:
> s/om te vormen een (.*)/om een \1 te vormen/g;
> things go wrong: 'om een \1' is replaced well, but the last two words
> ('te vormen') are placed after the last word in the text, instead of
> following the \1.
Right, because \1 actually ended up containing the entire rest of the
line, because that's what .* matched. (BTW, you want to use $1 instead of
\1 when you're refering to a parens on the right side of s///; \1 is only
for the left side.)
What you want is to use something more restrictive than .*. \w* will
match any number of "word characters", which excludes punctuation (except
underscores for mildly obscure reasons if you're not a C programmer) and
high-bit characters (unless you're using locales). \S* will match any
number of non-space characters. One or the other of those will probably
be what you want.
--
#!/usr/bin/perl -- Russ Allbery, Just Another Perl Hacker
$^=q;@!>~|{>krw>yn{u<$$<[~||<Juukn{=,<S~|}<Jwx}qn{<Yn{u<Qjltn{ > 0gFzD gD,
00Fz, 0,,( 0hF 0g)F/=, 0> "L$/GEIFewe{,$/ 0C$~> "@=,m,|,(e 0.), 01,pnn,y{
rw} >;,$0=q,$,,($_=$^)=~y,$/ C-~><@=\n\r,-~$:-u/ #y,d,s,(\$.),$1,gee,print
------------------------------
Date: Wed, 16 Dec 1998 12:31:48 GMT
From: Eric Bohlman <ebohlman@netcom.com>
Subject: Re: Inconsistent behavior s/// ?
Message-Id: <ebohlmanF425H0.6sJ@netcom.com>
Eric Bohlman <ebohlman@netcom.com> wrote:
: 2) a '.*' at the end of a pattern will match a null string, and therefore
: a '(*.)' at the end of a pattern will capture nothing.
My tests show that I was wrong; it will in fact capture to the end of the
string being matched even if it isn't anchored by a '$'. It *will* match
a null string if forced into non-greedy mode. I'm starting to wonder if
I had Perl's behavior confused with that of some other language where
closures default to non-greedy.
------------------------------
Date: 12 Dec 98 21:33:47 GMT (Last modified)
From: Perl-Request@ruby.oce.orst.edu (Perl-Users-Digest Admin)
Subject: Special: Digest Administrivia (Last modified: 12 Dec 98)
Message-Id: <null>
Administrivia:
Well, after 6 months, here's the answer to the quiz: what do we do about
comp.lang.perl.moderated. Answer: nothing.
]From: Russ Allbery <rra@stanford.edu>
]Date: 21 Sep 1998 19:53:43 -0700
]Subject: comp.lang.perl.moderated available via e-mail
]
]It is possible to subscribe to comp.lang.perl.moderated as a mailing list.
]To do so, send mail to majordomo@eyrie.org with "subscribe clpm" in the
]body. Majordomo will then send you instructions on how to confirm your
]subscription. This is provided as a general service for those people who
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]receive messages via e-mail.
The Perl-Users Digest is a retransmission of the USENET newsgroup
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To submit articles to comp.lang.perl.misc (and this Digest), send your
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The Meta-FAQ, an article containing information about the FAQ, is
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The "mini-FAQ", which is an updated version of the Meta-FAQ, is
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------------------------------
End of Perl-Users Digest V8 Issue 4437
**************************************
